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[linux-btrfs-devel.git] / fs / splice.c

/*
 * "splice": joining two ropes together by interweaving their strands.
 *
 * This is the "extended pipe" functionality, where a pipe is used as
 * an arbitrary in-memory buffer. Think of a pipe as a small kernel
 * buffer that you can use to transfer data from one end to the other.
 *
 * The traditional unix read/write is extended with a "splice()" operation
 * that transfers data buffers to or from a pipe buffer.
 *
 * Named by Larry McVoy, original implementation from Linus, extended by
 * Jens to support splicing to files, network, direct splicing, etc and
 * fixing lots of bugs.
 *
 * Copyright (C) 2005-2006 Jens Axboe <axboe@kernel.dk>
 * Copyright (C) 2005-2006 Linus Torvalds <torvalds@osdl.org>
 * Copyright (C) 2006 Ingo Molnar <mingo@elte.hu>
 *
 */
#include <linux/fs.h>
#include <linux/file.h>
#include <linux/pagemap.h>
#include <linux/splice.h>
#include <linux/memcontrol.h>
#include <linux/mm_inline.h>
#include <linux/swap.h>
#include <linux/writeback.h>
#include <linux/buffer_head.h>
#include <linux/module.h>
#include <linux/syscalls.h>
#include <linux/uio.h>
#include <linux/security.h>
#include <linux/gfp.h>

/*
 * Attempt to steal a page from a pipe buffer. This should perhaps go into
 * a vm helper function, it's already simplified quite a bit by the
 * addition of remove_mapping(). If success is returned, the caller may
 * attempt to reuse this page for another destination.
 */
static int page_cache_pipe_buf_steal(struct pipe_inode_info *pipe,
                                     struct pipe_buffer *buf)
{
        struct page *page = buf->page;
        struct address_space *mapping;

        lock_page(page);

        mapping = page_mapping(page);
        if (mapping) {
                WARN_ON(!PageUptodate(page));

                /*
                 * At least for ext2 with nobh option, we need to wait on
                 * writeback completing on this page, since we'll remove it
                 * from the pagecache.  Otherwise truncate wont wait on the
                 * page, allowing the disk blocks to be reused by someone else
                 * before we actually wrote our data to them. fs corruption
                 * ensues.
                 */
                wait_on_page_writeback(page);

                if (page_has_private(page) &&
                    !try_to_release_page(page, GFP_KERNEL))
                        goto out_unlock;

                /*
                 * If we succeeded in removing the mapping, set LRU flag
                 * and return good.
                 */
                if (remove_mapping(mapping, page)) {
                        buf->flags |= PIPE_BUF_FLAG_LRU;
                        return 0;
                }
        }

        /*
         * Raced with truncate or failed to remove page from current
         * address space, unlock and return failure.
         */
out_unlock:
        unlock_page(page);
        return 1;
}

static void page_cache_pipe_buf_release(struct pipe_inode_info *pipe,
                                        struct pipe_buffer *buf)
{
        page_cache_release(buf->page);
        buf->flags &= ~PIPE_BUF_FLAG_LRU;
}

/*
 * Check whether the contents of buf is OK to access. Since the content
 * is a page cache page, IO may be in flight.
 */
static int page_cache_pipe_buf_confirm(struct pipe_inode_info *pipe,
                                       struct pipe_buffer *buf)
{
        struct page *page = buf->page;
        int err;

        if (!PageUptodate(page)) {
                lock_page(page);

                /*
                 * Page got truncated/unhashed. This will cause a 0-byte
                 * splice, if this is the first page.
                 */
                if (!page->mapping) {
                        err = -ENODATA;
                        goto error;
                }

                /*
                 * Uh oh, read-error from disk.
                 */
                if (!PageUptodate(page)) {
                        err = -EIO;
                        goto error;
                }

                /*
                 * Page is ok afterall, we are done.
                 */
                unlock_page(page);
        }

        return 0;
error:
        unlock_page(page);
        return err;
}

const struct pipe_buf_operations page_cache_pipe_buf_ops = {
        .can_merge = 0,
        .map = generic_pipe_buf_map,
        .unmap = generic_pipe_buf_unmap,
        .confirm = page_cache_pipe_buf_confirm,
        .release = page_cache_pipe_buf_release,
        .steal = page_cache_pipe_buf_steal,
        .get = generic_pipe_buf_get,
};

static int user_page_pipe_buf_steal(struct pipe_inode_info *pipe,
                                    struct pipe_buffer *buf)
{
        if (!(buf->flags & PIPE_BUF_FLAG_GIFT))
                return 1;

        buf->flags |= PIPE_BUF_FLAG_LRU;
        return generic_pipe_buf_steal(pipe, buf);
}

static const struct pipe_buf_operations user_page_pipe_buf_ops = {
        .can_merge = 0,
        .map = generic_pipe_buf_map,
        .unmap = generic_pipe_buf_unmap,
        .confirm = generic_pipe_buf_confirm,
        .release = page_cache_pipe_buf_release,
        .steal = user_page_pipe_buf_steal,
        .get = generic_pipe_buf_get,
};

static void wakeup_pipe_readers(struct pipe_inode_info *pipe)
{
        smp_mb();
        if (waitqueue_active(&pipe->wait))
                wake_up_interruptible(&pipe->wait);
        kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
}

/**
 * splice_to_pipe - fill passed data into a pipe
 * @pipe:       pipe to fill
 * @spd:        data to fill
 *
 * Description:
 *    @spd contains a map of pages and len/offset tuples, along with
 *    the struct pipe_buf_operations associated with these pages. This
 *    function will link that data to the pipe.
 *
 */
ssize_t splice_to_pipe(struct pipe_inode_info *pipe,
                       struct splice_pipe_desc *spd)
{
        unsigned int spd_pages = spd->nr_pages;
        int ret, do_wakeup, page_nr;

        ret = 0;
        do_wakeup = 0;
        page_nr = 0;

        pipe_lock(pipe);

        for (;;) {
                if (!pipe->readers) {
                        send_sig(SIGPIPE, current, 0);
                        if (!ret)
                                ret = -EPIPE;
                        break;
                }

                if (pipe->nrbufs < pipe->buffers) {
                        int newbuf = (pipe->curbuf + pipe->nrbufs) & (pipe->buffers - 1);
                        struct pipe_buffer *buf = pipe->bufs + newbuf;

                        buf->page = spd->pages[page_nr];
                        buf->offset = spd->partial[page_nr].offset;
                        buf->len = spd->partial[page_nr].len;
                        buf->private = spd->partial[page_nr].private;
                        buf->ops = spd->ops;
                        if (spd->flags & SPLICE_F_GIFT)
                                buf->flags |= PIPE_BUF_FLAG_GIFT;

                        pipe->nrbufs++;
                        page_nr++;
                        ret += buf->len;

                        if (pipe->inode)
                                do_wakeup = 1;

                        if (!--spd->nr_pages)
                                break;
                        if (pipe->nrbufs < pipe->buffers)
                                continue;

                        break;
                }

                if (spd->flags & SPLICE_F_NONBLOCK) {
                        if (!ret)
                                ret = -EAGAIN;
                        break;
                }

                if (signal_pending(current)) {
                        if (!ret)
                                ret = -ERESTARTSYS;
                        break;
                }

                if (do_wakeup) {
                        smp_mb();
                        if (waitqueue_active(&pipe->wait))
                                wake_up_interruptible_sync(&pipe->wait);
                        kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
                        do_wakeup = 0;
                }

                pipe->waiting_writers++;
                pipe_wait(pipe);
                pipe->waiting_writers--;
        }

        pipe_unlock(pipe);

        if (do_wakeup)
                wakeup_pipe_readers(pipe);

        while (page_nr < spd_pages)
                spd->spd_release(spd, page_nr++);

        return ret;
}

void spd_release_page(struct splice_pipe_desc *spd, unsigned int i)
{
        page_cache_release(spd->pages[i]);
}

/*
 * Check if we need to grow the arrays holding pages and partial page
 * descriptions.
 */
int splice_grow_spd(struct pipe_inode_info *pipe, struct splice_pipe_desc *spd)
{
        if (pipe->buffers <= PIPE_DEF_BUFFERS)
                return 0;

        spd->pages = kmalloc(pipe->buffers * sizeof(struct page *), GFP_KERNEL);
        spd->partial = kmalloc(pipe->buffers * sizeof(struct partial_page), GFP_KERNEL);

        if (spd->pages && spd->partial)
                return 0;

        kfree(spd->pages);
        kfree(spd->partial);
        return -ENOMEM;
}

void splice_shrink_spd(struct pipe_inode_info *pipe,
                       struct splice_pipe_desc *spd)
{
        if (pipe->buffers <= PIPE_DEF_BUFFERS)
                return;

        kfree(spd->pages);
        kfree(spd->partial);
}

static int
__generic_file_splice_read(struct file *in, loff_t *ppos,
                           struct pipe_inode_info *pipe, size_t len,
                           unsigned int flags)
{
        struct address_space *mapping = in->f_mapping;
        unsigned int loff, nr_pages, req_pages;
        struct page *pages[PIPE_DEF_BUFFERS];
        struct partial_page partial[PIPE_DEF_BUFFERS];
        struct page *page;
        pgoff_t index, end_index;
        loff_t isize;
        int error, page_nr;
        struct splice_pipe_desc spd = {
                .pages = pages,
                .partial = partial,
                .flags = flags,
                .ops = &page_cache_pipe_buf_ops,
                .spd_release = spd_release_page,
        };

        if (splice_grow_spd(pipe, &spd))
                return -ENOMEM;

        index = *ppos >> PAGE_CACHE_SHIFT;
        loff = *ppos & ~PAGE_CACHE_MASK;
        req_pages = (len + loff + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
        nr_pages = min(req_pages, pipe->buffers);

        /*
         * Lookup the (hopefully) full range of pages we need.
         */
        spd.nr_pages = find_get_pages_contig(mapping, index, nr_pages, spd.pages);
        index += spd.nr_pages;

        /*
         * If find_get_pages_contig() returned fewer pages than we needed,
         * readahead/allocate the rest and fill in the holes.
         */
        if (spd.nr_pages < nr_pages)
                page_cache_sync_readahead(mapping, &in->f_ra, in,
                                index, req_pages - spd.nr_pages);

        error = 0;
        while (spd.nr_pages < nr_pages) {
                /*
                 * Page could be there, find_get_pages_contig() breaks on
                 * the first hole.
                 */
                page = find_get_page(mapping, index);
                if (!page) {
                        /*
                         * page didn't exist, allocate one.
                         */
                        page = page_cache_alloc_cold(mapping);
                        if (!page)
                                break;

                        error = add_to_page_cache_lru(page, mapping, index,
                                                GFP_KERNEL);
                        if (unlikely(error)) {
                                page_cache_release(page);
                                if (error == -EEXIST)
                                        continue;
                                break;
                        }
                        /*
                         * add_to_page_cache() locks the page, unlock it
                         * to avoid convoluting the logic below even more.
                         */
                        unlock_page(page);
                }

                spd.pages[spd.nr_pages++] = page;
                index++;
        }

        /*
         * Now loop over the map and see if we need to start IO on any
         * pages, fill in the partial map, etc.
         */
        index = *ppos >> PAGE_CACHE_SHIFT;
        nr_pages = spd.nr_pages;
        spd.nr_pages = 0;
        for (page_nr = 0; page_nr < nr_pages; page_nr++) {
                unsigned int this_len;

                if (!len)
                        break;

                /*
                 * this_len is the max we'll use from this page
                 */
                this_len = min_t(unsigned long, len, PAGE_CACHE_SIZE - loff);
                page = spd.pages[page_nr];

                if (PageReadahead(page))
                        page_cache_async_readahead(mapping, &in->f_ra, in,
                                        page, index, req_pages - page_nr);

                /*
                 * If the page isn't uptodate, we may need to start io on it
                 */
                if (!PageUptodate(page)) {
                        lock_page(page);

                        /*
                         * Page was truncated, or invalidated by the
                         * filesystem.  Redo the find/create, but this time the
                         * page is kept locked, so there's no chance of another
                         * race with truncate/invalidate.
                         */
                        if (!page->mapping) {
                                unlock_page(page);
                                page = find_or_create_page(mapping, index,
                                                mapping_gfp_mask(mapping));

                                if (!page) {
                                        error = -ENOMEM;
                                        break;
                                }
                                page_cache_release(spd.pages[page_nr]);
                                spd.pages[page_nr] = page;
                        }
                        /*
                         * page was already under io and is now done, great
                         */
                        if (PageUptodate(page)) {
                                unlock_page(page);
                                goto fill_it;
                        }

                        /*
                         * need to read in the page
                         */
                        error = mapping->a_ops->readpage(in, page);
                        if (unlikely(error)) {
                                /*
                                 * We really should re-lookup the page here,
                                 * but it complicates things a lot. Instead
                                 * lets just do what we already stored, and
                                 * we'll get it the next time we are called.
                                 */
                                if (error == AOP_TRUNCATED_PAGE)
                                        error = 0;

                                break;
                        }
                }
fill_it:
                /*
                 * i_size must be checked after PageUptodate.
                 */
                isize = i_size_read(mapping->host);
                end_index = (isize - 1) >> PAGE_CACHE_SHIFT;
                if (unlikely(!isize || index > end_index))
                        break;

                /*
                 * if this is the last page, see if we need to shrink
                 * the length and stop
                 */
                if (end_index == index) {
                        unsigned int plen;

                        /*
                         * max good bytes in this page
                         */
                        plen = ((isize - 1) & ~PAGE_CACHE_MASK) + 1;
                        if (plen <= loff)
                                break;

                        /*
                         * force quit after adding this page
                         */
                        this_len = min(this_len, plen - loff);
                        len = this_len;
                }

                spd.partial[page_nr].offset = loff;
                spd.partial[page_nr].len = this_len;
                len -= this_len;
                loff = 0;
                spd.nr_pages++;
                index++;
        }

        /*
         * Release any pages at the end, if we quit early. 'page_nr' is how far
         * we got, 'nr_pages' is how many pages are in the map.
         */
        while (page_nr < nr_pages)
                page_cache_release(spd.pages[page_nr++]);
        in->f_ra.prev_pos = (loff_t)index << PAGE_CACHE_SHIFT;

        if (spd.nr_pages)
                error = splice_to_pipe(pipe, &spd);

        splice_shrink_spd(pipe, &spd);
        return error;
}

/**
 * generic_file_splice_read - splice data from file to a pipe
 * @in:         file to splice from
 * @ppos:       position in @in
 * @pipe:       pipe to splice to
 * @len:        number of bytes to splice
 * @flags:      splice modifier flags
 *
 * Description:
 *    Will read pages from given file and fill them into a pipe. Can be
 *    used as long as the address_space operations for the source implements
 *    a readpage() hook.
 *
 */
ssize_t generic_file_splice_read(struct file *in, loff_t *ppos,
                                 struct pipe_inode_info *pipe, size_t len,
                                 unsigned int flags)
{
        loff_t isize, left;
        int ret;

        isize = i_size_read(in->f_mapping->host);
        if (unlikely(*ppos >= isize))
                return 0;

        left = isize - *ppos;
        if (unlikely(left < len))
                len = left;

        ret = __generic_file_splice_read(in, ppos, pipe, len, flags);
        if (ret > 0) {
                *ppos += ret;
                file_accessed(in);
        }

        return ret;
}
EXPORT_SYMBOL(generic_file_splice_read);

static const struct pipe_buf_operations default_pipe_buf_ops = {
        .can_merge = 0,
        .map = generic_pipe_buf_map,
        .unmap = generic_pipe_buf_unmap,
        .confirm = generic_pipe_buf_confirm,
        .release = generic_pipe_buf_release,
        .steal = generic_pipe_buf_steal,
        .get = generic_pipe_buf_get,
};

static ssize_t kernel_readv(struct file *file, const struct iovec *vec,
                            unsigned long vlen, loff_t offset)
{
        mm_segment_t old_fs;
        loff_t pos = offset;
        ssize_t res;

        old_fs = get_fs();
        set_fs(get_ds());
        /* The cast to a user pointer is valid due to the set_fs() */
        res = vfs_readv(file, (const struct iovec __user *)vec, vlen, &pos);
        set_fs(old_fs);

        return res;
}

static ssize_t kernel_write(struct file *file, const char *buf, size_t count,
                            loff_t pos)
{
        mm_segment_t old_fs;
        ssize_t res;

        old_fs = get_fs();
        set_fs(get_ds());
        /* The cast to a user pointer is valid due to the set_fs() */
        res = vfs_write(file, (const char __user *)buf, count, &pos);
        set_fs(old_fs);

        return res;
}

ssize_t default_file_splice_read(struct file *in, loff_t *ppos,
                                 struct pipe_inode_info *pipe, size_t len,
                                 unsigned int flags)
{
        unsigned int nr_pages;
        unsigned int nr_freed;
        size_t offset;
        struct page *pages[PIPE_DEF_BUFFERS];
        struct partial_page partial[PIPE_DEF_BUFFERS];
        struct iovec *vec, __vec[PIPE_DEF_BUFFERS];
        ssize_t res;
        size_t this_len;
        int error;
        int i;
        struct splice_pipe_desc spd = {
                .pages = pages,
                .partial = partial,
                .flags = flags,
                .ops = &default_pipe_buf_ops,
                .spd_release = spd_release_page,
        };

        if (splice_grow_spd(pipe, &spd))
                return -ENOMEM;

        res = -ENOMEM;
        vec = __vec;
        if (pipe->buffers > PIPE_DEF_BUFFERS) {
                vec = kmalloc(pipe->buffers * sizeof(struct iovec), GFP_KERNEL);
                if (!vec)
                        goto shrink_ret;
        }

        offset = *ppos & ~PAGE_CACHE_MASK;
        nr_pages = (len + offset + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;

        for (i = 0; i < nr_pages && i < pipe->buffers && len; i++) {
                struct page *page;

                page = alloc_page(GFP_USER);
                error = -ENOMEM;
                if (!page)
                        goto err;

                this_len = min_t(size_t, len, PAGE_CACHE_SIZE - offset);
                vec[i].iov_base = (void __user *) page_address(page);
                vec[i].iov_len = this_len;
                spd.pages[i] = page;
                spd.nr_pages++;
                len -= this_len;
                offset = 0;
        }

        res = kernel_readv(in, vec, spd.nr_pages, *ppos);
        if (res < 0) {
                error = res;
                goto err;
        }

        error = 0;
        if (!res)
                goto err;

        nr_freed = 0;
        for (i = 0; i < spd.nr_pages; i++) {
                this_len = min_t(size_t, vec[i].iov_len, res);
                spd.partial[i].offset = 0;
                spd.partial[i].len = this_len;
                if (!this_len) {
                        __free_page(spd.pages[i]);
                        spd.pages[i] = NULL;
                        nr_freed++;
                }
                res -= this_len;
        }
        spd.nr_pages -= nr_freed;

        res = splice_to_pipe(pipe, &spd);
        if (res > 0)
                *ppos += res;

shrink_ret:
        if (vec != __vec)
                kfree(vec);
        splice_shrink_spd(pipe, &spd);
        return res;

err:
        for (i = 0; i < spd.nr_pages; i++)
                __free_page(spd.pages[i]);

        res = error;
        goto shrink_ret;
}
EXPORT_SYMBOL(default_file_splice_read);

/*
 * Send 'sd->len' bytes to socket from 'sd->file' at position 'sd->pos'
 * using sendpage(). Return the number of bytes sent.
 */
static int pipe_to_sendpage(struct pipe_inode_info *pipe,
                            struct pipe_buffer *buf, struct splice_desc *sd)
{
        struct file *file = sd->u.file;
        loff_t pos = sd->pos;
        int more;

        if (!likely(file->f_op && file->f_op->sendpage))
                return -EINVAL;

        more = (sd->flags & SPLICE_F_MORE) || sd->len < sd->total_len;
        return file->f_op->sendpage(file, buf->page, buf->offset,
                                    sd->len, &pos, more);
}

/*
 * This is a little more tricky than the file -> pipe splicing. There are
 * basically three cases:
 *
 *      - Destination page already exists in the address space and there
 *        are users of it. For that case we have no other option that
 *        copying the data. Tough luck.
 *      - Destination page already exists in the address space, but there
 *        are no users of it. Make sure it's uptodate, then drop it. Fall
 *        through to last case.
 *      - Destination page does not exist, we can add the pipe page to
 *        the page cache and avoid the copy.
 *
 * If asked to move pages to the output file (SPLICE_F_MOVE is set in
 * sd->flags), we attempt to migrate pages from the pipe to the output
 * file address space page cache. This is possible if no one else has
 * the pipe page referenced outside of the pipe and page cache. If
 * SPLICE_F_MOVE isn't set, or we cannot move the page, we simply create
 * a new page in the output file page cache and fill/dirty that.
 */
int pipe_to_file(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
                 struct splice_desc *sd)
{
        struct file *file = sd->u.file;
        struct address_space *mapping = file->f_mapping;
        unsigned int offset, this_len;
        struct page *page;
        void *fsdata;
        int ret;

        offset = sd->pos & ~PAGE_CACHE_MASK;

        this_len = sd->len;
        if (this_len + offset > PAGE_CACHE_SIZE)
                this_len = PAGE_CACHE_SIZE - offset;

        ret = pagecache_write_begin(file, mapping, sd->pos, this_len,
                                AOP_FLAG_UNINTERRUPTIBLE, &page, &fsdata);
        if (unlikely(ret))
                goto out;

        if (buf->page != page) {
                /*
                 * Careful, ->map() uses KM_USER0!
                 */
                char *src = buf->ops->map(pipe, buf, 1);
                char *dst = kmap_atomic(page, KM_USER1);

                memcpy(dst + offset, src + buf->offset, this_len);
                flush_dcache_page(page);
                kunmap_atomic(dst, KM_USER1);
                buf->ops->unmap(pipe, buf, src);
        }
        ret = pagecache_write_end(file, mapping, sd->pos, this_len, this_len,
                                page, fsdata);
out:
        return ret;
}
EXPORT_SYMBOL(pipe_to_file);

static void wakeup_pipe_writers(struct pipe_inode_info *pipe)
{
        smp_mb();
        if (waitqueue_active(&pipe->wait))
                wake_up_interruptible(&pipe->wait);
        kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
}

/**
 * splice_from_pipe_feed - feed available data from a pipe to a file
 * @pipe:       pipe to splice from
 * @sd:         information to @actor
 * @actor:      handler that splices the data
 *
 * Description:
 *    This function loops over the pipe and calls @actor to do the
 *    actual moving of a single struct pipe_buffer to the desired
 *    destination.  It returns when there's no more buffers left in
 *    the pipe or if the requested number of bytes (@sd->total_len)
 *    have been copied.  It returns a positive number (one) if the
 *    pipe needs to be filled with more data, zero if the required
 *    number of bytes have been copied and -errno on error.
 *
 *    This, together with splice_from_pipe_{begin,end,next}, may be
 *    used to implement the functionality of __splice_from_pipe() when
 *    locking is required around copying the pipe buffers to the
 *    destination.
 */
int splice_from_pipe_feed(struct pipe_inode_info *pipe, struct splice_desc *sd,
                          splice_actor *actor)
{
        int ret;

        while (pipe->nrbufs) {
                struct pipe_buffer *buf = pipe->bufs + pipe->curbuf;
                const struct pipe_buf_operations *ops = buf->ops;

                sd->len = buf->len;
                if (sd->len > sd->total_len)
                        sd->len = sd->total_len;

                ret = buf->ops->confirm(pipe, buf);
                if (unlikely(ret)) {
                        if (ret == -ENODATA)
                                ret = 0;
                        return ret;
                }

                ret = actor(pipe, buf, sd);
                if (ret <= 0)
                        return ret;

                buf->offset += ret;
                buf->len -= ret;

                sd->num_spliced += ret;
                sd->len -= ret;
                sd->pos += ret;
                sd->total_len -= ret;

                if (!buf->len) {
                        buf->ops = NULL;
                        ops->release(pipe, buf);
                        pipe->curbuf = (pipe->curbuf + 1) & (pipe->buffers - 1);
                        pipe->nrbufs--;
                        if (pipe->inode)
                                sd->need_wakeup = true;
                }

                if (!sd->total_len)
                        return 0;
        }

        return 1;
}
EXPORT_SYMBOL(splice_from_pipe_feed);

/**
 * splice_from_pipe_next - wait for some data to splice from
 * @pipe:       pipe to splice from
 * @sd:         information about the splice operation
 *
 * Description:
 *    This function will wait for some data and return a positive
 *    value (one) if pipe buffers are available.  It will return zero
 *    or -errno if no more data needs to be spliced.
 */
int splice_from_pipe_next(struct pipe_inode_info *pipe, struct splice_desc *sd)
{
        while (!pipe->nrbufs) {
                if (!pipe->writers)
                        return 0;

                if (!pipe->waiting_writers && sd->num_spliced)
                        return 0;

                if (sd->flags & SPLICE_F_NONBLOCK)
                        return -EAGAIN;

                if (signal_pending(current))
                        return -ERESTARTSYS;

                if (sd->need_wakeup) {
                        wakeup_pipe_writers(pipe);
                        sd->need_wakeup = false;
                }

                pipe_wait(pipe);
        }

        return 1;
}
EXPORT_SYMBOL(splice_from_pipe_next);

/**
 * splice_from_pipe_begin - start splicing from pipe
 * @sd:         information about the splice operation
 *
 * Description:
 *    This function should be called before a loop containing
 *    splice_from_pipe_next() and splice_from_pipe_feed() to
 *    initialize the necessary fields of @sd.
 */
void splice_from_pipe_begin(struct splice_desc *sd)
{
        sd->num_spliced = 0;
        sd->need_wakeup = false;
}
EXPORT_SYMBOL(splice_from_pipe_begin);

/**
 * splice_from_pipe_end - finish splicing from pipe
 * @pipe:       pipe to splice from
 * @sd:         information about the splice operation
 *
 * Description:
 *    This function will wake up pipe writers if necessary.  It should
 *    be called after a loop containing splice_from_pipe_next() and
 *    splice_from_pipe_feed().
 */
void splice_from_pipe_end(struct pipe_inode_info *pipe, struct splice_desc *sd)
{
        if (sd->need_wakeup)
                wakeup_pipe_writers(pipe);
}
EXPORT_SYMBOL(splice_from_pipe_end);

/**
 * __splice_from_pipe - splice data from a pipe to given actor
 * @pipe:       pipe to splice from
 * @sd:         information to @actor
 * @actor:      handler that splices the data
 *
 * Description:
 *    This function does little more than loop over the pipe and call
 *    @actor to do the actual moving of a single struct pipe_buffer to
 *    the desired destination. See pipe_to_file, pipe_to_sendpage, or
 *    pipe_to_user.
 *
 */
ssize_t __splice_from_pipe(struct pipe_inode_info *pipe, struct splice_desc *sd,
                           splice_actor *actor)
{
        int ret;

        splice_from_pipe_begin(sd);
        do {
                ret = splice_from_pipe_next(pipe, sd);
                if (ret > 0)
                        ret = splice_from_pipe_feed(pipe, sd, actor);
        } while (ret > 0);
        splice_from_pipe_end(pipe, sd);

        return sd->num_spliced ? sd->num_spliced : ret;
}
EXPORT_SYMBOL(__splice_from_pipe);

/**
 * splice_from_pipe - splice data from a pipe to a file
 * @pipe:       pipe to splice from
 * @out:        file to splice to
 * @ppos:       position in @out
 * @len:        how many bytes to splice
 * @flags:      splice modifier flags
 * @actor:      handler that splices the data
 *
 * Description:
 *    See __splice_from_pipe. This function locks the pipe inode,
 *    otherwise it's identical to __splice_from_pipe().
 *
 */
ssize_t splice_from_pipe(struct pipe_inode_info *pipe, struct file *out,
                         loff_t *ppos, size_t len, unsigned int flags,
                         splice_actor *actor)
{
        ssize_t ret;
        struct splice_desc sd = {
                .total_len = len,
                .flags = flags,
                .pos = *ppos,
                .u.file = out,
        };

        pipe_lock(pipe);
        ret = __splice_from_pipe(pipe, &sd, actor);
        pipe_unlock(pipe);

        return ret;
}

/**
 * generic_file_splice_write - splice data from a pipe to a file
 * @pipe:       pipe info
 * @out:        file to write to
 * @ppos:       position in @out
 * @len:        number of bytes to splice
 * @flags:      splice modifier flags
 *
 * Description:
 *    Will either move or copy pages (determined by @flags options) from
 *    the given pipe inode to the given file.
 *
 */
ssize_t
generic_file_splice_write(struct pipe_inode_info *pipe, struct file *out,
                          loff_t *ppos, size_t len, unsigned int flags)
{
        struct address_space *mapping = out->f_mapping;
        struct inode *inode = mapping->host;
        struct splice_desc sd = {
                .total_len = len,
                .flags = flags,
                .pos = *ppos,
                .u.file = out,
        };
        ssize_t ret;

        pipe_lock(pipe);

        splice_from_pipe_begin(&sd);
        do {
                ret = splice_from_pipe_next(pipe, &sd);
                if (ret <= 0)
                        break;

                mutex_lock_nested(&inode->i_mutex, I_MUTEX_CHILD);
                ret = file_remove_suid(out);
                if (!ret) {
                        file_update_time(out);
                        ret = splice_from_pipe_feed(pipe, &sd, pipe_to_file);
                }
                mutex_unlock(&inode->i_mutex);
        } while (ret > 0);
        splice_from_pipe_end(pipe, &sd);

        pipe_unlock(pipe);

        if (sd.num_spliced)
                ret = sd.num_spliced;

        if (ret > 0) {
                unsigned long nr_pages;
                int err;

                nr_pages = (ret + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;

                err = generic_write_sync(out, *ppos, ret);
                if (err)
                        ret = err;
                else
                        *ppos += ret;
                balance_dirty_pages_ratelimited_nr(mapping, nr_pages);
        }

        return ret;
}

EXPORT_SYMBOL(generic_file_splice_write);

static int write_pipe_buf(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
                          struct splice_desc *sd)
{
        int ret;
        void *data;

        data = buf->ops->map(pipe, buf, 0);
        ret = kernel_write(sd->u.file, data + buf->offset, sd->len, sd->pos);
        buf->ops->unmap(pipe, buf, data);

        return ret;
}

static ssize_t default_file_splice_write(struct pipe_inode_info *pipe,
                                         struct file *out, loff_t *ppos,
                                         size_t len, unsigned int flags)
{
        ssize_t ret;

        ret = splice_from_pipe(pipe, out, ppos, len, flags, write_pipe_buf);
        if (ret > 0)
                *ppos += ret;

        return ret;
}

/**
 * generic_splice_sendpage - splice data from a pipe to a socket
 * @pipe:       pipe to splice from
 * @out:        socket to write to
 * @ppos:       position in @out
 * @len:        number of bytes to splice
 * @flags:      splice modifier flags
 *
 * Description:
 *    Will send @len bytes from the pipe to a network socket. No data copying
 *    is involved.
 *
 */
ssize_t generic_splice_sendpage(struct pipe_inode_info *pipe, struct file *out,
                                loff_t *ppos, size_t len, unsigned int flags)
{
        return splice_from_pipe(pipe, out, ppos, len, flags, pipe_to_sendpage);
}

EXPORT_SYMBOL(generic_splice_sendpage);

/*
 * Attempt to initiate a splice from pipe to file.
 */
static long do_splice_from(struct pipe_inode_info *pipe, struct file *out,
                           loff_t *ppos, size_t len, unsigned int flags)
{
        ssize_t (*splice_write)(struct pipe_inode_info *, struct file *,
                                loff_t *, size_t, unsigned int);
        int ret;

        if (unlikely(!(out->f_mode & FMODE_WRITE)))
                return -EBADF;

        if (unlikely(out->f_flags & O_APPEND))
                return -EINVAL;

        ret = rw_verify_area(WRITE, out, ppos, len);
        if (unlikely(ret < 0))
                return ret;

        if (out->f_op && out->f_op->splice_write)
                splice_write = out->f_op->splice_write;
        else
                splice_write = default_file_splice_write;

        return splice_write(pipe, out, ppos, len, flags);
}

/*
 * Attempt to initiate a splice from a file to a pipe.
 */
static long do_splice_to(struct file *in, loff_t *ppos,
                         struct pipe_inode_info *pipe, size_t len,
                         unsigned int flags)
{
        ssize_t (*splice_read)(struct file *, loff_t *,
                               struct pipe_inode_info *, size_t, unsigned int);
        int ret;

        if (unlikely(!(in->f_mode & FMODE_READ)))
                return -EBADF;

        ret = rw_verify_area(READ, in, ppos, len);
        if (unlikely(ret < 0))
                return ret;

        if (in->f_op && in->f_op->splice_read)
                splice_read = in->f_op->splice_read;
        else
                splice_read = default_file_splice_read;

        return splice_read(in, ppos, pipe, len, flags);
}

/**
 * splice_direct_to_actor - splices data directly between two non-pipes
 * @in:         file to splice from
 * @sd:         actor information on where to splice to
 * @actor:      handles the data splicing
 *
 * Description:
 *    This is a special case helper to splice directly between two
 *    points, without requiring an explicit pipe. Internally an allocated
 *    pipe is cached in the process, and reused during the lifetime of
 *    that process.
 *
 */
ssize_t splice_direct_to_actor(struct file *in, struct splice_desc *sd,
                               splice_direct_actor *actor)
{
        struct pipe_inode_info *pipe;
        long ret, bytes;
        umode_t i_mode;
        size_t len;
        int i, flags;

        /*
         * We require the input being a regular file, as we don't want to
         * randomly drop data for eg socket -> socket splicing. Use the
         * piped splicing for that!
         */
        i_mode = in->f_path.dentry->d_inode->i_mode;
        if (unlikely(!S_ISREG(i_mode) && !S_ISBLK(i_mode)))
                return -EINVAL;

        /*
         * neither in nor out is a pipe, setup an internal pipe attached to
         * 'out' and transfer the wanted data from 'in' to 'out' through that
         */
        pipe = current->splice_pipe;
        if (unlikely(!pipe)) {
                pipe = alloc_pipe_info(NULL);
                if (!pipe)
                        return -ENOMEM;

                /*
                 * We don't have an immediate reader, but we'll read the stuff
                 * out of the pipe right after the splice_to_pipe(). So set
                 * PIPE_READERS appropriately.
                 */
                pipe->readers = 1;

                current->splice_pipe = pipe;
        }

        /*
         * Do the splice.
         */
        ret = 0;
        bytes = 0;
        len = sd->total_len;
        flags = sd->flags;

        /*
         * Don't block on output, we have to drain the direct pipe.
         */
        sd->flags &= ~SPLICE_F_NONBLOCK;

        while (len) {
                size_t read_len;
                loff_t pos = sd->pos, prev_pos = pos;

                ret = do_splice_to(in, &pos, pipe, len, flags);
                if (unlikely(ret <= 0))
                        goto out_release;

                read_len = ret;
                sd->total_len = read_len;

                /*
                 * NOTE: nonblocking mode only applies to the input. We
                 * must not do the output in nonblocking mode as then we
                 * could get stuck data in the internal pipe:
                 */
                ret = actor(pipe, sd);
                if (unlikely(ret <= 0)) {
                        sd->pos = prev_pos;
                        goto out_release;
                }

                bytes += ret;
                len -= ret;
                sd->pos = pos;

                if (ret < read_len) {
                        sd->pos = prev_pos + ret;
                        goto out_release;
                }
        }

done:
        pipe->nrbufs = pipe->curbuf = 0;
        file_accessed(in);
        return bytes;

out_release:
        /*
         * If we did an incomplete transfer we must release
         * the pipe buffers in question:
         */
        for (i = 0; i < pipe->buffers; i++) {
                struct pipe_buffer *buf = pipe->bufs + i;

                if (buf->ops) {
                        buf->ops->release(pipe, buf);
                        buf->ops = NULL;
                }
        }

        if (!bytes)
                bytes = ret;

        goto done;
}
EXPORT_SYMBOL(splice_direct_to_actor);

static int direct_splice_actor(struct pipe_inode_info *pipe,
                               struct splice_desc *sd)
{
        struct file *file = sd->u.file;

        return do_splice_from(pipe, file, &file->f_pos, sd->total_len,
                              sd->flags);
}

/**
 * do_splice_direct - splices data directly between two files
 * @in:         file to splice from
 * @ppos:       input file offset
 * @out:        file to splice to
 * @len:        number of bytes to splice
 * @flags:      splice modifier flags
 *
 * Description:
 *    For use by do_sendfile(). splice can easily emulate sendfile, but
 *    doing it in the application would incur an extra system call
 *    (splice in + splice out, as compared to just sendfile()). So this helper
 *    can splice directly through a process-private pipe.
 *
 */
long do_splice_direct(struct file *in, loff_t *ppos, struct file *out,
                      size_t len, unsigned int flags)
{
        struct splice_desc sd = {
                .len            = len,
                .total_len      = len,
                .flags          = flags,
                .pos            = *ppos,
                .u.file         = out,
        };
        long ret;

        ret = splice_direct_to_actor(in, &sd, direct_splice_actor);
        if (ret > 0)
                *ppos = sd.pos;

        return ret;
}

static int splice_pipe_to_pipe(struct pipe_inode_info *ipipe,
                               struct pipe_inode_info *opipe,
                               size_t len, unsigned int flags);

/*
 * Determine where to splice to/from.
 */
static long do_splice(struct file *in, loff_t __user *off_in,
                      struct file *out, loff_t __user *off_out,
                      size_t len, unsigned int flags)
{
        struct pipe_inode_info *ipipe;
        struct pipe_inode_info *opipe;
        loff_t offset, *off;
        long ret;

        ipipe = get_pipe_info(in);
        opipe = get_pipe_info(out);

        if (ipipe && opipe) {
                if (off_in || off_out)
                        return -ESPIPE;

                if (!(in->f_mode & FMODE_READ))
                        return -EBADF;

                if (!(out->f_mode & FMODE_WRITE))
                        return -EBADF;

                /* Splicing to self would be fun, but... */
                if (ipipe == opipe)
                        return -EINVAL;

                return splice_pipe_to_pipe(ipipe, opipe, len, flags);
        }

        if (ipipe) {
                if (off_in)
                        return -ESPIPE;
                if (off_out) {
                        if (!(out->f_mode & FMODE_PWRITE))
                                return -EINVAL;
                        if (copy_from_user(&offset, off_out, sizeof(loff_t)))
                                return -EFAULT;
                        off = &offset;
                } else
                        off = &out->f_pos;

                ret = do_splice_from(ipipe, out, off, len, flags);

                if (off_out && copy_to_user(off_out, off, sizeof(loff_t)))
                        ret = -EFAULT;

                return ret;
        }

        if (opipe) {
                if (off_out)
                        return -ESPIPE;
                if (off_in) {
                        if (!(in->f_mode & FMODE_PREAD))
                                return -EINVAL;
                        if (copy_from_user(&offset, off_in, sizeof(loff_t)))
                                return -EFAULT;
                        off = &offset;
                } else
                        off = &in->f_pos;

                ret = do_splice_to(in, off, opipe, len, flags);

                if (off_in && copy_to_user(off_in, off, sizeof(loff_t)))
                        ret = -EFAULT;

                return ret;
        }

        return -EINVAL;
}

/*
 * Map an iov into an array of pages and offset/length tupples. With the
 * partial_page structure, we can map several non-contiguous ranges into
 * our ones pages[] map instead of splitting that operation into pieces.
 * Could easily be exported as a generic helper for other users, in which
 * case one would probably want to add a 'max_nr_pages' parameter as well.
 */
static int get_iovec_page_array(const struct iovec __user *iov,
                                unsigned int nr_vecs, struct page **pages,
                                struct partial_page *partial, int aligned,
                                unsigned int pipe_buffers)
{
        int buffers = 0, error = 0;

        while (nr_vecs) {
                unsigned long off, npages;
                struct iovec entry;
                void __user *base;
                size_t len;
                int i;

                error = -EFAULT;
                if (copy_from_user(&entry, iov, sizeof(entry)))
                        break;

                base = entry.iov_base;
                len = entry.iov_len;

                /*
                 * Sanity check this iovec. 0 read succeeds.
                 */
                error = 0;
                if (unlikely(!len))
                        break;
                error = -EFAULT;
                if (!access_ok(VERIFY_READ, base, len))
                        break;

                /*
                 * Get this base offset and number of pages, then map
                 * in the user pages.
                 */
                off = (unsigned long) base & ~PAGE_MASK;

                /*
                 * If asked for alignment, the offset must be zero and the
                 * length a multiple of the PAGE_SIZE.
                 */
                error = -EINVAL;
                if (aligned && (off || len & ~PAGE_MASK))
                        break;

                npages = (off + len + PAGE_SIZE - 1) >> PAGE_SHIFT;
                if (npages > pipe_buffers - buffers)
                        npages = pipe_buffers - buffers;

                error = get_user_pages_fast((unsigned long)base, npages,
                                        0, &pages[buffers]);

                if (unlikely(error <= 0))
                        break;

                /*
                 * Fill this contiguous range into the partial page map.
                 */
                for (i = 0; i < error; i++) {
                        const int plen = min_t(size_t, len, PAGE_SIZE - off);

                        partial[buffers].offset = off;
                        partial[buffers].len = plen;

                        off = 0;
                        len -= plen;
                        buffers++;
                }

                /*
                 * We didn't complete this iov, stop here since it probably
                 * means we have to move some of this into a pipe to
                 * be able to continue.
                 */
                if (len)
                        break;

                /*
                 * Don't continue if we mapped fewer pages than we asked for,
                 * or if we mapped the max number of pages that we have
                 * room for.
                 */
                if (error < npages || buffers == pipe_buffers)
                        break;

                nr_vecs--;
                iov++;
        }

        if (buffers)
                return buffers;

        return error;
}

static int pipe_to_user(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
                        struct splice_desc *sd)
{
        char *src;
        int ret;

        /*
         * See if we can use the atomic maps, by prefaulting in the
         * pages and doing an atomic copy
         */
        if (!fault_in_pages_writeable(sd->u.userptr, sd->len)) {
                src = buf->ops->map(pipe, buf, 1);
                ret = __copy_to_user_inatomic(sd->u.userptr, src + buf->offset,
                                                        sd->len);
                buf->ops->unmap(pipe, buf, src);
                if (!ret) {
                        ret = sd->len;
                        goto out;
                }
        }

        /*
         * No dice, use slow non-atomic map and copy
         */
        src = buf->ops->map(pipe, buf, 0);

        ret = sd->len;
        if (copy_to_user(sd->u.userptr, src + buf->offset, sd->len))
                ret = -EFAULT;

        buf->ops->unmap(pipe, buf, src);
out:
        if (ret > 0)
                sd->u.userptr += ret;
        return ret;
}

/*
 * For lack of a better implementation, implement vmsplice() to userspace
 * as a simple copy of the pipes pages to the user iov.
 */
static long vmsplice_to_user(struct file *file, const struct iovec __user *iov,
                             unsigned long nr_segs, unsigned int flags)
{
        struct pipe_inode_info *pipe;
        struct splice_desc sd;
        ssize_t size;
        int error;
        long ret;

        pipe = get_pipe_info(file);
        if (!pipe)
                return -EBADF;

        pipe_lock(pipe);

        error = ret = 0;
        while (nr_segs) {
                void __user *base;
                size_t len;

                /*
                 * Get user address base and length for this iovec.
                 */
                error = get_user(base, &iov->iov_base);
                if (unlikely(error))
                        break;
                error = get_user(len, &iov->iov_len);
                if (unlikely(error))
                        break;

                /*
                 * Sanity check this iovec. 0 read succeeds.
                 */
                if (unlikely(!len))
                        break;
                if (unlikely(!base)) {
                        error = -EFAULT;
                        break;
                }

                if (unlikely(!access_ok(VERIFY_WRITE, base, len))) {
                        error = -EFAULT;
                        break;
                }

                sd.len = 0;
                sd.total_len = len;
                sd.flags = flags;
                sd.u.userptr = base;
                sd.pos = 0;

                size = __splice_from_pipe(pipe, &sd, pipe_to_user);
                if (size < 0) {
                        if (!ret)
                                ret = size;

                        break;
                }

                ret += size;

                if (size < len)
                        break;

                nr_segs--;
                iov++;
        }

        pipe_unlock(pipe);

        if (!ret)
                ret = error;

        return ret;
}

/*
 * vmsplice splices a user address range into a pipe. It can be thought of
 * as splice-from-memory, where the regular splice is splice-from-file (or
 * to file). In both cases the output is a pipe, naturally.
 */
static long vmsplice_to_pipe(struct file *file, const struct iovec __user *iov,
                             unsigned long nr_segs, unsigned int flags)
{
        struct pipe_inode_info *pipe;
        struct page *pages[PIPE_DEF_BUFFERS];
        struct partial_page partial[PIPE_DEF_BUFFERS];
        struct splice_pipe_desc spd = {
                .pages = pages,
                .partial = partial,
                .flags = flags,
                .ops = &user_page_pipe_buf_ops,
                .spd_release = spd_release_page,
        };
        long ret;

        pipe = get_pipe_info(file);
        if (!pipe)
                return -EBADF;

        if (splice_grow_spd(pipe, &spd))
                return -ENOMEM;

        spd.nr_pages = get_iovec_page_array(iov, nr_segs, spd.pages,
                                            spd.partial, flags & SPLICE_F_GIFT,
                                            pipe->buffers);
        if (spd.nr_pages <= 0)
                ret = spd.nr_pages;
        else
                ret = splice_to_pipe(pipe, &spd);

        splice_shrink_spd(pipe, &spd);
        return ret;
}

/*
 * Note that vmsplice only really supports true splicing _from_ user memory
 * to a pipe, not the other way around. Splicing from user memory is a simple
 * operation that can be supported without any funky alignment restrictions
 * or nasty vm tricks. We simply map in the user memory and fill them into
 * a pipe. The reverse isn't quite as easy, though. There are two possible
 * solutions for that:
 *
 *      - memcpy() the data internally, at which point we might as well just
 *        do a regular read() on the buffer anyway.
 *      - Lots of nasty vm tricks, that are neither fast nor flexible (it
 *        has restriction limitations on both ends of the pipe).
 *
 * Currently we punt and implement it as a normal copy, see pipe_to_user().
 *
 */
SYSCALL_DEFINE4(vmsplice, int, fd, const struct iovec __user *, iov,
                unsigned long, nr_segs, unsigned int, flags)
{
        struct file *file;
        long error;
        int fput;

        if (unlikely(nr_segs > UIO_MAXIOV))
                return -EINVAL;
        else if (unlikely(!nr_segs))
                return 0;

        error = -EBADF;
        file = fget_light(fd, &fput);
        if (file) {
                if (file->f_mode & FMODE_WRITE)
                        error = vmsplice_to_pipe(file, iov, nr_segs, flags);
                else if (file->f_mode & FMODE_READ)
                        error = vmsplice_to_user(file, iov, nr_segs, flags);

                fput_light(file, fput);
        }

        return error;
}

SYSCALL_DEFINE6(splice, int, fd_in, loff_t __user *, off_in,
                int, fd_out, loff_t __user *, off_out,
                size_t, len, unsigned int, flags)
{
        long error;
        struct file *in, *out;
        int fput_in, fput_out;

        if (unlikely(!len))
                return 0;

        error = -EBADF;
        in = fget_light(fd_in, &fput_in);
        if (in) {
                if (in->f_mode & FMODE_READ) {
                        out = fget_light(fd_out, &fput_out);
                        if (out) {
                                if (out->f_mode & FMODE_WRITE)
                                        error = do_splice(in, off_in,
                                                          out, off_out,
                                                          len, flags);
                                fput_light(out, fput_out);
                        }
                }

                fput_light(in, fput_in);
        }

        return error;
}

/*
 * Make sure there's data to read. Wait for input if we can, otherwise
 * return an appropriate error.
 */
static int ipipe_prep(struct pipe_inode_info *pipe, unsigned int flags)
{
        int ret;

        /*
         * Check ->nrbufs without the inode lock first. This function
         * is speculative anyways, so missing one is ok.
         */
        if (pipe->nrbufs)
                return 0;

        ret = 0;
        pipe_lock(pipe);

        while (!pipe->nrbufs) {
                if (signal_pending(current)) {
                        ret = -ERESTARTSYS;
                        break;
                }
                if (!pipe->writers)
                        break;
                if (!pipe->waiting_writers) {
                        if (flags & SPLICE_F_NONBLOCK) {
                                ret = -EAGAIN;
                                break;
                        }
                }
                pipe_wait(pipe);
        }

        pipe_unlock(pipe);
        return ret;
}

/*
 * Make sure there's writeable room. Wait for room if we can, otherwise
 * return an appropriate error.
 */
static int opipe_prep(struct pipe_inode_info *pipe, unsigned int flags)
{
        int ret;

        /*
         * Check ->nrbufs without the inode lock first. This function
         * is speculative anyways, so missing one is ok.
         */
        if (pipe->nrbufs < pipe->buffers)
                return 0;

        ret = 0;
        pipe_lock(pipe);

        while (pipe->nrbufs >= pipe->buffers) {
                if (!pipe->readers) {
                        send_sig(SIGPIPE, current, 0);
                        ret = -EPIPE;
                        break;
                }
                if (flags & SPLICE_F_NONBLOCK) {
                        ret = -EAGAIN;
                        break;
                }
                if (signal_pending(current)) {
                        ret = -ERESTARTSYS;
                        break;
                }
                pipe->waiting_writers++;
                pipe_wait(pipe);
                pipe->waiting_writers--;
        }

        pipe_unlock(pipe);
        return ret;
}

/*
 * Splice contents of ipipe to opipe.
 */
static int splice_pipe_to_pipe(struct pipe_inode_info *ipipe,
                               struct pipe_inode_info *opipe,
                               size_t len, unsigned int flags)
{
        struct pipe_buffer *ibuf, *obuf;
        int ret = 0, nbuf;
        bool input_wakeup = false;


retry:
        ret = ipipe_prep(ipipe, flags);
        if (ret)
                return ret;

        ret = opipe_prep(opipe, flags);
        if (ret)
                return ret;

        /*
         * Potential ABBA deadlock, work around it by ordering lock
         * grabbing by pipe info address. Otherwise two different processes
         * could deadlock (one doing tee from A -> B, the other from B -> A).
         */
        pipe_double_lock(ipipe, opipe);

        do {
                if (!opipe->readers) {
                        send_sig(SIGPIPE, current, 0);
                        if (!ret)
                                ret = -EPIPE;
                        break;
                }

                if (!ipipe->nrbufs && !ipipe->writers)
                        break;

                /*
                 * Cannot make any progress, because either the input
                 * pipe is empty or the output pipe is full.
                 */
                if (!ipipe->nrbufs || opipe->nrbufs >= opipe->buffers) {
                        /* Already processed some buffers, break */
                        if (ret)
                                break;

                        if (flags & SPLICE_F_NONBLOCK) {
                                ret = -EAGAIN;
                                break;
                        }

                        /*
                         * We raced with another reader/writer and haven't
                         * managed to process any buffers.  A zero return
                         * value means EOF, so retry instead.
                         */
                        pipe_unlock(ipipe);
                        pipe_unlock(opipe);
                        goto retry;
                }

                ibuf = ipipe->bufs + ipipe->curbuf;
                nbuf = (opipe->curbuf + opipe->nrbufs) & (opipe->buffers - 1);
                obuf = opipe->bufs + nbuf;

                if (len >= ibuf->len) {
                        /*
                         * Simply move the whole buffer from ipipe to opipe
                         */
                        *obuf = *ibuf;
                        ibuf->ops = NULL;
                        opipe->nrbufs++;
                        ipipe->curbuf = (ipipe->curbuf + 1) & (ipipe->buffers - 1);
                        ipipe->nrbufs--;
                        input_wakeup = true;
                } else {
                        /*
                         * Get a reference to this pipe buffer,
                         * so we can copy the contents over.
                         */
                        ibuf->ops->get(ipipe, ibuf);
                        *obuf = *ibuf;

                        /*
                         * Don't inherit the gift flag, we need to
                         * prevent multiple steals of this page.
                         */
                        obuf->flags &= ~PIPE_BUF_FLAG_GIFT;

                        obuf->len = len;
                        opipe->nrbufs++;
                        ibuf->offset += obuf->len;
                        ibuf->len -= obuf->len;
                }
                ret += obuf->len;
                len -= obuf->len;
        } while (len);

        pipe_unlock(ipipe);
        pipe_unlock(opipe);

        /*
         * If we put data in the output pipe, wakeup any potential readers.
         */
        if (ret > 0)
                wakeup_pipe_readers(opipe);

        if (input_wakeup)
                wakeup_pipe_writers(ipipe);

        return ret;
}

/*
 * Link contents of ipipe to opipe.
 */
static int link_pipe(struct pipe_inode_info *ipipe,
                     struct pipe_inode_info *opipe,
                     size_t len, unsigned int flags)
{
        struct pipe_buffer *ibuf, *obuf;
        int ret = 0, i = 0, nbuf;

        /*
         * Potential ABBA deadlock, work around it by ordering lock
         * grabbing by pipe info address. Otherwise two different processes
         * could deadlock (one doing tee from A -> B, the other from B -> A).
         */
        pipe_double_lock(ipipe, opipe);

        do {
                if (!opipe->readers) {
                        send_sig(SIGPIPE, current, 0);
                        if (!ret)
                                ret = -EPIPE;
                        break;
                }

                /*
                 * If we have iterated all input buffers or ran out of
                 * output room, break.
                 */
                if (i >= ipipe->nrbufs || opipe->nrbufs >= opipe->buffers)
                        break;

                ibuf = ipipe->bufs + ((ipipe->curbuf + i) & (ipipe->buffers-1));
                nbuf = (opipe->curbuf + opipe->nrbufs) & (opipe->buffers - 1);

                /*
                 * Get a reference to this pipe buffer,
                 * so we can copy the contents over.
                 */
                ibuf->ops->get(ipipe, ibuf);

                obuf = opipe->bufs + nbuf;
                *obuf = *ibuf;

                /*
                 * Don't inherit the gift flag, we need to
                 * prevent multiple steals of this page.
                 */
                obuf->flags &= ~PIPE_BUF_FLAG_GIFT;

                if (obuf->len > len)
                        obuf->len = len;

                opipe->nrbufs++;
                ret += obuf->len;
                len -= obuf->len;
                i++;
        } while (len);

        /*
         * return EAGAIN if we have the potential of some data in the
         * future, otherwise just return 0
         */
        if (!ret && ipipe->waiting_writers && (flags & SPLICE_F_NONBLOCK))
                ret = -EAGAIN;

        pipe_unlock(ipipe);
        pipe_unlock(opipe);

        /*
         * If we put data in the output pipe, wakeup any potential readers.
         */
        if (ret > 0)
                wakeup_pipe_readers(opipe);

        return ret;
}

/*
 * This is a tee(1) implementation that works on pipes. It doesn't copy
 * any data, it simply references the 'in' pages on the 'out' pipe.
 * The 'flags' used are the SPLICE_F_* variants, currently the only
 * applicable one is SPLICE_F_NONBLOCK.
 */
static long do_tee(struct file *in, struct file *out, size_t len,
                   unsigned int flags)
{
        struct pipe_inode_info *ipipe = get_pipe_info(in);
        struct pipe_inode_info *opipe = get_pipe_info(out);
        int ret = -EINVAL;

        /*
         * Duplicate the contents of ipipe to opipe without actually
         * copying the data.
         */
        if (ipipe && opipe && ipipe != opipe) {
                /*
                 * Keep going, unless we encounter an error. The ipipe/opipe
                 * ordering doesn't really matter.
                 */
                ret = ipipe_prep(ipipe, flags);
                if (!ret) {
                        ret = opipe_prep(opipe, flags);
                        if (!ret)
                                ret = link_pipe(ipipe, opipe, len, flags);
                }
        }

        return ret;
}

SYSCALL_DEFINE4(tee, int, fdin, int, fdout, size_t, len, unsigned int, flags)
{
        struct file *in;
        int error, fput_in;

        if (unlikely(!len))
                return 0;

        error = -EBADF;
        in = fget_light(fdin, &fput_in);
        if (in) {
                if (in->f_mode & FMODE_READ) {
                        int fput_out;
                        struct file *out = fget_light(fdout, &fput_out);

                        if (out) {
                                if (out->f_mode & FMODE_WRITE)
                                        error = do_tee(in, out, len, flags);
                                fput_light(out, fput_out);
                        }
                }
                fput_light(in, fput_in);
        }

        return error;
}