ext4: fix a potential fiemap/page fault deadlock w/ inline_data

[linux/fpc-iii.git] / fs / dax.c

/*
 * fs/dax.c - Direct Access filesystem code
 * Copyright (c) 2013-2014 Intel Corporation
 * Author: Matthew Wilcox <matthew.r.wilcox@intel.com>
 * Author: Ross Zwisler <ross.zwisler@linux.intel.com>
 *
 * This program is free software; you can redistribute it and/or modify it
 * under the terms and conditions of the GNU General Public License,
 * version 2, as published by the Free Software Foundation.
 *
 * This program is distributed in the hope it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
 * more details.
 */

#include <linux/atomic.h>
#include <linux/blkdev.h>
#include <linux/buffer_head.h>
#include <linux/dax.h>
#include <linux/fs.h>
#include <linux/genhd.h>
#include <linux/highmem.h>
#include <linux/memcontrol.h>
#include <linux/mm.h>
#include <linux/mutex.h>
#include <linux/pmem.h>
#include <linux/sched.h>
#include <linux/uio.h>
#include <linux/vmstat.h>

/*
 * dax_clear_blocks() is called from within transaction context from XFS,
 * and hence this means the stack from this point must follow GFP_NOFS
 * semantics for all operations.
 */
int dax_clear_blocks(struct inode *inode, sector_t block, long size)
{
        struct block_device *bdev = inode->i_sb->s_bdev;
        sector_t sector = block << (inode->i_blkbits - 9);

        might_sleep();
        do {
                void __pmem *addr;
                unsigned long pfn;
                long count;

                count = bdev_direct_access(bdev, sector, &addr, &pfn, size);
                if (count < 0)
                        return count;
                BUG_ON(size < count);
                while (count > 0) {
                        unsigned pgsz = PAGE_SIZE - offset_in_page(addr);
                        if (pgsz > count)
                                pgsz = count;
                        clear_pmem(addr, pgsz);
                        addr += pgsz;
                        size -= pgsz;
                        count -= pgsz;
                        BUG_ON(pgsz & 511);
                        sector += pgsz / 512;
                        cond_resched();
                }
        } while (size);

        wmb_pmem();
        return 0;
}
EXPORT_SYMBOL_GPL(dax_clear_blocks);

static long dax_get_addr(struct buffer_head *bh, void __pmem **addr,
                unsigned blkbits)
{
        unsigned long pfn;
        sector_t sector = bh->b_blocknr << (blkbits - 9);
        return bdev_direct_access(bh->b_bdev, sector, addr, &pfn, bh->b_size);
}

/* the clear_pmem() calls are ordered by a wmb_pmem() in the caller */
static void dax_new_buf(void __pmem *addr, unsigned size, unsigned first,
                loff_t pos, loff_t end)
{
        loff_t final = end - pos + first; /* The final byte of the buffer */

        if (first > 0)
                clear_pmem(addr, first);
        if (final < size)
                clear_pmem(addr + final, size - final);
}

static bool buffer_written(struct buffer_head *bh)
{
        return buffer_mapped(bh) && !buffer_unwritten(bh);
}

/*
 * When ext4 encounters a hole, it returns without modifying the buffer_head
 * which means that we can't trust b_size.  To cope with this, we set b_state
 * to 0 before calling get_block and, if any bit is set, we know we can trust
 * b_size.  Unfortunate, really, since ext4 knows precisely how long a hole is
 * and would save us time calling get_block repeatedly.
 */
static bool buffer_size_valid(struct buffer_head *bh)
{
        return bh->b_state != 0;
}

static ssize_t dax_io(struct inode *inode, struct iov_iter *iter,
                      loff_t start, loff_t end, get_block_t get_block,
                      struct buffer_head *bh)
{
        ssize_t retval = 0;
        loff_t pos = start;
        loff_t max = start;
        loff_t bh_max = start;
        void __pmem *addr;
        bool hole = false;
        bool need_wmb = false;

        if (iov_iter_rw(iter) != WRITE)
                end = min(end, i_size_read(inode));

        while (pos < end) {
                size_t len;
                if (pos == max) {
                        unsigned blkbits = inode->i_blkbits;
                        long page = pos >> PAGE_SHIFT;
                        sector_t block = page << (PAGE_SHIFT - blkbits);
                        unsigned first = pos - (block << blkbits);
                        long size;

                        if (pos == bh_max) {
                                bh->b_size = PAGE_ALIGN(end - pos);
                                bh->b_state = 0;
                                retval = get_block(inode, block, bh,
                                                   iov_iter_rw(iter) == WRITE);
                                if (retval)
                                        break;
                                if (!buffer_size_valid(bh))
                                        bh->b_size = 1 << blkbits;
                                bh_max = pos - first + bh->b_size;
                        } else {
                                unsigned done = bh->b_size -
                                                (bh_max - (pos - first));
                                bh->b_blocknr += done >> blkbits;
                                bh->b_size -= done;
                        }

                        hole = iov_iter_rw(iter) != WRITE && !buffer_written(bh);
                        if (hole) {
                                addr = NULL;
                                size = bh->b_size - first;
                        } else {
                                retval = dax_get_addr(bh, &addr, blkbits);
                                if (retval < 0)
                                        break;
                                if (buffer_unwritten(bh) || buffer_new(bh)) {
                                        dax_new_buf(addr, retval, first, pos,
                                                                        end);
                                        need_wmb = true;
                                }
                                addr += first;
                                size = retval - first;
                        }
                        max = min(pos + size, end);
                }

                if (iov_iter_rw(iter) == WRITE) {
                        len = copy_from_iter_pmem(addr, max - pos, iter);
                        need_wmb = true;
                } else if (!hole)
                        len = copy_to_iter((void __force *)addr, max - pos,
                                        iter);
                else
                        len = iov_iter_zero(max - pos, iter);

                if (!len) {
                        retval = -EFAULT;
                        break;
                }

                pos += len;
                addr += len;
        }

        if (need_wmb)
                wmb_pmem();

        return (pos == start) ? retval : pos - start;
}

/**
 * dax_do_io - Perform I/O to a DAX file
 * @iocb: The control block for this I/O
 * @inode: The file which the I/O is directed at
 * @iter: The addresses to do I/O from or to
 * @pos: The file offset where the I/O starts
 * @get_block: The filesystem method used to translate file offsets to blocks
 * @end_io: A filesystem callback for I/O completion
 * @flags: See below
 *
 * This function uses the same locking scheme as do_blockdev_direct_IO:
 * If @flags has DIO_LOCKING set, we assume that the i_mutex is held by the
 * caller for writes.  For reads, we take and release the i_mutex ourselves.
 * If DIO_LOCKING is not set, the filesystem takes care of its own locking.
 * As with do_blockdev_direct_IO(), we increment i_dio_count while the I/O
 * is in progress.
 */
ssize_t dax_do_io(struct kiocb *iocb, struct inode *inode,
                  struct iov_iter *iter, loff_t pos, get_block_t get_block,
                  dio_iodone_t end_io, int flags)
{
        struct buffer_head bh;
        ssize_t retval = -EINVAL;
        loff_t end = pos + iov_iter_count(iter);

        memset(&bh, 0, sizeof(bh));

        if ((flags & DIO_LOCKING) && iov_iter_rw(iter) == READ) {
                struct address_space *mapping = inode->i_mapping;
                mutex_lock(&inode->i_mutex);
                retval = filemap_write_and_wait_range(mapping, pos, end - 1);
                if (retval) {
                        mutex_unlock(&inode->i_mutex);
                        goto out;
                }
        }

        /* Protects against truncate */
        if (!(flags & DIO_SKIP_DIO_COUNT))
                inode_dio_begin(inode);

        retval = dax_io(inode, iter, pos, end, get_block, &bh);

        if ((flags & DIO_LOCKING) && iov_iter_rw(iter) == READ)
                mutex_unlock(&inode->i_mutex);

        if ((retval > 0) && end_io)
                end_io(iocb, pos, retval, bh.b_private);

        if (!(flags & DIO_SKIP_DIO_COUNT))
                inode_dio_end(inode);
 out:
        return retval;
}
EXPORT_SYMBOL_GPL(dax_do_io);

/*
 * The user has performed a load from a hole in the file.  Allocating
 * a new page in the file would cause excessive storage usage for
 * workloads with sparse files.  We allocate a page cache page instead.
 * We'll kick it out of the page cache if it's ever written to,
 * otherwise it will simply fall out of the page cache under memory
 * pressure without ever having been dirtied.
 */
static int dax_load_hole(struct address_space *mapping, struct page *page,
                                                        struct vm_fault *vmf)
{
        unsigned long size;
        struct inode *inode = mapping->host;
        if (!page)
                page = find_or_create_page(mapping, vmf->pgoff,
                                                GFP_KERNEL | __GFP_ZERO);
        if (!page)
                return VM_FAULT_OOM;
        /* Recheck i_size under page lock to avoid truncate race */
        size = (i_size_read(inode) + PAGE_SIZE - 1) >> PAGE_SHIFT;
        if (vmf->pgoff >= size) {
                unlock_page(page);
                page_cache_release(page);
                return VM_FAULT_SIGBUS;
        }

        vmf->page = page;
        return VM_FAULT_LOCKED;
}

static int copy_user_bh(struct page *to, struct buffer_head *bh,
                        unsigned blkbits, unsigned long vaddr)
{
        void __pmem *vfrom;
        void *vto;

        if (dax_get_addr(bh, &vfrom, blkbits) < 0)
                return -EIO;
        vto = kmap_atomic(to);
        copy_user_page(vto, (void __force *)vfrom, vaddr, to);
        kunmap_atomic(vto);
        return 0;
}

static int dax_insert_mapping(struct inode *inode, struct buffer_head *bh,
                        struct vm_area_struct *vma, struct vm_fault *vmf)
{
        struct address_space *mapping = inode->i_mapping;
        sector_t sector = bh->b_blocknr << (inode->i_blkbits - 9);
        unsigned long vaddr = (unsigned long)vmf->virtual_address;
        void __pmem *addr;
        unsigned long pfn;
        pgoff_t size;
        int error;

        i_mmap_lock_read(mapping);

        /*
         * Check truncate didn't happen while we were allocating a block.
         * If it did, this block may or may not be still allocated to the
         * file.  We can't tell the filesystem to free it because we can't
         * take i_mutex here.  In the worst case, the file still has blocks
         * allocated past the end of the file.
         */
        size = (i_size_read(inode) + PAGE_SIZE - 1) >> PAGE_SHIFT;
        if (unlikely(vmf->pgoff >= size)) {
                error = -EIO;
                goto out;
        }

        error = bdev_direct_access(bh->b_bdev, sector, &addr, &pfn, bh->b_size);
        if (error < 0)
                goto out;
        if (error < PAGE_SIZE) {
                error = -EIO;
                goto out;
        }

        if (buffer_unwritten(bh) || buffer_new(bh)) {
                clear_pmem(addr, PAGE_SIZE);
                wmb_pmem();
        }

        error = vm_insert_mixed(vma, vaddr, pfn);

 out:
        i_mmap_unlock_read(mapping);

        return error;
}

/**
 * __dax_fault - handle a page fault on a DAX file
 * @vma: The virtual memory area where the fault occurred
 * @vmf: The description of the fault
 * @get_block: The filesystem method used to translate file offsets to blocks
 * @complete_unwritten: The filesystem method used to convert unwritten blocks
 *      to written so the data written to them is exposed. This is required for
 *      required by write faults for filesystems that will return unwritten
 *      extent mappings from @get_block, but it is optional for reads as
 *      dax_insert_mapping() will always zero unwritten blocks. If the fs does
 *      not support unwritten extents, the it should pass NULL.
 *
 * When a page fault occurs, filesystems may call this helper in their
 * fault handler for DAX files. __dax_fault() assumes the caller has done all
 * the necessary locking for the page fault to proceed successfully.
 */
int __dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf,
                        get_block_t get_block, dax_iodone_t complete_unwritten)
{
        struct file *file = vma->vm_file;
        struct address_space *mapping = file->f_mapping;
        struct inode *inode = mapping->host;
        struct page *page;
        struct buffer_head bh;
        unsigned long vaddr = (unsigned long)vmf->virtual_address;
        unsigned blkbits = inode->i_blkbits;
        sector_t block;
        pgoff_t size;
        int error;
        int major = 0;

        size = (i_size_read(inode) + PAGE_SIZE - 1) >> PAGE_SHIFT;
        if (vmf->pgoff >= size)
                return VM_FAULT_SIGBUS;

        memset(&bh, 0, sizeof(bh));
        block = (sector_t)vmf->pgoff << (PAGE_SHIFT - blkbits);
        bh.b_size = PAGE_SIZE;

 repeat:
        page = find_get_page(mapping, vmf->pgoff);
        if (page) {
                if (!lock_page_or_retry(page, vma->vm_mm, vmf->flags)) {
                        page_cache_release(page);
                        return VM_FAULT_RETRY;
                }
                if (unlikely(page->mapping != mapping)) {
                        unlock_page(page);
                        page_cache_release(page);
                        goto repeat;
                }
                size = (i_size_read(inode) + PAGE_SIZE - 1) >> PAGE_SHIFT;
                if (unlikely(vmf->pgoff >= size)) {
                        /*
                         * We have a struct page covering a hole in the file
                         * from a read fault and we've raced with a truncate
                         */
                        error = -EIO;
                        goto unlock_page;
                }
        }

        error = get_block(inode, block, &bh, 0);
        if (!error && (bh.b_size < PAGE_SIZE))
                error = -EIO;           /* fs corruption? */
        if (error)
                goto unlock_page;

        if (!buffer_mapped(&bh) && !buffer_unwritten(&bh) && !vmf->cow_page) {
                if (vmf->flags & FAULT_FLAG_WRITE) {
                        error = get_block(inode, block, &bh, 1);
                        count_vm_event(PGMAJFAULT);
                        mem_cgroup_count_vm_event(vma->vm_mm, PGMAJFAULT);
                        major = VM_FAULT_MAJOR;
                        if (!error && (bh.b_size < PAGE_SIZE))
                                error = -EIO;
                        if (error)
                                goto unlock_page;
                } else {
                        return dax_load_hole(mapping, page, vmf);
                }
        }

        if (vmf->cow_page) {
                struct page *new_page = vmf->cow_page;
                if (buffer_written(&bh))
                        error = copy_user_bh(new_page, &bh, blkbits, vaddr);
                else
                        clear_user_highpage(new_page, vaddr);
                if (error)
                        goto unlock_page;
                vmf->page = page;
                if (!page) {
                        i_mmap_lock_read(mapping);
                        /* Check we didn't race with truncate */
                        size = (i_size_read(inode) + PAGE_SIZE - 1) >>
                                                                PAGE_SHIFT;
                        if (vmf->pgoff >= size) {
                                i_mmap_unlock_read(mapping);
                                error = -EIO;
                                goto out;
                        }
                }
                return VM_FAULT_LOCKED;
        }

        /* Check we didn't race with a read fault installing a new page */
        if (!page && major)
                page = find_lock_page(mapping, vmf->pgoff);

        if (page) {
                unmap_mapping_range(mapping, vmf->pgoff << PAGE_SHIFT,
                                                        PAGE_CACHE_SIZE, 0);
                delete_from_page_cache(page);
                unlock_page(page);
                page_cache_release(page);
        }

        /*
         * If we successfully insert the new mapping over an unwritten extent,
         * we need to ensure we convert the unwritten extent. If there is an
         * error inserting the mapping, the filesystem needs to leave it as
         * unwritten to prevent exposure of the stale underlying data to
         * userspace, but we still need to call the completion function so
         * the private resources on the mapping buffer can be released. We
         * indicate what the callback should do via the uptodate variable, same
         * as for normal BH based IO completions.
         */
        error = dax_insert_mapping(inode, &bh, vma, vmf);
        if (buffer_unwritten(&bh)) {
                if (complete_unwritten)
                        complete_unwritten(&bh, !error);
                else
                        WARN_ON_ONCE(!(vmf->flags & FAULT_FLAG_WRITE));
        }

 out:
        if (error == -ENOMEM)
                return VM_FAULT_OOM | major;
        /* -EBUSY is fine, somebody else faulted on the same PTE */
        if ((error < 0) && (error != -EBUSY))
                return VM_FAULT_SIGBUS | major;
        return VM_FAULT_NOPAGE | major;

 unlock_page:
        if (page) {
                unlock_page(page);
                page_cache_release(page);
        }
        goto out;
}
EXPORT_SYMBOL(__dax_fault);

/**
 * dax_fault - handle a page fault on a DAX file
 * @vma: The virtual memory area where the fault occurred
 * @vmf: The description of the fault
 * @get_block: The filesystem method used to translate file offsets to blocks
 *
 * When a page fault occurs, filesystems may call this helper in their
 * fault handler for DAX files.
 */
int dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf,
              get_block_t get_block, dax_iodone_t complete_unwritten)
{
        int result;
        struct super_block *sb = file_inode(vma->vm_file)->i_sb;

        if (vmf->flags & FAULT_FLAG_WRITE) {
                sb_start_pagefault(sb);
                file_update_time(vma->vm_file);
        }
        result = __dax_fault(vma, vmf, get_block, complete_unwritten);
        if (vmf->flags & FAULT_FLAG_WRITE)
                sb_end_pagefault(sb);

        return result;
}
EXPORT_SYMBOL_GPL(dax_fault);

#ifdef CONFIG_TRANSPARENT_HUGEPAGE
/*
 * The 'colour' (ie low bits) within a PMD of a page offset.  This comes up
 * more often than one might expect in the below function.
 */
#define PG_PMD_COLOUR   ((PMD_SIZE >> PAGE_SHIFT) - 1)

int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
                pmd_t *pmd, unsigned int flags, get_block_t get_block,
                dax_iodone_t complete_unwritten)
{
        struct file *file = vma->vm_file;
        struct address_space *mapping = file->f_mapping;
        struct inode *inode = mapping->host;
        struct buffer_head bh;
        unsigned blkbits = inode->i_blkbits;
        unsigned long pmd_addr = address & PMD_MASK;
        bool write = flags & FAULT_FLAG_WRITE;
        long length;
        void __pmem *kaddr;
        pgoff_t size, pgoff;
        sector_t block, sector;
        unsigned long pfn;
        int result = 0;

        /* dax pmd mappings are broken wrt gup and fork */
        if (!IS_ENABLED(CONFIG_FS_DAX_PMD))
                return VM_FAULT_FALLBACK;

        /* Fall back to PTEs if we're going to COW */
        if (write && !(vma->vm_flags & VM_SHARED))
                return VM_FAULT_FALLBACK;
        /* If the PMD would extend outside the VMA */
        if (pmd_addr < vma->vm_start)
                return VM_FAULT_FALLBACK;
        if ((pmd_addr + PMD_SIZE) > vma->vm_end)
                return VM_FAULT_FALLBACK;

        pgoff = linear_page_index(vma, pmd_addr);
        size = (i_size_read(inode) + PAGE_SIZE - 1) >> PAGE_SHIFT;
        if (pgoff >= size)
                return VM_FAULT_SIGBUS;
        /* If the PMD would cover blocks out of the file */
        if ((pgoff | PG_PMD_COLOUR) >= size)
                return VM_FAULT_FALLBACK;

        memset(&bh, 0, sizeof(bh));
        block = (sector_t)pgoff << (PAGE_SHIFT - blkbits);

        bh.b_size = PMD_SIZE;
        length = get_block(inode, block, &bh, write);
        if (length)
                return VM_FAULT_SIGBUS;
        i_mmap_lock_read(mapping);

        /*
         * If the filesystem isn't willing to tell us the length of a hole,
         * just fall back to PTEs.  Calling get_block 512 times in a loop
         * would be silly.
         */
        if (!buffer_size_valid(&bh) || bh.b_size < PMD_SIZE)
                goto fallback;

        /*
         * If we allocated new storage, make sure no process has any
         * zero pages covering this hole
         */
        if (buffer_new(&bh)) {
                i_mmap_unlock_read(mapping);
                unmap_mapping_range(mapping, pgoff << PAGE_SHIFT, PMD_SIZE, 0);
                i_mmap_lock_read(mapping);
        }

        /*
         * If a truncate happened while we were allocating blocks, we may
         * leave blocks allocated to the file that are beyond EOF.  We can't
         * take i_mutex here, so just leave them hanging; they'll be freed
         * when the file is deleted.
         */
        size = (i_size_read(inode) + PAGE_SIZE - 1) >> PAGE_SHIFT;
        if (pgoff >= size) {
                result = VM_FAULT_SIGBUS;
                goto out;
        }
        if ((pgoff | PG_PMD_COLOUR) >= size)
                goto fallback;

        if (!write && !buffer_mapped(&bh) && buffer_uptodate(&bh)) {
                spinlock_t *ptl;
                pmd_t entry;
                struct page *zero_page = get_huge_zero_page();

                if (unlikely(!zero_page))
                        goto fallback;

                ptl = pmd_lock(vma->vm_mm, pmd);
                if (!pmd_none(*pmd)) {
                        spin_unlock(ptl);
                        goto fallback;
                }

                entry = mk_pmd(zero_page, vma->vm_page_prot);
                entry = pmd_mkhuge(entry);
                set_pmd_at(vma->vm_mm, pmd_addr, pmd, entry);
                result = VM_FAULT_NOPAGE;
                spin_unlock(ptl);
        } else {
                sector = bh.b_blocknr << (blkbits - 9);
                length = bdev_direct_access(bh.b_bdev, sector, &kaddr, &pfn,
                                                bh.b_size);
                if (length < 0) {
                        result = VM_FAULT_SIGBUS;
                        goto out;
                }
                if ((length < PMD_SIZE) || (pfn & PG_PMD_COLOUR))
                        goto fallback;

                /*
                 * TODO: teach vmf_insert_pfn_pmd() to support
                 * 'pte_special' for pmds
                 */
                if (pfn_valid(pfn))
                        goto fallback;

                if (buffer_unwritten(&bh) || buffer_new(&bh)) {
                        int i;
                        for (i = 0; i < PTRS_PER_PMD; i++)
                                clear_pmem(kaddr + i * PAGE_SIZE, PAGE_SIZE);
                        wmb_pmem();
                        count_vm_event(PGMAJFAULT);
                        mem_cgroup_count_vm_event(vma->vm_mm, PGMAJFAULT);
                        result |= VM_FAULT_MAJOR;
                }

                result |= vmf_insert_pfn_pmd(vma, address, pmd, pfn, write);
        }

 out:
        i_mmap_unlock_read(mapping);

        if (buffer_unwritten(&bh))
                complete_unwritten(&bh, !(result & VM_FAULT_ERROR));

        return result;

 fallback:
        count_vm_event(THP_FAULT_FALLBACK);
        result = VM_FAULT_FALLBACK;
        goto out;
}
EXPORT_SYMBOL_GPL(__dax_pmd_fault);

/**
 * dax_pmd_fault - handle a PMD fault on a DAX file
 * @vma: The virtual memory area where the fault occurred
 * @vmf: The description of the fault
 * @get_block: The filesystem method used to translate file offsets to blocks
 *
 * When a page fault occurs, filesystems may call this helper in their
 * pmd_fault handler for DAX files.
 */
int dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
                        pmd_t *pmd, unsigned int flags, get_block_t get_block,
                        dax_iodone_t complete_unwritten)
{
        int result;
        struct super_block *sb = file_inode(vma->vm_file)->i_sb;

        if (flags & FAULT_FLAG_WRITE) {
                sb_start_pagefault(sb);
                file_update_time(vma->vm_file);
        }
        result = __dax_pmd_fault(vma, address, pmd, flags, get_block,
                                complete_unwritten);
        if (flags & FAULT_FLAG_WRITE)
                sb_end_pagefault(sb);

        return result;
}
EXPORT_SYMBOL_GPL(dax_pmd_fault);
#endif /* CONFIG_TRANSPARENT_HUGEPAGE */

/**
 * dax_pfn_mkwrite - handle first write to DAX page
 * @vma: The virtual memory area where the fault occurred
 * @vmf: The description of the fault
 *
 */
int dax_pfn_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
{
        struct super_block *sb = file_inode(vma->vm_file)->i_sb;

        sb_start_pagefault(sb);
        file_update_time(vma->vm_file);
        sb_end_pagefault(sb);
        return VM_FAULT_NOPAGE;
}
EXPORT_SYMBOL_GPL(dax_pfn_mkwrite);

/**
 * dax_zero_page_range - zero a range within a page of a DAX file
 * @inode: The file being truncated
 * @from: The file offset that is being truncated to
 * @length: The number of bytes to zero
 * @get_block: The filesystem method used to translate file offsets to blocks
 *
 * This function can be called by a filesystem when it is zeroing part of a
 * page in a DAX file.  This is intended for hole-punch operations.  If
 * you are truncating a file, the helper function dax_truncate_page() may be
 * more convenient.
 *
 * We work in terms of PAGE_CACHE_SIZE here for commonality with
 * block_truncate_page(), but we could go down to PAGE_SIZE if the filesystem
 * took care of disposing of the unnecessary blocks.  Even if the filesystem
 * block size is smaller than PAGE_SIZE, we have to zero the rest of the page
 * since the file might be mmapped.
 */
int dax_zero_page_range(struct inode *inode, loff_t from, unsigned length,
                                                        get_block_t get_block)
{
        struct buffer_head bh;
        pgoff_t index = from >> PAGE_CACHE_SHIFT;
        unsigned offset = from & (PAGE_CACHE_SIZE-1);
        int err;

        /* Block boundary? Nothing to do */
        if (!length)
                return 0;
        BUG_ON((offset + length) > PAGE_CACHE_SIZE);

        memset(&bh, 0, sizeof(bh));
        bh.b_size = PAGE_CACHE_SIZE;
        err = get_block(inode, index, &bh, 0);
        if (err < 0)
                return err;
        if (buffer_written(&bh)) {
                void __pmem *addr;
                err = dax_get_addr(&bh, &addr, inode->i_blkbits);
                if (err < 0)
                        return err;
                clear_pmem(addr + offset, length);
                wmb_pmem();
        }

        return 0;
}
EXPORT_SYMBOL_GPL(dax_zero_page_range);

/**
 * dax_truncate_page - handle a partial page being truncated in a DAX file
 * @inode: The file being truncated
 * @from: The file offset that is being truncated to
 * @get_block: The filesystem method used to translate file offsets to blocks
 *
 * Similar to block_truncate_page(), this function can be called by a
 * filesystem when it is truncating a DAX file to handle the partial page.
 *
 * We work in terms of PAGE_CACHE_SIZE here for commonality with
 * block_truncate_page(), but we could go down to PAGE_SIZE if the filesystem
 * took care of disposing of the unnecessary blocks.  Even if the filesystem
 * block size is smaller than PAGE_SIZE, we have to zero the rest of the page
 * since the file might be mmapped.
 */
int dax_truncate_page(struct inode *inode, loff_t from, get_block_t get_block)
{
        unsigned length = PAGE_CACHE_ALIGN(from) - from;
        return dax_zero_page_range(inode, from, length, get_block);
}
EXPORT_SYMBOL_GPL(dax_truncate_page);