Linux 6.12 compat: META

[zfs.git] / module / os / linux / zfs / abd_os.c

/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License (the "License").
 * You may not use this file except in compliance with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or https://opensource.org/licenses/CDDL-1.0.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */
/*
 * Copyright (c) 2014 by Chunwei Chen. All rights reserved.
 * Copyright (c) 2019 by Delphix. All rights reserved.
 * Copyright (c) 2023, 2024, Klara Inc.
 */

/*
 * See abd.c for a general overview of the arc buffered data (ABD).
 *
 * Linear buffers act exactly like normal buffers and are always mapped into the
 * kernel's virtual memory space, while scattered ABD data chunks are allocated
 * as physical pages and then mapped in only while they are actually being
 * accessed through one of the abd_* library functions. Using scattered ABDs
 * provides several benefits:
 *
 *  (1) They avoid use of kmem_*, preventing performance problems where running
 *      kmem_reap on very large memory systems never finishes and causes
 *      constant TLB shootdowns.
 *
 *  (2) Fragmentation is less of an issue since when we are at the limit of
 *      allocatable space, we won't have to search around for a long free
 *      hole in the VA space for large ARC allocations. Each chunk is mapped in
 *      individually, so even if we are using HIGHMEM (see next point) we
 *      wouldn't need to worry about finding a contiguous address range.
 *
 *  (3) If we are not using HIGHMEM, then all physical memory is always
 *      mapped into the kernel's address space, so we also avoid the map /
 *      unmap costs on each ABD access.
 *
 * If we are not using HIGHMEM, scattered buffers which have only one chunk
 * can be treated as linear buffers, because they are contiguous in the
 * kernel's virtual address space.  See abd_alloc_chunks() for details.
 */

#include <sys/abd_impl.h>
#include <sys/param.h>
#include <sys/zio.h>
#include <sys/arc.h>
#include <sys/zfs_context.h>
#include <sys/zfs_znode.h>
#include <linux/kmap_compat.h>
#include <linux/mm_compat.h>
#include <linux/scatterlist.h>
#include <linux/version.h>

#if defined(MAX_ORDER)
#define ABD_MAX_ORDER   (MAX_ORDER)
#elif defined(MAX_PAGE_ORDER)
#define ABD_MAX_ORDER   (MAX_PAGE_ORDER)
#endif

typedef struct abd_stats {
        kstat_named_t abdstat_struct_size;
        kstat_named_t abdstat_linear_cnt;
        kstat_named_t abdstat_linear_data_size;
        kstat_named_t abdstat_scatter_cnt;
        kstat_named_t abdstat_scatter_data_size;
        kstat_named_t abdstat_scatter_chunk_waste;
        kstat_named_t abdstat_scatter_orders[ABD_MAX_ORDER];
        kstat_named_t abdstat_scatter_page_multi_chunk;
        kstat_named_t abdstat_scatter_page_multi_zone;
        kstat_named_t abdstat_scatter_page_alloc_retry;
        kstat_named_t abdstat_scatter_sg_table_retry;
} abd_stats_t;

static abd_stats_t abd_stats = {
        /* Amount of memory occupied by all of the abd_t struct allocations */
        { "struct_size",                        KSTAT_DATA_UINT64 },
        /*
         * The number of linear ABDs which are currently allocated, excluding
         * ABDs which don't own their data (for instance the ones which were
         * allocated through abd_get_offset() and abd_get_from_buf()). If an
         * ABD takes ownership of its buf then it will become tracked.
         */
        { "linear_cnt",                         KSTAT_DATA_UINT64 },
        /* Amount of data stored in all linear ABDs tracked by linear_cnt */
        { "linear_data_size",                   KSTAT_DATA_UINT64 },
        /*
         * The number of scatter ABDs which are currently allocated, excluding
         * ABDs which don't own their data (for instance the ones which were
         * allocated through abd_get_offset()).
         */
        { "scatter_cnt",                        KSTAT_DATA_UINT64 },
        /* Amount of data stored in all scatter ABDs tracked by scatter_cnt */
        { "scatter_data_size",                  KSTAT_DATA_UINT64 },
        /*
         * The amount of space wasted at the end of the last chunk across all
         * scatter ABDs tracked by scatter_cnt.
         */
        { "scatter_chunk_waste",                KSTAT_DATA_UINT64 },
        /*
         * The number of compound allocations of a given order.  These
         * allocations are spread over all currently allocated ABDs, and
         * act as a measure of memory fragmentation.
         */
        { { "scatter_order_N",                  KSTAT_DATA_UINT64 } },
        /*
         * The number of scatter ABDs which contain multiple chunks.
         * ABDs are preferentially allocated from the minimum number of
         * contiguous multi-page chunks, a single chunk is optimal.
         */
        { "scatter_page_multi_chunk",           KSTAT_DATA_UINT64 },
        /*
         * The number of scatter ABDs which are split across memory zones.
         * ABDs are preferentially allocated using pages from a single zone.
         */
        { "scatter_page_multi_zone",            KSTAT_DATA_UINT64 },
        /*
         *  The total number of retries encountered when attempting to
         *  allocate the pages to populate the scatter ABD.
         */
        { "scatter_page_alloc_retry",           KSTAT_DATA_UINT64 },
        /*
         *  The total number of retries encountered when attempting to
         *  allocate the sg table for an ABD.
         */
        { "scatter_sg_table_retry",             KSTAT_DATA_UINT64 },
};

static struct {
        wmsum_t abdstat_struct_size;
        wmsum_t abdstat_linear_cnt;
        wmsum_t abdstat_linear_data_size;
        wmsum_t abdstat_scatter_cnt;
        wmsum_t abdstat_scatter_data_size;
        wmsum_t abdstat_scatter_chunk_waste;
        wmsum_t abdstat_scatter_orders[ABD_MAX_ORDER];
        wmsum_t abdstat_scatter_page_multi_chunk;
        wmsum_t abdstat_scatter_page_multi_zone;
        wmsum_t abdstat_scatter_page_alloc_retry;
        wmsum_t abdstat_scatter_sg_table_retry;
} abd_sums;

#define abd_for_each_sg(abd, sg, n, i)  \
        for_each_sg(ABD_SCATTER(abd).abd_sgl, sg, n, i)

/*
 * zfs_abd_scatter_min_size is the minimum allocation size to use scatter
 * ABD's.  Smaller allocations will use linear ABD's which uses
 * zio_[data_]buf_alloc().
 *
 * Scatter ABD's use at least one page each, so sub-page allocations waste
 * some space when allocated as scatter (e.g. 2KB scatter allocation wastes
 * half of each page).  Using linear ABD's for small allocations means that
 * they will be put on slabs which contain many allocations.  This can
 * improve memory efficiency, but it also makes it much harder for ARC
 * evictions to actually free pages, because all the buffers on one slab need
 * to be freed in order for the slab (and underlying pages) to be freed.
 * Typically, 512B and 1KB kmem caches have 16 buffers per slab, so it's
 * possible for them to actually waste more memory than scatter (one page per
 * buf = wasting 3/4 or 7/8th; one buf per slab = wasting 15/16th).
 *
 * Spill blocks are typically 512B and are heavily used on systems running
 * selinux with the default dnode size and the `xattr=sa` property set.
 *
 * By default we use linear allocations for 512B and 1KB, and scatter
 * allocations for larger (1.5KB and up).
 */
static int zfs_abd_scatter_min_size = 512 * 3;

/*
 * We use a scattered SPA_MAXBLOCKSIZE sized ABD whose pages are
 * just a single zero'd page. This allows us to conserve memory by
 * only using a single zero page for the scatterlist.
 */
abd_t *abd_zero_scatter = NULL;

struct page;

/*
 * abd_zero_page is assigned to each of the pages of abd_zero_scatter. It will
 * point to ZERO_PAGE if it is available or it will be an allocated zero'd
 * PAGESIZE buffer.
 */
static struct page *abd_zero_page = NULL;

static kmem_cache_t *abd_cache = NULL;
static kstat_t *abd_ksp;

static uint_t
abd_chunkcnt_for_bytes(size_t size)
{
        return (P2ROUNDUP(size, PAGESIZE) / PAGESIZE);
}

abd_t *
abd_alloc_struct_impl(size_t size)
{
        /*
         * In Linux we do not use the size passed in during ABD
         * allocation, so we just ignore it.
         */
        (void) size;
        abd_t *abd = kmem_cache_alloc(abd_cache, KM_PUSHPAGE);
        ASSERT3P(abd, !=, NULL);
        ABDSTAT_INCR(abdstat_struct_size, sizeof (abd_t));

        return (abd);
}

void
abd_free_struct_impl(abd_t *abd)
{
        kmem_cache_free(abd_cache, abd);
        ABDSTAT_INCR(abdstat_struct_size, -(int)sizeof (abd_t));
}

static unsigned zfs_abd_scatter_max_order = ABD_MAX_ORDER - 1;

/*
 * Mark zfs data pages so they can be excluded from kernel crash dumps
 */
#ifdef _LP64
#define ABD_FILE_CACHE_PAGE     0x2F5ABDF11ECAC4E

static inline void
abd_mark_zfs_page(struct page *page)
{
        get_page(page);
        SetPagePrivate(page);
        set_page_private(page, ABD_FILE_CACHE_PAGE);
}

static inline void
abd_unmark_zfs_page(struct page *page)
{
        set_page_private(page, 0UL);
        ClearPagePrivate(page);
        put_page(page);
}
#else
#define abd_mark_zfs_page(page)
#define abd_unmark_zfs_page(page)
#endif /* _LP64 */

#ifndef CONFIG_HIGHMEM

#ifndef __GFP_RECLAIM
#define __GFP_RECLAIM           __GFP_WAIT
#endif

/*
 * The goal is to minimize fragmentation by preferentially populating ABDs
 * with higher order compound pages from a single zone.  Allocation size is
 * progressively decreased until it can be satisfied without performing
 * reclaim or compaction.  When necessary this function will degenerate to
 * allocating individual pages and allowing reclaim to satisfy allocations.
 */
void
abd_alloc_chunks(abd_t *abd, size_t size)
{
        struct list_head pages;
        struct sg_table table;
        struct scatterlist *sg;
        struct page *page, *tmp_page = NULL;
        gfp_t gfp = __GFP_RECLAIMABLE | __GFP_NOWARN | GFP_NOIO;
        gfp_t gfp_comp = (gfp | __GFP_NORETRY | __GFP_COMP) & ~__GFP_RECLAIM;
        unsigned int max_order = MIN(zfs_abd_scatter_max_order,
            ABD_MAX_ORDER - 1);
        unsigned int nr_pages = abd_chunkcnt_for_bytes(size);
        unsigned int chunks = 0, zones = 0;
        size_t remaining_size;
        int nid = NUMA_NO_NODE;
        unsigned int alloc_pages = 0;

        INIT_LIST_HEAD(&pages);

        ASSERT3U(alloc_pages, <, nr_pages);

        while (alloc_pages < nr_pages) {
                unsigned int chunk_pages;
                unsigned int order;

                order = MIN(highbit64(nr_pages - alloc_pages) - 1, max_order);
                chunk_pages = (1U << order);

                page = alloc_pages_node(nid, order ? gfp_comp : gfp, order);
                if (page == NULL) {
                        if (order == 0) {
                                ABDSTAT_BUMP(abdstat_scatter_page_alloc_retry);
                                schedule_timeout_interruptible(1);
                        } else {
                                max_order = MAX(0, order - 1);
                        }
                        continue;
                }

                list_add_tail(&page->lru, &pages);

                if ((nid != NUMA_NO_NODE) && (page_to_nid(page) != nid))
                        zones++;

                nid = page_to_nid(page);
                ABDSTAT_BUMP(abdstat_scatter_orders[order]);
                chunks++;
                alloc_pages += chunk_pages;
        }

        ASSERT3S(alloc_pages, ==, nr_pages);

        while (sg_alloc_table(&table, chunks, gfp)) {
                ABDSTAT_BUMP(abdstat_scatter_sg_table_retry);
                schedule_timeout_interruptible(1);
        }

        sg = table.sgl;
        remaining_size = size;
        list_for_each_entry_safe(page, tmp_page, &pages, lru) {
                size_t sg_size = MIN(PAGESIZE << compound_order(page),
                    remaining_size);
                sg_set_page(sg, page, sg_size, 0);
                abd_mark_zfs_page(page);
                remaining_size -= sg_size;

                sg = sg_next(sg);
                list_del(&page->lru);
        }

        /*
         * These conditions ensure that a possible transformation to a linear
         * ABD would be valid.
         */
        ASSERT(!PageHighMem(sg_page(table.sgl)));
        ASSERT0(ABD_SCATTER(abd).abd_offset);

        if (table.nents == 1) {
                /*
                 * Since there is only one entry, this ABD can be represented
                 * as a linear buffer.  All single-page (4K) ABD's can be
                 * represented this way.  Some multi-page ABD's can also be
                 * represented this way, if we were able to allocate a single
                 * "chunk" (higher-order "page" which represents a power-of-2
                 * series of physically-contiguous pages).  This is often the
                 * case for 2-page (8K) ABD's.
                 *
                 * Representing a single-entry scatter ABD as a linear ABD
                 * has the performance advantage of avoiding the copy (and
                 * allocation) in abd_borrow_buf_copy / abd_return_buf_copy.
                 * A performance increase of around 5% has been observed for
                 * ARC-cached reads (of small blocks which can take advantage
                 * of this).
                 *
                 * Note that this optimization is only possible because the
                 * pages are always mapped into the kernel's address space.
                 * This is not the case for highmem pages, so the
                 * optimization can not be made there.
                 */
                abd->abd_flags |= ABD_FLAG_LINEAR;
                abd->abd_flags |= ABD_FLAG_LINEAR_PAGE;
                abd->abd_u.abd_linear.abd_sgl = table.sgl;
                ABD_LINEAR_BUF(abd) = page_address(sg_page(table.sgl));
        } else if (table.nents > 1) {
                ABDSTAT_BUMP(abdstat_scatter_page_multi_chunk);
                abd->abd_flags |= ABD_FLAG_MULTI_CHUNK;

                if (zones) {
                        ABDSTAT_BUMP(abdstat_scatter_page_multi_zone);
                        abd->abd_flags |= ABD_FLAG_MULTI_ZONE;
                }

                ABD_SCATTER(abd).abd_sgl = table.sgl;
                ABD_SCATTER(abd).abd_nents = table.nents;
        }
}
#else

/*
 * Allocate N individual pages to construct a scatter ABD.  This function
 * makes no attempt to request contiguous pages and requires the minimal
 * number of kernel interfaces.  It's designed for maximum compatibility.
 */
void
abd_alloc_chunks(abd_t *abd, size_t size)
{
        struct scatterlist *sg = NULL;
        struct sg_table table;
        struct page *page;
        gfp_t gfp = __GFP_RECLAIMABLE | __GFP_NOWARN | GFP_NOIO;
        int nr_pages = abd_chunkcnt_for_bytes(size);
        int i = 0;

        while (sg_alloc_table(&table, nr_pages, gfp)) {
                ABDSTAT_BUMP(abdstat_scatter_sg_table_retry);
                schedule_timeout_interruptible(1);
        }

        ASSERT3U(table.nents, ==, nr_pages);
        ABD_SCATTER(abd).abd_sgl = table.sgl;
        ABD_SCATTER(abd).abd_nents = nr_pages;

        abd_for_each_sg(abd, sg, nr_pages, i) {
                while ((page = __page_cache_alloc(gfp)) == NULL) {
                        ABDSTAT_BUMP(abdstat_scatter_page_alloc_retry);
                        schedule_timeout_interruptible(1);
                }

                ABDSTAT_BUMP(abdstat_scatter_orders[0]);
                sg_set_page(sg, page, PAGESIZE, 0);
                abd_mark_zfs_page(page);
        }

        if (nr_pages > 1) {
                ABDSTAT_BUMP(abdstat_scatter_page_multi_chunk);
                abd->abd_flags |= ABD_FLAG_MULTI_CHUNK;
        }
}
#endif /* !CONFIG_HIGHMEM */

/*
 * This must be called if any of the sg_table allocation functions
 * are called.
 */
static void
abd_free_sg_table(abd_t *abd)
{
        struct sg_table table;

        table.sgl = ABD_SCATTER(abd).abd_sgl;
        table.nents = table.orig_nents = ABD_SCATTER(abd).abd_nents;
        sg_free_table(&table);
}

void
abd_free_chunks(abd_t *abd)
{
        struct scatterlist *sg = NULL;
        struct page *page;
        int nr_pages = ABD_SCATTER(abd).abd_nents;
        int order, i = 0;

        if (abd->abd_flags & ABD_FLAG_MULTI_ZONE)
                ABDSTAT_BUMPDOWN(abdstat_scatter_page_multi_zone);

        if (abd->abd_flags & ABD_FLAG_MULTI_CHUNK)
                ABDSTAT_BUMPDOWN(abdstat_scatter_page_multi_chunk);

        /*
         * Scatter ABDs may be constructed by abd_alloc_from_pages() from
         * an array of pages. In which case they should not be freed.
         */
        if (!abd_is_from_pages(abd)) {
                abd_for_each_sg(abd, sg, nr_pages, i) {
                        page = sg_page(sg);
                        abd_unmark_zfs_page(page);
                        order = compound_order(page);
                        __free_pages(page, order);
                        ASSERT3U(sg->length, <=, PAGE_SIZE << order);
                        ABDSTAT_BUMPDOWN(abdstat_scatter_orders[order]);
                }
        }

        abd_free_sg_table(abd);
}

/*
 * Allocate scatter ABD of size SPA_MAXBLOCKSIZE, where each page in
 * the scatterlist will be set to the zero'd out buffer abd_zero_page.
 */
static void
abd_alloc_zero_scatter(void)
{
        struct scatterlist *sg = NULL;
        struct sg_table table;
        gfp_t gfp = __GFP_NOWARN | GFP_NOIO;
        int nr_pages = abd_chunkcnt_for_bytes(SPA_MAXBLOCKSIZE);
        int i = 0;

#if defined(HAVE_ZERO_PAGE_GPL_ONLY)
        gfp_t gfp_zero_page = gfp | __GFP_ZERO;
        while ((abd_zero_page = __page_cache_alloc(gfp_zero_page)) == NULL) {
                ABDSTAT_BUMP(abdstat_scatter_page_alloc_retry);
                schedule_timeout_interruptible(1);
        }
        abd_mark_zfs_page(abd_zero_page);
#else
        abd_zero_page = ZERO_PAGE(0);
#endif /* HAVE_ZERO_PAGE_GPL_ONLY */

        while (sg_alloc_table(&table, nr_pages, gfp)) {
                ABDSTAT_BUMP(abdstat_scatter_sg_table_retry);
                schedule_timeout_interruptible(1);
        }
        ASSERT3U(table.nents, ==, nr_pages);

        abd_zero_scatter = abd_alloc_struct(SPA_MAXBLOCKSIZE);
        abd_zero_scatter->abd_flags |= ABD_FLAG_OWNER;
        ABD_SCATTER(abd_zero_scatter).abd_offset = 0;
        ABD_SCATTER(abd_zero_scatter).abd_sgl = table.sgl;
        ABD_SCATTER(abd_zero_scatter).abd_nents = nr_pages;
        abd_zero_scatter->abd_size = SPA_MAXBLOCKSIZE;
        abd_zero_scatter->abd_flags |= ABD_FLAG_MULTI_CHUNK;

        abd_for_each_sg(abd_zero_scatter, sg, nr_pages, i) {
                sg_set_page(sg, abd_zero_page, PAGESIZE, 0);
        }

        ABDSTAT_BUMP(abdstat_scatter_cnt);
        ABDSTAT_INCR(abdstat_scatter_data_size, PAGESIZE);
        ABDSTAT_BUMP(abdstat_scatter_page_multi_chunk);
}

boolean_t
abd_size_alloc_linear(size_t size)
{
        return (!zfs_abd_scatter_enabled || size < zfs_abd_scatter_min_size);
}

void
abd_update_scatter_stats(abd_t *abd, abd_stats_op_t op)
{
        ASSERT(op == ABDSTAT_INCR || op == ABDSTAT_DECR);
        int waste = P2ROUNDUP(abd->abd_size, PAGESIZE) - abd->abd_size;
        if (op == ABDSTAT_INCR) {
                ABDSTAT_BUMP(abdstat_scatter_cnt);
                ABDSTAT_INCR(abdstat_scatter_data_size, abd->abd_size);
                ABDSTAT_INCR(abdstat_scatter_chunk_waste, waste);
                arc_space_consume(waste, ARC_SPACE_ABD_CHUNK_WASTE);
        } else {
                ABDSTAT_BUMPDOWN(abdstat_scatter_cnt);
                ABDSTAT_INCR(abdstat_scatter_data_size, -(int)abd->abd_size);
                ABDSTAT_INCR(abdstat_scatter_chunk_waste, -waste);
                arc_space_return(waste, ARC_SPACE_ABD_CHUNK_WASTE);
        }
}

void
abd_update_linear_stats(abd_t *abd, abd_stats_op_t op)
{
        ASSERT(op == ABDSTAT_INCR || op == ABDSTAT_DECR);
        if (op == ABDSTAT_INCR) {
                ABDSTAT_BUMP(abdstat_linear_cnt);
                ABDSTAT_INCR(abdstat_linear_data_size, abd->abd_size);
        } else {
                ABDSTAT_BUMPDOWN(abdstat_linear_cnt);
                ABDSTAT_INCR(abdstat_linear_data_size, -(int)abd->abd_size);
        }
}

void
abd_verify_scatter(abd_t *abd)
{
        ASSERT3U(ABD_SCATTER(abd).abd_nents, >, 0);
        ASSERT3U(ABD_SCATTER(abd).abd_offset, <,
            ABD_SCATTER(abd).abd_sgl->length);

#ifdef ZFS_DEBUG
        struct scatterlist *sg = NULL;
        size_t n = ABD_SCATTER(abd).abd_nents;
        int i = 0;

        abd_for_each_sg(abd, sg, n, i) {
                ASSERT3P(sg_page(sg), !=, NULL);
        }
#endif
}

static void
abd_free_zero_scatter(void)
{
        ABDSTAT_BUMPDOWN(abdstat_scatter_cnt);
        ABDSTAT_INCR(abdstat_scatter_data_size, -(int)PAGESIZE);
        ABDSTAT_BUMPDOWN(abdstat_scatter_page_multi_chunk);

        abd_free_sg_table(abd_zero_scatter);
        abd_free_struct(abd_zero_scatter);
        abd_zero_scatter = NULL;
        ASSERT3P(abd_zero_page, !=, NULL);
#if defined(HAVE_ZERO_PAGE_GPL_ONLY)
        abd_unmark_zfs_page(abd_zero_page);
        __free_page(abd_zero_page);
#endif /* HAVE_ZERO_PAGE_GPL_ONLY */
}

static int
abd_kstats_update(kstat_t *ksp, int rw)
{
        abd_stats_t *as = ksp->ks_data;

        if (rw == KSTAT_WRITE)
                return (EACCES);
        as->abdstat_struct_size.value.ui64 =
            wmsum_value(&abd_sums.abdstat_struct_size);
        as->abdstat_linear_cnt.value.ui64 =
            wmsum_value(&abd_sums.abdstat_linear_cnt);
        as->abdstat_linear_data_size.value.ui64 =
            wmsum_value(&abd_sums.abdstat_linear_data_size);
        as->abdstat_scatter_cnt.value.ui64 =
            wmsum_value(&abd_sums.abdstat_scatter_cnt);
        as->abdstat_scatter_data_size.value.ui64 =
            wmsum_value(&abd_sums.abdstat_scatter_data_size);
        as->abdstat_scatter_chunk_waste.value.ui64 =
            wmsum_value(&abd_sums.abdstat_scatter_chunk_waste);
        for (int i = 0; i < ABD_MAX_ORDER; i++) {
                as->abdstat_scatter_orders[i].value.ui64 =
                    wmsum_value(&abd_sums.abdstat_scatter_orders[i]);
        }
        as->abdstat_scatter_page_multi_chunk.value.ui64 =
            wmsum_value(&abd_sums.abdstat_scatter_page_multi_chunk);
        as->abdstat_scatter_page_multi_zone.value.ui64 =
            wmsum_value(&abd_sums.abdstat_scatter_page_multi_zone);
        as->abdstat_scatter_page_alloc_retry.value.ui64 =
            wmsum_value(&abd_sums.abdstat_scatter_page_alloc_retry);
        as->abdstat_scatter_sg_table_retry.value.ui64 =
            wmsum_value(&abd_sums.abdstat_scatter_sg_table_retry);
        return (0);
}

void
abd_init(void)
{
        int i;

        abd_cache = kmem_cache_create("abd_t", sizeof (abd_t),
            0, NULL, NULL, NULL, NULL, NULL, KMC_RECLAIMABLE);

        wmsum_init(&abd_sums.abdstat_struct_size, 0);
        wmsum_init(&abd_sums.abdstat_linear_cnt, 0);
        wmsum_init(&abd_sums.abdstat_linear_data_size, 0);
        wmsum_init(&abd_sums.abdstat_scatter_cnt, 0);
        wmsum_init(&abd_sums.abdstat_scatter_data_size, 0);
        wmsum_init(&abd_sums.abdstat_scatter_chunk_waste, 0);
        for (i = 0; i < ABD_MAX_ORDER; i++)
                wmsum_init(&abd_sums.abdstat_scatter_orders[i], 0);
        wmsum_init(&abd_sums.abdstat_scatter_page_multi_chunk, 0);
        wmsum_init(&abd_sums.abdstat_scatter_page_multi_zone, 0);
        wmsum_init(&abd_sums.abdstat_scatter_page_alloc_retry, 0);
        wmsum_init(&abd_sums.abdstat_scatter_sg_table_retry, 0);

        abd_ksp = kstat_create("zfs", 0, "abdstats", "misc", KSTAT_TYPE_NAMED,
            sizeof (abd_stats) / sizeof (kstat_named_t), KSTAT_FLAG_VIRTUAL);
        if (abd_ksp != NULL) {
                for (i = 0; i < ABD_MAX_ORDER; i++) {
                        snprintf(abd_stats.abdstat_scatter_orders[i].name,
                            KSTAT_STRLEN, "scatter_order_%d", i);
                        abd_stats.abdstat_scatter_orders[i].data_type =
                            KSTAT_DATA_UINT64;
                }
                abd_ksp->ks_data = &abd_stats;
                abd_ksp->ks_update = abd_kstats_update;
                kstat_install(abd_ksp);
        }

        abd_alloc_zero_scatter();
}

void
abd_fini(void)
{
        abd_free_zero_scatter();

        if (abd_ksp != NULL) {
                kstat_delete(abd_ksp);
                abd_ksp = NULL;
        }

        wmsum_fini(&abd_sums.abdstat_struct_size);
        wmsum_fini(&abd_sums.abdstat_linear_cnt);
        wmsum_fini(&abd_sums.abdstat_linear_data_size);
        wmsum_fini(&abd_sums.abdstat_scatter_cnt);
        wmsum_fini(&abd_sums.abdstat_scatter_data_size);
        wmsum_fini(&abd_sums.abdstat_scatter_chunk_waste);
        for (int i = 0; i < ABD_MAX_ORDER; i++)
                wmsum_fini(&abd_sums.abdstat_scatter_orders[i]);
        wmsum_fini(&abd_sums.abdstat_scatter_page_multi_chunk);
        wmsum_fini(&abd_sums.abdstat_scatter_page_multi_zone);
        wmsum_fini(&abd_sums.abdstat_scatter_page_alloc_retry);
        wmsum_fini(&abd_sums.abdstat_scatter_sg_table_retry);

        if (abd_cache) {
                kmem_cache_destroy(abd_cache);
                abd_cache = NULL;
        }
}

void
abd_free_linear_page(abd_t *abd)
{
        /* Transform it back into a scatter ABD for freeing */
        struct scatterlist *sg = abd->abd_u.abd_linear.abd_sgl;

        /* When backed by user page unmap it */
        if (abd_is_from_pages(abd))
                zfs_kunmap(sg_page(sg));
        else
                abd_update_scatter_stats(abd, ABDSTAT_DECR);

        abd->abd_flags &= ~ABD_FLAG_LINEAR;
        abd->abd_flags &= ~ABD_FLAG_LINEAR_PAGE;
        ABD_SCATTER(abd).abd_nents = 1;
        ABD_SCATTER(abd).abd_offset = 0;
        ABD_SCATTER(abd).abd_sgl = sg;
        abd_free_chunks(abd);
}

/*
 * Allocate a scatter ABD structure from user pages. The pages must be
 * pinned with get_user_pages, or similiar, but need not be mapped via
 * the kmap interfaces.
 */
abd_t *
abd_alloc_from_pages(struct page **pages, unsigned long offset, uint64_t size)
{
        uint_t npages = DIV_ROUND_UP(size, PAGE_SIZE);
        struct sg_table table;

        VERIFY3U(size, <=, DMU_MAX_ACCESS);
        ASSERT3U(offset, <, PAGE_SIZE);
        ASSERT3P(pages, !=, NULL);

        /*
         * Even if this buf is filesystem metadata, we only track that we
         * own the underlying data buffer, which is not true in this case.
         * Therefore, we don't ever use ABD_FLAG_META here.
         */
        abd_t *abd = abd_alloc_struct(0);
        abd->abd_flags |= ABD_FLAG_FROM_PAGES | ABD_FLAG_OWNER;
        abd->abd_size = size;

        while (sg_alloc_table_from_pages(&table, pages, npages, offset,
            size, __GFP_NOWARN | GFP_NOIO) != 0) {
                ABDSTAT_BUMP(abdstat_scatter_sg_table_retry);
                schedule_timeout_interruptible(1);
        }

        if ((offset + size) <= PAGE_SIZE) {
                /*
                 * Since there is only one entry, this ABD can be represented
                 * as a linear buffer. All single-page (4K) ABD's constructed
                 * from a user page can be represented this way as long as the
                 * page is mapped to a virtual address. This allows us to
                 * apply an offset in to the mapped page.
                 *
                 * Note that kmap() must be used, not kmap_atomic(), because
                 * the mapping needs to bet set up on all CPUs. Using kmap()
                 * also enables the user of highmem pages when required.
                 */
                abd->abd_flags |= ABD_FLAG_LINEAR | ABD_FLAG_LINEAR_PAGE;
                abd->abd_u.abd_linear.abd_sgl = table.sgl;
                zfs_kmap(sg_page(table.sgl));
                ABD_LINEAR_BUF(abd) = sg_virt(table.sgl);
        } else {
                ABDSTAT_BUMP(abdstat_scatter_page_multi_chunk);
                abd->abd_flags |= ABD_FLAG_MULTI_CHUNK;

                ABD_SCATTER(abd).abd_offset = offset;
                ABD_SCATTER(abd).abd_sgl = table.sgl;
                ABD_SCATTER(abd).abd_nents = table.nents;

                ASSERT0(ABD_SCATTER(abd).abd_offset);
        }

        return (abd);
}

/*
 * If we're going to use this ABD for doing I/O using the block layer, the
 * consumer of the ABD data doesn't care if it's scattered or not, and we don't
 * plan to store this ABD in memory for a long period of time, we should
 * allocate the ABD type that requires the least data copying to do the I/O.
 *
 * On Linux the optimal thing to do would be to use abd_get_offset() and
 * construct a new ABD which shares the original pages thereby eliminating
 * the copy.  But for the moment a new linear ABD is allocated until this
 * performance optimization can be implemented.
 */
abd_t *
abd_alloc_for_io(size_t size, boolean_t is_metadata)
{
        return (abd_alloc(size, is_metadata));
}

abd_t *
abd_get_offset_scatter(abd_t *abd, abd_t *sabd, size_t off,
    size_t size)
{
        (void) size;
        int i = 0;
        struct scatterlist *sg = NULL;

        abd_verify(sabd);
        ASSERT3U(off, <=, sabd->abd_size);

        size_t new_offset = ABD_SCATTER(sabd).abd_offset + off;

        if (abd == NULL)
                abd = abd_alloc_struct(0);

        /*
         * Even if this buf is filesystem metadata, we only track that
         * if we own the underlying data buffer, which is not true in
         * this case. Therefore, we don't ever use ABD_FLAG_META here.
         */

        abd_for_each_sg(sabd, sg, ABD_SCATTER(sabd).abd_nents, i) {
                if (new_offset < sg->length)
                        break;
                new_offset -= sg->length;
        }

        ABD_SCATTER(abd).abd_sgl = sg;
        ABD_SCATTER(abd).abd_offset = new_offset;
        ABD_SCATTER(abd).abd_nents = ABD_SCATTER(sabd).abd_nents - i;

        if (abd_is_from_pages(sabd))
                abd->abd_flags |= ABD_FLAG_FROM_PAGES;

        return (abd);
}

/*
 * Initialize the abd_iter.
 */
void
abd_iter_init(struct abd_iter *aiter, abd_t *abd)
{
        ASSERT(!abd_is_gang(abd));
        abd_verify(abd);
        memset(aiter, 0, sizeof (struct abd_iter));
        aiter->iter_abd = abd;
        if (!abd_is_linear(abd)) {
                aiter->iter_offset = ABD_SCATTER(abd).abd_offset;
                aiter->iter_sg = ABD_SCATTER(abd).abd_sgl;
        }
}

/*
 * This is just a helper function to see if we have exhausted the
 * abd_iter and reached the end.
 */
boolean_t
abd_iter_at_end(struct abd_iter *aiter)
{
        ASSERT3U(aiter->iter_pos, <=, aiter->iter_abd->abd_size);
        return (aiter->iter_pos == aiter->iter_abd->abd_size);
}

/*
 * Advance the iterator by a certain amount. Cannot be called when a chunk is
 * in use. This can be safely called when the aiter has already exhausted, in
 * which case this does nothing.
 */
void
abd_iter_advance(struct abd_iter *aiter, size_t amount)
{
        /*
         * Ensure that last chunk is not in use. abd_iterate_*() must clear
         * this state (directly or abd_iter_unmap()) before advancing.
         */
        ASSERT3P(aiter->iter_mapaddr, ==, NULL);
        ASSERT0(aiter->iter_mapsize);
        ASSERT3P(aiter->iter_page, ==, NULL);
        ASSERT0(aiter->iter_page_doff);
        ASSERT0(aiter->iter_page_dsize);

        /* There's nothing left to advance to, so do nothing */
        if (abd_iter_at_end(aiter))
                return;

        aiter->iter_pos += amount;
        aiter->iter_offset += amount;
        if (!abd_is_linear(aiter->iter_abd)) {
                while (aiter->iter_offset >= aiter->iter_sg->length) {
                        aiter->iter_offset -= aiter->iter_sg->length;
                        aiter->iter_sg = sg_next(aiter->iter_sg);
                        if (aiter->iter_sg == NULL) {
                                ASSERT0(aiter->iter_offset);
                                break;
                        }
                }
        }
}

/*
 * Map the current chunk into aiter. This can be safely called when the aiter
 * has already exhausted, in which case this does nothing.
 */
void
abd_iter_map(struct abd_iter *aiter)
{
        void *paddr;
        size_t offset = 0;

        ASSERT3P(aiter->iter_mapaddr, ==, NULL);
        ASSERT0(aiter->iter_mapsize);

        /* There's nothing left to iterate over, so do nothing */
        if (abd_iter_at_end(aiter))
                return;

        if (abd_is_linear(aiter->iter_abd)) {
                ASSERT3U(aiter->iter_pos, ==, aiter->iter_offset);
                offset = aiter->iter_offset;
                aiter->iter_mapsize = aiter->iter_abd->abd_size - offset;
                paddr = ABD_LINEAR_BUF(aiter->iter_abd);
        } else {
                offset = aiter->iter_offset;
                aiter->iter_mapsize = MIN(aiter->iter_sg->length - offset,
                    aiter->iter_abd->abd_size - aiter->iter_pos);

                paddr = zfs_kmap_local(sg_page(aiter->iter_sg));
        }

        aiter->iter_mapaddr = (char *)paddr + offset;
}

/*
 * Unmap the current chunk from aiter. This can be safely called when the aiter
 * has already exhausted, in which case this does nothing.
 */
void
abd_iter_unmap(struct abd_iter *aiter)
{
        /* There's nothing left to unmap, so do nothing */
        if (abd_iter_at_end(aiter))
                return;

        if (!abd_is_linear(aiter->iter_abd)) {
                /* LINTED E_FUNC_SET_NOT_USED */
                zfs_kunmap_local(aiter->iter_mapaddr - aiter->iter_offset);
        }

        ASSERT3P(aiter->iter_mapaddr, !=, NULL);
        ASSERT3U(aiter->iter_mapsize, >, 0);

        aiter->iter_mapaddr = NULL;
        aiter->iter_mapsize = 0;
}

void
abd_cache_reap_now(void)
{
}

/*
 * Borrow a raw buffer from an ABD without copying the contents of the ABD
 * into the buffer. If the ABD is scattered, this will allocate a raw buffer
 * whose contents are undefined. To copy over the existing data in the ABD, use
 * abd_borrow_buf_copy() instead.
 */
void *
abd_borrow_buf(abd_t *abd, size_t n)
{
        void *buf;
        abd_verify(abd);
        ASSERT3U(abd->abd_size, >=, 0);
        /*
         * In the event the ABD is composed of a single user page from Direct
         * I/O we can not direclty return the raw buffer. This is a consequence
         * of not being able to write protect the page and the contents of the
         * page can be changed at any time by the user.
         */
        if (abd_is_from_pages(abd)) {
                buf = zio_buf_alloc(n);
        } else if (abd_is_linear(abd)) {
                buf = abd_to_buf(abd);
        } else {
                buf = zio_buf_alloc(n);
        }

#ifdef ZFS_DEBUG
        (void) zfs_refcount_add_many(&abd->abd_children, n, buf);
#endif
        return (buf);
}

void *
abd_borrow_buf_copy(abd_t *abd, size_t n)
{
        void *buf = abd_borrow_buf(abd, n);

        /*
         * In the event the ABD is composed of a single user page from Direct
         * I/O we must make sure copy the data over into the newly allocated
         * buffer. This is a consequence of the fact that we can not write
         * protect the user page and there is a risk the contents of the page
         * could be changed by the user at any moment.
         */
        if (!abd_is_linear(abd) || abd_is_from_pages(abd)) {
                abd_copy_to_buf(buf, abd, n);
        }
        return (buf);
}

/*
 * Return a borrowed raw buffer to an ABD. If the ABD is scatterd, this will
 * not change the contents of the ABD. If you want any changes you made to
 * buf to be copied back to abd, use abd_return_buf_copy() instead. If the
 * ABD is not constructed from user pages for Direct I/O then an ASSERT
 * checks to make sure the contents of buffer have not changed since it was
 * borrowed. We can not ASSERT that the contents of the buffer have not changed
 * if it is composed of user pages because the pages can not be placed under
 * write protection and the user could have possibly changed the contents in
 * the pages at any time. This is also an issue for Direct I/O reads. Checksum
 * verifications in the ZIO pipeline check for this issue and handle it by
 * returning an error on checksum verification failure.
 */
void
abd_return_buf(abd_t *abd, void *buf, size_t n)
{
        abd_verify(abd);
        ASSERT3U(abd->abd_size, >=, n);
#ifdef ZFS_DEBUG
        (void) zfs_refcount_remove_many(&abd->abd_children, n, buf);
#endif
        if (abd_is_from_pages(abd)) {
                zio_buf_free(buf, n);
        } else if (abd_is_linear(abd)) {
                ASSERT3P(buf, ==, abd_to_buf(abd));
        } else if (abd_is_gang(abd)) {
#ifdef ZFS_DEBUG
                /*
                 * We have to be careful with gang ABD's that we do not ASSERT0
                 * for any ABD's that contain user pages from Direct I/O. In
                 * order to handle this, we just iterate through the gang ABD
                 * and only verify ABDs that are not from user pages.
                 */
                void *cmp_buf = buf;

                for (abd_t *cabd = list_head(&ABD_GANG(abd).abd_gang_chain);
                    cabd != NULL;
                    cabd = list_next(&ABD_GANG(abd).abd_gang_chain, cabd)) {
                        if (!abd_is_from_pages(cabd)) {
                                ASSERT0(abd_cmp_buf(cabd, cmp_buf,
                                    cabd->abd_size));
                        }
                        cmp_buf = (char *)cmp_buf + cabd->abd_size;
                }
#endif
                zio_buf_free(buf, n);
        } else {
                ASSERT0(abd_cmp_buf(abd, buf, n));
                zio_buf_free(buf, n);
        }
}

void
abd_return_buf_copy(abd_t *abd, void *buf, size_t n)
{
        if (!abd_is_linear(abd) || abd_is_from_pages(abd)) {
                abd_copy_from_buf(abd, buf, n);
        }
        abd_return_buf(abd, buf, n);
}

/*
 * This is abd_iter_page(), the function underneath abd_iterate_page_func().
 * It yields the next page struct and data offset and size within it, without
 * mapping it into the address space.
 */

/*
 * "Compound pages" are a group of pages that can be referenced from a single
 * struct page *. Its organised as a "head" page, followed by a series of
 * "tail" pages.
 *
 * In OpenZFS, compound pages are allocated using the __GFP_COMP flag, which we
 * get from scatter ABDs and SPL vmalloc slabs (ie >16K allocations). So a
 * great many of the IO buffers we get are going to be of this type.
 *
 * The tail pages are just regular PAGESIZE pages, and can be safely used
 * as-is. However, the head page has length covering itself and all the tail
 * pages. If the ABD chunk spans multiple pages, then we can use the head page
 * and a >PAGESIZE length, which is far more efficient.
 *
 * Before kernel 4.5 however, compound page heads were refcounted separately
 * from tail pages, such that moving back to the head page would require us to
 * take a reference to it and releasing it once we're completely finished with
 * it. In practice, that meant when our caller is done with the ABD, which we
 * have no insight into from here. Rather than contort this API to track head
 * page references on such ancient kernels, we disabled this special compound
 * page handling on kernels before 4.5, instead just using treating each page
 * within it as a regular PAGESIZE page (which it is). This is slightly less
 * efficient, but makes everything far simpler.
 *
 * We no longer support kernels before 4.5, so in theory none of this is
 * necessary. However, this code is still relatively new in the grand scheme of
 * things, so I'm leaving the ability to compile this out for the moment.
 *
 * Setting/clearing ABD_ITER_COMPOUND_PAGES below enables/disables the special
 * handling, by defining the ABD_ITER_PAGE_SIZE(page) macro to understand
 * compound pages, or not, and compiling in/out the support to detect compound
 * tail pages and move back to the start.
 */

/* On by default */
#define ABD_ITER_COMPOUND_PAGES

#ifdef ABD_ITER_COMPOUND_PAGES
#define ABD_ITER_PAGE_SIZE(page)        \
        (PageCompound(page) ? page_size(page) : PAGESIZE)
#else
#define ABD_ITER_PAGE_SIZE(page)        (PAGESIZE)
#endif

void
abd_iter_page(struct abd_iter *aiter)
{
        if (abd_iter_at_end(aiter)) {
                aiter->iter_page = NULL;
                aiter->iter_page_doff = 0;
                aiter->iter_page_dsize = 0;
                return;
        }

        struct page *page;
        size_t doff, dsize;

        /*
         * Find the page, and the start of the data within it. This is computed
         * differently for linear and scatter ABDs; linear is referenced by
         * virtual memory location, while scatter is referenced by page
         * pointer.
         */
        if (abd_is_linear(aiter->iter_abd)) {
                ASSERT3U(aiter->iter_pos, ==, aiter->iter_offset);

                /* memory address at iter_pos */
                void *paddr = ABD_LINEAR_BUF(aiter->iter_abd) + aiter->iter_pos;

                /* struct page for address */
                page = is_vmalloc_addr(paddr) ?
                    vmalloc_to_page(paddr) : virt_to_page(paddr);

                /* offset of address within the page */
                doff = offset_in_page(paddr);
        } else {
                ASSERT(!abd_is_gang(aiter->iter_abd));

                /* current scatter page */
                page = nth_page(sg_page(aiter->iter_sg),
                    aiter->iter_offset >> PAGE_SHIFT);

                /* position within page */
                doff = aiter->iter_offset & (PAGESIZE - 1);
        }

#ifdef ABD_ITER_COMPOUND_PAGES
        if (PageTail(page)) {
                /*
                 * If this is a compound tail page, move back to the head, and
                 * adjust the offset to match. This may let us yield a much
                 * larger amount of data from a single logical page, and so
                 * leave our caller with fewer pages to process.
                 */
                struct page *head = compound_head(page);
                doff += ((page - head) * PAGESIZE);
                page = head;
        }
#endif

        ASSERT(page);

        /*
         * Compute the maximum amount of data we can take from this page. This
         * is the smaller of:
         * - the remaining space in the page
         * - the remaining space in this scatterlist entry (which may not cover
         *   the entire page)
         * - the remaining space in the abd (which may not cover the entire
         *   scatterlist entry)
         */
        dsize = MIN(ABD_ITER_PAGE_SIZE(page) - doff,
            aiter->iter_abd->abd_size - aiter->iter_pos);
        if (!abd_is_linear(aiter->iter_abd))
                dsize = MIN(dsize, aiter->iter_sg->length - aiter->iter_offset);
        ASSERT3U(dsize, >, 0);

        /* final iterator outputs */
        aiter->iter_page = page;
        aiter->iter_page_doff = doff;
        aiter->iter_page_dsize = dsize;
}

/*
 * Note: ABD BIO functions only needed to support vdev_classic. See comments in
 * vdev_disk.c.
 */

/*
 * bio_nr_pages for ABD.
 * @off is the offset in @abd
 */
unsigned long
abd_nr_pages_off(abd_t *abd, unsigned int size, size_t off)
{
        unsigned long pos;

        if (abd_is_gang(abd)) {
                unsigned long count = 0;

                for (abd_t *cabd = abd_gang_get_offset(abd, &off);
                    cabd != NULL && size != 0;
                    cabd = list_next(&ABD_GANG(abd).abd_gang_chain, cabd)) {
                        ASSERT3U(off, <, cabd->abd_size);
                        int mysize = MIN(size, cabd->abd_size - off);
                        count += abd_nr_pages_off(cabd, mysize, off);
                        size -= mysize;
                        off = 0;
                }
                return (count);
        }

        if (abd_is_linear(abd))
                pos = (unsigned long)abd_to_buf(abd) + off;
        else
                pos = ABD_SCATTER(abd).abd_offset + off;

        return (((pos + size + PAGESIZE - 1) >> PAGE_SHIFT) -
            (pos >> PAGE_SHIFT));
}

static unsigned int
bio_map(struct bio *bio, void *buf_ptr, unsigned int bio_size)
{
        unsigned int offset, size, i;
        struct page *page;

        offset = offset_in_page(buf_ptr);
        for (i = 0; i < bio->bi_max_vecs; i++) {
                size = PAGE_SIZE - offset;

                if (bio_size <= 0)
                        break;

                if (size > bio_size)
                        size = bio_size;

                if (is_vmalloc_addr(buf_ptr))
                        page = vmalloc_to_page(buf_ptr);
                else
                        page = virt_to_page(buf_ptr);

                /*
                 * Some network related block device uses tcp_sendpage, which
                 * doesn't behave well when using 0-count page, this is a
                 * safety net to catch them.
                 */
                ASSERT3S(page_count(page), >, 0);

                if (bio_add_page(bio, page, size, offset) != size)
                        break;

                buf_ptr += size;
                bio_size -= size;
                offset = 0;
        }

        return (bio_size);
}

/*
 * bio_map for gang ABD.
 */
static unsigned int
abd_gang_bio_map_off(struct bio *bio, abd_t *abd,
    unsigned int io_size, size_t off)
{
        ASSERT(abd_is_gang(abd));

        for (abd_t *cabd = abd_gang_get_offset(abd, &off);
            cabd != NULL;
            cabd = list_next(&ABD_GANG(abd).abd_gang_chain, cabd)) {
                ASSERT3U(off, <, cabd->abd_size);
                int size = MIN(io_size, cabd->abd_size - off);
                int remainder = abd_bio_map_off(bio, cabd, size, off);
                io_size -= (size - remainder);
                if (io_size == 0 || remainder > 0)
                        return (io_size);
                off = 0;
        }
        ASSERT0(io_size);
        return (io_size);
}

/*
 * bio_map for ABD.
 * @off is the offset in @abd
 * Remaining IO size is returned
 */
unsigned int
abd_bio_map_off(struct bio *bio, abd_t *abd,
    unsigned int io_size, size_t off)
{
        struct abd_iter aiter;

        ASSERT3U(io_size, <=, abd->abd_size - off);
        if (abd_is_linear(abd))
                return (bio_map(bio, ((char *)abd_to_buf(abd)) + off, io_size));

        ASSERT(!abd_is_linear(abd));
        if (abd_is_gang(abd))
                return (abd_gang_bio_map_off(bio, abd, io_size, off));

        abd_iter_init(&aiter, abd);
        abd_iter_advance(&aiter, off);

        for (int i = 0; i < bio->bi_max_vecs; i++) {
                struct page *pg;
                size_t len, sgoff, pgoff;
                struct scatterlist *sg;

                if (io_size <= 0)
                        break;

                sg = aiter.iter_sg;
                sgoff = aiter.iter_offset;
                pgoff = sgoff & (PAGESIZE - 1);
                len = MIN(io_size, PAGESIZE - pgoff);
                ASSERT(len > 0);

                pg = nth_page(sg_page(sg), sgoff >> PAGE_SHIFT);
                if (bio_add_page(bio, pg, len, pgoff) != len)
                        break;

                io_size -= len;
                abd_iter_advance(&aiter, len);
        }

        return (io_size);
}

/* Tunable Parameters */
module_param(zfs_abd_scatter_enabled, int, 0644);
MODULE_PARM_DESC(zfs_abd_scatter_enabled,
        "Toggle whether ABD allocations must be linear.");
module_param(zfs_abd_scatter_min_size, int, 0644);
MODULE_PARM_DESC(zfs_abd_scatter_min_size,
        "Minimum size of scatter allocations.");
/* CSTYLED */
module_param(zfs_abd_scatter_max_order, uint, 0644);
MODULE_PARM_DESC(zfs_abd_scatter_max_order,
        "Maximum order allocation used for a scatter ABD.");