Merge commit '37e84ab74e939caf52150fc3352081786ecc0c29' into merges

[unleashed.git] / kernel / fs / zfs / abd.c

/*
 * This file and its contents are supplied under the terms of the
 * Common Development and Distribution License ("CDDL"), version 1.0.
 * You may only use this file in accordance with the terms of version
 * 1.0 of the CDDL.
 *
 * A full copy of the text of the CDDL should have accompanied this
 * source.  A copy of the CDDL is also available via the Internet at
 * http://www.illumos.org/license/CDDL.
 */

/*
 * Copyright (c) 2014 by Chunwei Chen. All rights reserved.
 * Copyright (c) 2016 by Delphix. All rights reserved.
 */

/*
 * ARC buffer data (ABD).
 *
 * ABDs are an abstract data structure for the ARC which can use two
 * different ways of storing the underlying data:
 *
 * (a) Linear buffer. In this case, all the data in the ABD is stored in one
 *     contiguous buffer in memory (from a zio_[data_]buf_* kmem cache).
 *
 *         +-------------------+
 *         | ABD (linear)      |
 *         |   abd_flags = ... |
 *         |   abd_size = ...  |     +--------------------------------+
 *         |   abd_buf ------------->| raw buffer of size abd_size    |
 *         +-------------------+     +--------------------------------+
 *              no abd_chunks
 *
 * (b) Scattered buffer. In this case, the data in the ABD is split into
 *     equal-sized chunks (from the abd_chunk_cache kmem_cache), with pointers
 *     to the chunks recorded in an array at the end of the ABD structure.
 *
 *         +-------------------+
 *         | ABD (scattered)   |
 *         |   abd_flags = ... |
 *         |   abd_size = ...  |
 *         |   abd_offset = 0  |                           +-----------+
 *         |   abd_chunks[0] ----------------------------->| chunk 0   |
 *         |   abd_chunks[1] ---------------------+        +-----------+
 *         |   ...             |                  |        +-----------+
 *         |   abd_chunks[N-1] ---------+         +------->| chunk 1   |
 *         +-------------------+        |                  +-----------+
 *                                      |                      ...
 *                                      |                  +-----------+
 *                                      +----------------->| chunk N-1 |
 *                                                         +-----------+
 *
 * Using a large proportion of scattered ABDs decreases ARC fragmentation since
 * when we are at the limit of allocatable space, using equal-size chunks will
 * allow us to quickly reclaim enough space for a new large allocation (assuming
 * it is also scattered).
 *
 * In addition to directly allocating a linear or scattered ABD, it is also
 * possible to create an ABD by requesting the "sub-ABD" starting at an offset
 * within an existing ABD. In linear buffers this is simple (set abd_buf of
 * the new ABD to the starting point within the original raw buffer), but
 * scattered ABDs are a little more complex. The new ABD makes a copy of the
 * relevant abd_chunks pointers (but not the underlying data). However, to
 * provide arbitrary rather than only chunk-aligned starting offsets, it also
 * tracks an abd_offset field which represents the starting point of the data
 * within the first chunk in abd_chunks. For both linear and scattered ABDs,
 * creating an offset ABD marks the original ABD as the offset's parent, and the
 * original ABD's abd_children refcount is incremented. This data allows us to
 * ensure the root ABD isn't deleted before its children.
 *
 * Most consumers should never need to know what type of ABD they're using --
 * the ABD public API ensures that it's possible to transparently switch from
 * using a linear ABD to a scattered one when doing so would be beneficial.
 *
 * If you need to use the data within an ABD directly, if you know it's linear
 * (because you allocated it) you can use abd_to_buf() to access the underlying
 * raw buffer. Otherwise, you should use one of the abd_borrow_buf* functions
 * which will allocate a raw buffer if necessary. Use the abd_return_buf*
 * functions to return any raw buffers that are no longer necessary when you're
 * done using them.
 *
 * There are a variety of ABD APIs that implement basic buffer operations:
 * compare, copy, read, write, and fill with zeroes. If you need a custom
 * function which progressively accesses the whole ABD, use the abd_iterate_*
 * functions.
 */

#include <sys/abd.h>
#include <sys/param.h>
#include <sys/zio.h>
#include <sys/zfs_context.h>
#include <sys/zfs_znode.h>

typedef struct abd_stats {
        kstat_named_t abdstat_struct_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_linear_cnt;
        kstat_named_t abdstat_linear_data_size;
} 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 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 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 },
};

#define ABDSTAT(stat)           (abd_stats.stat.value.ui64)
#define ABDSTAT_INCR(stat, val) \
        atomic_add_64(&abd_stats.stat.value.ui64, (val))
#define ABDSTAT_BUMP(stat)      ABDSTAT_INCR(stat, 1)
#define ABDSTAT_BUMPDOWN(stat)  ABDSTAT_INCR(stat, -1)

/*
 * It is possible to make all future ABDs be linear by setting this to B_FALSE.
 * Otherwise, ABDs are allocated scattered by default unless the caller uses
 * abd_alloc_linear().
 */
boolean_t zfs_abd_scatter_enabled = B_TRUE;

/*
 * The size of the chunks ABD allocates. Because the sizes allocated from the
 * kmem_cache can't change, this tunable can only be modified at boot. Changing
 * it at runtime would cause ABD iteration to work incorrectly for ABDs which
 * were allocated with the old size, so a safeguard has been put in place which
 * will cause the machine to panic if you change it and try to access the data
 * within a scattered ABD.
 */
size_t zfs_abd_chunk_size = 4096;

#ifdef _KERNEL
extern vmem_t *zio_alloc_arena;
#endif

kmem_cache_t *abd_chunk_cache;
static kstat_t *abd_ksp;

extern inline boolean_t abd_is_linear(abd_t *abd);
extern inline void abd_copy(abd_t *dabd, abd_t *sabd, size_t size);
extern inline void abd_copy_from_buf(abd_t *abd, const void *buf, size_t size);
extern inline void abd_copy_to_buf(void* buf, abd_t *abd, size_t size);
extern inline int abd_cmp_buf(abd_t *abd, const void *buf, size_t size);
extern inline void abd_zero(abd_t *abd, size_t size);

static void *
abd_alloc_chunk()
{
        void *c = kmem_cache_alloc(abd_chunk_cache, KM_PUSHPAGE);
        ASSERT3P(c, !=, NULL);
        return (c);
}

static void
abd_free_chunk(void *c)
{
        kmem_cache_free(abd_chunk_cache, c);
}

void
abd_init(void)
{
        vmem_t *data_alloc_arena = NULL;

#ifdef _KERNEL
        data_alloc_arena = zio_alloc_arena;
#endif

        /*
         * Since ABD chunks do not appear in crash dumps, we pass KMC_NOTOUCH
         * so that no allocator metadata is stored with the buffers.
         */
        abd_chunk_cache = kmem_cache_create("abd_chunk", zfs_abd_chunk_size, 0,
            NULL, NULL, NULL, NULL, data_alloc_arena, KMC_NOTOUCH);

        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) {
                abd_ksp->ks_data = &abd_stats;
                kstat_install(abd_ksp);
        }
}

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

        kmem_cache_destroy(abd_chunk_cache);
        abd_chunk_cache = NULL;
}

static inline size_t
abd_chunkcnt_for_bytes(size_t size)
{
        return (P2ROUNDUP(size, zfs_abd_chunk_size) / zfs_abd_chunk_size);
}

static inline size_t
abd_scatter_chunkcnt(abd_t *abd)
{
        ASSERT(!abd_is_linear(abd));
        return (abd_chunkcnt_for_bytes(
            abd->abd_u.abd_scatter.abd_offset + abd->abd_size));
}

static inline void
abd_verify(abd_t *abd)
{
        ASSERT3U(abd->abd_size, >, 0);
        ASSERT3U(abd->abd_size, <=, SPA_MAXBLOCKSIZE);
        ASSERT3U(abd->abd_flags, ==, abd->abd_flags & (ABD_FLAG_LINEAR |
            ABD_FLAG_OWNER | ABD_FLAG_META));
        IMPLY(abd->abd_parent != NULL, !(abd->abd_flags & ABD_FLAG_OWNER));
        IMPLY(abd->abd_flags & ABD_FLAG_META, abd->abd_flags & ABD_FLAG_OWNER);
        if (abd_is_linear(abd)) {
                ASSERT3P(abd->abd_u.abd_linear.abd_buf, !=, NULL);
        } else {
                ASSERT3U(abd->abd_u.abd_scatter.abd_offset, <,
                    zfs_abd_chunk_size);
                size_t n = abd_scatter_chunkcnt(abd);
                for (int i = 0; i < n; i++) {
                        ASSERT3P(
                            abd->abd_u.abd_scatter.abd_chunks[i], !=, NULL);
                }
        }
}

static inline abd_t *
abd_alloc_struct(size_t chunkcnt)
{
        size_t size = offsetof(abd_t, abd_u.abd_scatter.abd_chunks[chunkcnt]);
        abd_t *abd = kmem_alloc(size, KM_PUSHPAGE);
        ASSERT3P(abd, !=, NULL);
        ABDSTAT_INCR(abdstat_struct_size, size);

        return (abd);
}

static inline void
abd_free_struct(abd_t *abd)
{
        size_t chunkcnt = abd_is_linear(abd) ? 0 : abd_scatter_chunkcnt(abd);
        int size = offsetof(abd_t, abd_u.abd_scatter.abd_chunks[chunkcnt]);
        kmem_free(abd, size);
        ABDSTAT_INCR(abdstat_struct_size, -size);
}

/*
 * Allocate an ABD, along with its own underlying data buffers. Use this if you
 * don't care whether the ABD is linear or not.
 */
abd_t *
abd_alloc(size_t size, boolean_t is_metadata)
{
        if (!zfs_abd_scatter_enabled)
                return (abd_alloc_linear(size, is_metadata));

        VERIFY3U(size, <=, SPA_MAXBLOCKSIZE);

        size_t n = abd_chunkcnt_for_bytes(size);
        abd_t *abd = abd_alloc_struct(n);

        abd->abd_flags = ABD_FLAG_OWNER;
        if (is_metadata) {
                abd->abd_flags |= ABD_FLAG_META;
        }
        abd->abd_size = size;
        abd->abd_parent = NULL;
        refcount_create(&abd->abd_children);

        abd->abd_u.abd_scatter.abd_offset = 0;
        abd->abd_u.abd_scatter.abd_chunk_size = zfs_abd_chunk_size;

        for (int i = 0; i < n; i++) {
                void *c = abd_alloc_chunk();
                ASSERT3P(c, !=, NULL);
                abd->abd_u.abd_scatter.abd_chunks[i] = c;
        }

        ABDSTAT_BUMP(abdstat_scatter_cnt);
        ABDSTAT_INCR(abdstat_scatter_data_size, size);
        ABDSTAT_INCR(abdstat_scatter_chunk_waste,
            n * zfs_abd_chunk_size - size);

        return (abd);
}

static void
abd_free_scatter(abd_t *abd)
{
        size_t n = abd_scatter_chunkcnt(abd);
        for (int i = 0; i < n; i++) {
                abd_free_chunk(abd->abd_u.abd_scatter.abd_chunks[i]);
        }

        refcount_destroy(&abd->abd_children);
        ABDSTAT_BUMPDOWN(abdstat_scatter_cnt);
        ABDSTAT_INCR(abdstat_scatter_data_size, -(int)abd->abd_size);
        ABDSTAT_INCR(abdstat_scatter_chunk_waste,
            abd->abd_size - n * zfs_abd_chunk_size);

        abd_free_struct(abd);
}

/*
 * Allocate an ABD that must be linear, along with its own underlying data
 * buffer. Only use this when it would be very annoying to write your ABD
 * consumer with a scattered ABD.
 */
abd_t *
abd_alloc_linear(size_t size, boolean_t is_metadata)
{
        abd_t *abd = abd_alloc_struct(0);

        VERIFY3U(size, <=, SPA_MAXBLOCKSIZE);

        abd->abd_flags = ABD_FLAG_LINEAR | ABD_FLAG_OWNER;
        if (is_metadata) {
                abd->abd_flags |= ABD_FLAG_META;
        }
        abd->abd_size = size;
        abd->abd_parent = NULL;
        refcount_create(&abd->abd_children);

        if (is_metadata) {
                abd->abd_u.abd_linear.abd_buf = zio_buf_alloc(size);
        } else {
                abd->abd_u.abd_linear.abd_buf = zio_data_buf_alloc(size);
        }

        ABDSTAT_BUMP(abdstat_linear_cnt);
        ABDSTAT_INCR(abdstat_linear_data_size, size);

        return (abd);
}

static void
abd_free_linear(abd_t *abd)
{
        if (abd->abd_flags & ABD_FLAG_META) {
                zio_buf_free(abd->abd_u.abd_linear.abd_buf, abd->abd_size);
        } else {
                zio_data_buf_free(abd->abd_u.abd_linear.abd_buf, abd->abd_size);
        }

        refcount_destroy(&abd->abd_children);
        ABDSTAT_BUMPDOWN(abdstat_linear_cnt);
        ABDSTAT_INCR(abdstat_linear_data_size, -(int)abd->abd_size);

        abd_free_struct(abd);
}

/*
 * Free an ABD. Only use this on ABDs allocated with abd_alloc() or
 * abd_alloc_linear().
 */
void
abd_free(abd_t *abd)
{
        abd_verify(abd);
        ASSERT3P(abd->abd_parent, ==, NULL);
        ASSERT(abd->abd_flags & ABD_FLAG_OWNER);
        if (abd_is_linear(abd))
                abd_free_linear(abd);
        else
                abd_free_scatter(abd);
}

/*
 * Allocate an ABD of the same format (same metadata flag, same scatterize
 * setting) as another ABD.
 */
abd_t *
abd_alloc_sametype(abd_t *sabd, size_t size)
{
        boolean_t is_metadata = (sabd->abd_flags & ABD_FLAG_META) != 0;
        if (abd_is_linear(sabd)) {
                return (abd_alloc_linear(size, is_metadata));
        } else {
                return (abd_alloc(size, is_metadata));
        }
}

/*
 * 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.
 *
 * Currently this is linear ABDs, however if ldi_strategy() can ever issue I/Os
 * using a scatter/gather list we should switch to that and replace this call
 * with vanilla abd_alloc().
 */
abd_t *
abd_alloc_for_io(size_t size, boolean_t is_metadata)
{
        return (abd_alloc_linear(size, is_metadata));
}

/*
 * Allocate a new ABD to point to offset off of sabd. It shares the underlying
 * buffer data with sabd. Use abd_put() to free. sabd must not be freed while
 * any derived ABDs exist.
 */
abd_t *
abd_get_offset(abd_t *sabd, size_t off)
{
        abd_t *abd;

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

        if (abd_is_linear(sabd)) {
                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->abd_flags = ABD_FLAG_LINEAR;

                abd->abd_u.abd_linear.abd_buf =
                    (char *)sabd->abd_u.abd_linear.abd_buf + off;
        } else {
                size_t new_offset = sabd->abd_u.abd_scatter.abd_offset + off;
                size_t chunkcnt = abd_scatter_chunkcnt(sabd) -
                    (new_offset / zfs_abd_chunk_size);

                abd = abd_alloc_struct(chunkcnt);

                /*
                 * 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->abd_flags = 0;

                abd->abd_u.abd_scatter.abd_offset =
                    new_offset % zfs_abd_chunk_size;
                abd->abd_u.abd_scatter.abd_chunk_size = zfs_abd_chunk_size;

                /* Copy the scatterlist starting at the correct offset */
                (void) memcpy(&abd->abd_u.abd_scatter.abd_chunks,
                    &sabd->abd_u.abd_scatter.abd_chunks[new_offset /
                    zfs_abd_chunk_size],
                    chunkcnt * sizeof (void *));
        }

        abd->abd_size = sabd->abd_size - off;
        abd->abd_parent = sabd;
        refcount_create(&abd->abd_children);
        (void) refcount_add_many(&sabd->abd_children, abd->abd_size, abd);

        return (abd);
}

/*
 * Allocate a linear ABD structure for buf. You must free this with abd_put()
 * since the resulting ABD doesn't own its own buffer.
 */
abd_t *
abd_get_from_buf(void *buf, size_t size)
{
        abd_t *abd = abd_alloc_struct(0);

        VERIFY3U(size, <=, SPA_MAXBLOCKSIZE);

        /*
         * 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->abd_flags = ABD_FLAG_LINEAR;
        abd->abd_size = size;
        abd->abd_parent = NULL;
        refcount_create(&abd->abd_children);

        abd->abd_u.abd_linear.abd_buf = buf;

        return (abd);
}

/*
 * Free an ABD allocated from abd_get_offset() or abd_get_from_buf(). Will not
 * free the underlying scatterlist or buffer.
 */
void
abd_put(abd_t *abd)
{
        abd_verify(abd);
        ASSERT(!(abd->abd_flags & ABD_FLAG_OWNER));

        if (abd->abd_parent != NULL) {
                (void) refcount_remove_many(&abd->abd_parent->abd_children,
                    abd->abd_size, abd);
        }

        refcount_destroy(&abd->abd_children);
        abd_free_struct(abd);
}

/*
 * Get the raw buffer associated with a linear ABD.
 */
void *
abd_to_buf(abd_t *abd)
{
        ASSERT(abd_is_linear(abd));
        abd_verify(abd);
        return (abd->abd_u.abd_linear.abd_buf);
}

/*
 * 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, >=, n);
        if (abd_is_linear(abd)) {
                buf = abd_to_buf(abd);
        } else {
                buf = zio_buf_alloc(n);
        }
        (void) refcount_add_many(&abd->abd_children, n, buf);

        return (buf);
}

void *
abd_borrow_buf_copy(abd_t *abd, size_t n)
{
        void *buf = abd_borrow_buf(abd, n);
        if (!abd_is_linear(abd)) {
                abd_copy_to_buf(buf, abd, n);
        }
        return (buf);
}

/*
 * Return a borrowed raw buffer to an ABD. If the ABD is scattered, this will
 * not change the contents of the ABD and will ASSERT that you didn't modify
 * the buffer since it was borrowed. If you want any changes you made to buf to
 * be copied back to abd, use abd_return_buf_copy() instead.
 */
void
abd_return_buf(abd_t *abd, void *buf, size_t n)
{
        abd_verify(abd);
        ASSERT3U(abd->abd_size, >=, n);
        if (abd_is_linear(abd)) {
                ASSERT3P(buf, ==, abd_to_buf(abd));
        } else {
                ASSERT0(abd_cmp_buf(abd, buf, n));
                zio_buf_free(buf, n);
        }
        (void) refcount_remove_many(&abd->abd_children, n, buf);
}

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

/*
 * Give this ABD ownership of the buffer that it's storing. Can only be used on
 * linear ABDs which were allocated via abd_get_from_buf(), or ones allocated
 * with abd_alloc_linear() which subsequently released ownership of their buf
 * with abd_release_ownership_of_buf().
 */
void
abd_take_ownership_of_buf(abd_t *abd, boolean_t is_metadata)
{
        ASSERT(abd_is_linear(abd));
        ASSERT(!(abd->abd_flags & ABD_FLAG_OWNER));
        abd_verify(abd);

        abd->abd_flags |= ABD_FLAG_OWNER;
        if (is_metadata) {
                abd->abd_flags |= ABD_FLAG_META;
        }

        ABDSTAT_BUMP(abdstat_linear_cnt);
        ABDSTAT_INCR(abdstat_linear_data_size, abd->abd_size);
}

void
abd_release_ownership_of_buf(abd_t *abd)
{
        ASSERT(abd_is_linear(abd));
        ASSERT(abd->abd_flags & ABD_FLAG_OWNER);
        abd_verify(abd);

        abd->abd_flags &= ~ABD_FLAG_OWNER;
        /* Disable this flag since we no longer own the data buffer */
        abd->abd_flags &= ~ABD_FLAG_META;

        ABDSTAT_BUMPDOWN(abdstat_linear_cnt);
        ABDSTAT_INCR(abdstat_linear_data_size, -(int)abd->abd_size);
}

struct abd_iter {
        abd_t           *iter_abd;      /* ABD being iterated through */
        size_t          iter_pos;       /* position (relative to abd_offset) */
        void            *iter_mapaddr;  /* addr corresponding to iter_pos */
        size_t          iter_mapsize;   /* length of data valid at mapaddr */
};

static inline size_t
abd_iter_scatter_chunk_offset(struct abd_iter *aiter)
{
        ASSERT(!abd_is_linear(aiter->iter_abd));
        return ((aiter->iter_abd->abd_u.abd_scatter.abd_offset +
            aiter->iter_pos) % zfs_abd_chunk_size);
}

static inline size_t
abd_iter_scatter_chunk_index(struct abd_iter *aiter)
{
        ASSERT(!abd_is_linear(aiter->iter_abd));
        return ((aiter->iter_abd->abd_u.abd_scatter.abd_offset +
            aiter->iter_pos) / zfs_abd_chunk_size);
}

/*
 * Initialize the abd_iter.
 */
static void
abd_iter_init(struct abd_iter *aiter, abd_t *abd)
{
        abd_verify(abd);
        aiter->iter_abd = abd;
        aiter->iter_pos = 0;
        aiter->iter_mapaddr = NULL;
        aiter->iter_mapsize = 0;
}

/*
 * 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.
 */
static void
abd_iter_advance(struct abd_iter *aiter, size_t amount)
{
        ASSERT3P(aiter->iter_mapaddr, ==, NULL);
        ASSERT0(aiter->iter_mapsize);

        /* There's nothing left to advance to, so do nothing */
        if (aiter->iter_pos == aiter->iter_abd->abd_size)
                return;

        aiter->iter_pos += amount;
}

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

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

        /* Panic if someone has changed zfs_abd_chunk_size */
        IMPLY(!abd_is_linear(aiter->iter_abd), zfs_abd_chunk_size ==
            aiter->iter_abd->abd_u.abd_scatter.abd_chunk_size);

        /* There's nothing left to iterate over, so do nothing */
        if (aiter->iter_pos == aiter->iter_abd->abd_size)
                return;

        if (abd_is_linear(aiter->iter_abd)) {
                offset = aiter->iter_pos;
                aiter->iter_mapsize = aiter->iter_abd->abd_size - offset;
                paddr = aiter->iter_abd->abd_u.abd_linear.abd_buf;
        } else {
                size_t index = abd_iter_scatter_chunk_index(aiter);
                offset = abd_iter_scatter_chunk_offset(aiter);
                aiter->iter_mapsize = zfs_abd_chunk_size - offset;
                paddr = aiter->iter_abd->abd_u.abd_scatter.abd_chunks[index];
        }
        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.
 */
static void
abd_iter_unmap(struct abd_iter *aiter)
{
        /* There's nothing left to unmap, so do nothing */
        if (aiter->iter_pos == aiter->iter_abd->abd_size)
                return;

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

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

int
abd_iterate_func(abd_t *abd, size_t off, size_t size,
    abd_iter_func_t *func, void *private)
{
        int ret = 0;
        struct abd_iter aiter;

        abd_verify(abd);
        ASSERT3U(off + size, <=, abd->abd_size);

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

        while (size > 0) {
                abd_iter_map(&aiter);

                size_t len = MIN(aiter.iter_mapsize, size);
                ASSERT3U(len, >, 0);

                ret = func(aiter.iter_mapaddr, len, private);

                abd_iter_unmap(&aiter);

                if (ret != 0)
                        break;

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

        return (ret);
}

struct buf_arg {
        void *arg_buf;
};

static int
abd_copy_to_buf_off_cb(void *buf, size_t size, void *private)
{
        struct buf_arg *ba_ptr = private;

        (void) memcpy(ba_ptr->arg_buf, buf, size);
        ba_ptr->arg_buf = (char *)ba_ptr->arg_buf + size;

        return (0);
}

/*
 * Copy abd to buf. (off is the offset in abd.)
 */
void
abd_copy_to_buf_off(void *buf, abd_t *abd, size_t off, size_t size)
{
        struct buf_arg ba_ptr = { buf };

        (void) abd_iterate_func(abd, off, size, abd_copy_to_buf_off_cb,
            &ba_ptr);
}

static int
abd_cmp_buf_off_cb(void *buf, size_t size, void *private)
{
        int ret;
        struct buf_arg *ba_ptr = private;

        ret = memcmp(buf, ba_ptr->arg_buf, size);
        ba_ptr->arg_buf = (char *)ba_ptr->arg_buf + size;

        return (ret);
}

/*
 * Compare the contents of abd to buf. (off is the offset in abd.)
 */
int
abd_cmp_buf_off(abd_t *abd, const void *buf, size_t off, size_t size)
{
        struct buf_arg ba_ptr = { (void *) buf };

        return (abd_iterate_func(abd, off, size, abd_cmp_buf_off_cb, &ba_ptr));
}

static int
abd_copy_from_buf_off_cb(void *buf, size_t size, void *private)
{
        struct buf_arg *ba_ptr = private;

        (void) memcpy(buf, ba_ptr->arg_buf, size);
        ba_ptr->arg_buf = (char *)ba_ptr->arg_buf + size;

        return (0);
}

/*
 * Copy from buf to abd. (off is the offset in abd.)
 */
void
abd_copy_from_buf_off(abd_t *abd, const void *buf, size_t off, size_t size)
{
        struct buf_arg ba_ptr = { (void *) buf };

        (void) abd_iterate_func(abd, off, size, abd_copy_from_buf_off_cb,
            &ba_ptr);
}

/*ARGSUSED*/
static int
abd_zero_off_cb(void *buf, size_t size, void *private)
{
        (void) memset(buf, 0, size);
        return (0);
}

/*
 * Zero out the abd from a particular offset to the end.
 */
void
abd_zero_off(abd_t *abd, size_t off, size_t size)
{
        (void) abd_iterate_func(abd, off, size, abd_zero_off_cb, NULL);
}

/*
 * Iterate over two ABDs and call func incrementally on the two ABDs' data in
 * equal-sized chunks (passed to func as raw buffers). func could be called many
 * times during this iteration.
 */
int
abd_iterate_func2(abd_t *dabd, abd_t *sabd, size_t doff, size_t soff,
    size_t size, abd_iter_func2_t *func, void *private)
{
        int ret = 0;
        struct abd_iter daiter, saiter;

        abd_verify(dabd);
        abd_verify(sabd);

        ASSERT3U(doff + size, <=, dabd->abd_size);
        ASSERT3U(soff + size, <=, sabd->abd_size);

        abd_iter_init(&daiter, dabd);
        abd_iter_init(&saiter, sabd);
        abd_iter_advance(&daiter, doff);
        abd_iter_advance(&saiter, soff);

        while (size > 0) {
                abd_iter_map(&daiter);
                abd_iter_map(&saiter);

                size_t dlen = MIN(daiter.iter_mapsize, size);
                size_t slen = MIN(saiter.iter_mapsize, size);
                size_t len = MIN(dlen, slen);
                ASSERT(dlen > 0 || slen > 0);

                ret = func(daiter.iter_mapaddr, saiter.iter_mapaddr, len,
                    private);

                abd_iter_unmap(&saiter);
                abd_iter_unmap(&daiter);

                if (ret != 0)
                        break;

                size -= len;
                abd_iter_advance(&daiter, len);
                abd_iter_advance(&saiter, len);
        }

        return (ret);
}

/*ARGSUSED*/
static int
abd_copy_off_cb(void *dbuf, void *sbuf, size_t size, void *private)
{
        (void) memcpy(dbuf, sbuf, size);
        return (0);
}

/*
 * Copy from sabd to dabd starting from soff and doff.
 */
void
abd_copy_off(abd_t *dabd, abd_t *sabd, size_t doff, size_t soff, size_t size)
{
        (void) abd_iterate_func2(dabd, sabd, doff, soff, size,
            abd_copy_off_cb, NULL);
}

/*ARGSUSED*/
static int
abd_cmp_cb(void *bufa, void *bufb, size_t size, void *private)
{
        return (memcmp(bufa, bufb, size));
}

/*
 * Compares the first size bytes of two ABDs.
 */
int
abd_cmp(abd_t *dabd, abd_t *sabd, size_t size)
{
        return (abd_iterate_func2(dabd, sabd, 0, 0, size, abd_cmp_cb, NULL));
}