mirror_zfs/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 http://www.opensolaris.org/os/licensing.
 * 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.
 */

/*
 * 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/zfs_context.h>
#include <sys/zfs_znode.h>
#ifdef _KERNEL
#include <linux/kmap_compat.h>
#include <linux/scatterlist.h>
#else
#define	MAX_ORDER	1
#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[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 },
};

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

unsigned zfs_abd_scatter_max_order = MAX_ORDER - 1;

/*
 * 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).
 */
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;

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

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

abd_t *
abd_alloc_struct(size_t size)
{
	/*
	 * In Linux we do not use the size passed in during ABD
	 * allocation, so we just ignore it.
	 */
	abd_t *abd = kmem_cache_alloc(abd_cache, KM_PUSHPAGE);
	ASSERT3P(abd, !=, NULL);
	list_link_init(&abd->abd_gang_link);
	mutex_init(&abd->abd_mtx, NULL, MUTEX_DEFAULT, NULL);
	ABDSTAT_INCR(abdstat_struct_size, sizeof (abd_t));

	return (abd);
}

void
abd_free_struct(abd_t *abd)
{
	mutex_destroy(&abd->abd_mtx);
	ASSERT(!list_link_active(&abd->abd_gang_link));
	kmem_cache_free(abd_cache, abd);
	ABDSTAT_INCR(abdstat_struct_size, -(int)sizeof (abd_t));
}

#ifdef _KERNEL
/*
 * 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_NOWARN | GFP_NOIO;
	gfp_t gfp_comp = (gfp | __GFP_NORETRY | __GFP_COMP) & ~__GFP_RECLAIM;
	int max_order = MIN(zfs_abd_scatter_max_order, MAX_ORDER - 1);
	int nr_pages = abd_chunkcnt_for_bytes(size);
	int chunks = 0, zones = 0;
	size_t remaining_size;
	int nid = NUMA_NO_NODE;
	int alloc_pages = 0;

	INIT_LIST_HEAD(&pages);

	while (alloc_pages < nr_pages) {
		unsigned chunk_pages;
		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_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);

	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 ZERO_PAGE(0). ZERO_PAGE(0) returns
 * a global shared page that is always zero'd out.
 */
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;

	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_parent = NULL;
	abd_zero_scatter->abd_flags |= ABD_FLAG_MULTI_CHUNK | ABD_FLAG_ZEROS;
	zfs_refcount_create(&abd_zero_scatter->abd_children);

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

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

#else /* _KERNEL */

struct page;

/*
 * In user space abd_zero_page we will be an allocated zero'd PAGESIZE
 * buffer, which is assigned to set each of the pages of abd_zero_scatter.
 */
static struct page *abd_zero_page = NULL;

#ifndef PAGE_SHIFT
#define	PAGE_SHIFT (highbit64(PAGESIZE)-1)
#endif

#define	zfs_kmap_atomic(chunk, km)	((void *)chunk)
#define	zfs_kunmap_atomic(addr, km)	do { (void)(addr); } while (0)
#define	local_irq_save(flags)		do { (void)(flags); } while (0)
#define	local_irq_restore(flags)	do { (void)(flags); } while (0)
#define	nth_page(pg, i) \
	((struct page *)((void *)(pg) + (i) * PAGESIZE))

struct scatterlist {
	struct page *page;
	int length;
	int end;
};

static void
sg_init_table(struct scatterlist *sg, int nr)
{
	memset(sg, 0, nr * sizeof (struct scatterlist));
	sg[nr - 1].end = 1;
}

/*
 * This must be called if any of the sg_table allocation functions
 * are called.
 */
static void
abd_free_sg_table(abd_t *abd)
{
	int nents = ABD_SCATTER(abd).abd_nents;
	vmem_free(ABD_SCATTER(abd).abd_sgl,
	    nents * sizeof (struct scatterlist));
}

#define	for_each_sg(sgl, sg, nr, i)	\
	for ((i) = 0, (sg) = (sgl); (i) < (nr); (i)++, (sg) = sg_next(sg))

static inline void
sg_set_page(struct scatterlist *sg, struct page *page, unsigned int len,
    unsigned int offset)
{
	/* currently we don't use offset */
	ASSERT(offset == 0);
	sg->page = page;
	sg->length = len;
}

static inline struct page *
sg_page(struct scatterlist *sg)
{
	return (sg->page);
}

static inline struct scatterlist *
sg_next(struct scatterlist *sg)
{
	if (sg->end)
		return (NULL);

	return (sg + 1);
}

void
abd_alloc_chunks(abd_t *abd, size_t size)
{
	unsigned nr_pages = abd_chunkcnt_for_bytes(size);
	struct scatterlist *sg;
	int i;

	ABD_SCATTER(abd).abd_sgl = vmem_alloc(nr_pages *
	    sizeof (struct scatterlist), KM_SLEEP);
	sg_init_table(ABD_SCATTER(abd).abd_sgl, nr_pages);

	abd_for_each_sg(abd, sg, nr_pages, i) {
		struct page *p = umem_alloc_aligned(PAGESIZE, 64, KM_SLEEP);
		sg_set_page(sg, p, PAGESIZE, 0);
	}
	ABD_SCATTER(abd).abd_nents = nr_pages;
}

void
abd_free_chunks(abd_t *abd)
{
	int i, n = ABD_SCATTER(abd).abd_nents;
	struct scatterlist *sg;

	abd_for_each_sg(abd, sg, n, i) {
		for (int j = 0; j < sg->length; j += PAGESIZE) {
			struct page *p = nth_page(sg_page(sg), j >> PAGE_SHIFT);
			umem_free(p, PAGESIZE);
		}
	}
	abd_free_sg_table(abd);
}

static void
abd_alloc_zero_scatter(void)
{
	unsigned nr_pages = abd_chunkcnt_for_bytes(SPA_MAXBLOCKSIZE);
	struct scatterlist *sg;
	int i;

	abd_zero_page = umem_alloc_aligned(PAGESIZE, 64, KM_SLEEP);
	memset(abd_zero_page, 0, PAGESIZE);
	abd_zero_scatter = abd_alloc_struct(SPA_MAXBLOCKSIZE);
	abd_zero_scatter->abd_flags = ABD_FLAG_OWNER;
	abd_zero_scatter->abd_flags |= ABD_FLAG_MULTI_CHUNK | ABD_FLAG_ZEROS;
	ABD_SCATTER(abd_zero_scatter).abd_offset = 0;
	ABD_SCATTER(abd_zero_scatter).abd_nents = nr_pages;
	abd_zero_scatter->abd_size = SPA_MAXBLOCKSIZE;
	abd_zero_scatter->abd_parent = NULL;
	zfs_refcount_create(&abd_zero_scatter->abd_children);
	ABD_SCATTER(abd_zero_scatter).abd_sgl = vmem_alloc(nr_pages *
	    sizeof (struct scatterlist), KM_SLEEP);

	sg_init_table(ABD_SCATTER(abd_zero_scatter).abd_sgl, nr_pages);

	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);
}

#endif /* _KERNEL */

boolean_t
abd_size_alloc_linear(size_t size)
{
	return (size < zfs_abd_scatter_min_size ? B_TRUE : B_FALSE);
}

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

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)
{
	size_t n;
	int i = 0;
	struct scatterlist *sg = NULL;

	ASSERT3U(ABD_SCATTER(abd).abd_nents, >, 0);
	ASSERT3U(ABD_SCATTER(abd).abd_offset, <,
	    ABD_SCATTER(abd).abd_sgl->length);
	n = ABD_SCATTER(abd).abd_nents;
	abd_for_each_sg(abd, sg, n, i) {
		ASSERT3P(sg_page(sg), !=, NULL);
	}
}

static void
abd_free_zero_scatter(void)
{
	zfs_refcount_destroy(&abd_zero_scatter->abd_children);
	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;
#if !defined(_KERNEL)
	umem_free(abd_zero_page, PAGESIZE);
#endif /* _KERNEL */
}

void
abd_init(void)
{
	int i;

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

	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;
	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);

	zfs_refcount_destroy(&abd->abd_children);
	abd_update_scatter_stats(abd, ABDSTAT_DECR);
	abd_free_struct(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 *sabd, size_t off)
{
	abd_t *abd = NULL;
	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;

	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 = 0;

	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;

	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);
	aiter->iter_abd = abd;
	aiter->iter_mapaddr = NULL;
	aiter->iter_mapsize = 0;
	aiter->iter_pos = 0;
	if (abd_is_linear(abd)) {
		aiter->iter_offset = 0;
		aiter->iter_sg = NULL;
	} else {
		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)
{
	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)
{
	ASSERT3P(aiter->iter_mapaddr, ==, NULL);
	ASSERT0(aiter->iter_mapsize);

	/* 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_atomic(sg_page(aiter->iter_sg),
		    km_table[aiter->iter_km]);
	}

	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_atomic(aiter->iter_mapaddr - aiter->iter_offset,
		    km_table[aiter->iter_km]);
	}

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

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

#if defined(_KERNEL)
/*
 * 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;

	while (abd_is_gang(abd))
		abd = abd_gang_get_offset(abd, &off);

	ASSERT(!abd_is_gang(abd));
	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)
{
	int i;
	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 (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.");
#endif