abd_os: split userspace and Linux kernel code

The Linux abd_os.c serves double-duty as the userspace scatter abd
implementation, by carrying an emulation of kernel scatterlists. This
commit lifts common and userspace-specific parts out into a separate
abd_os.c for libzpool.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #16253

include/sys/abd.h

@@ -68,7 +68,9 @@ typedef struct abd {
 		} abd_scatter;
 		struct abd_linear {
 			void		*abd_buf;
+#if defined(__linux__) && defined(_KERNEL)
 			struct scatterlist *abd_sgl; /* for LINEAR_PAGE */
+#endif
 		} abd_linear;
 		struct abd_gang {
 			list_t abd_gang_chain;

lib/libzpool/Makefile.am

@@ -9,6 +9,7 @@ lib_LTLIBRARIES += libzpool.la
 CPPCHECKTARGETS += libzpool.la
 
 dist_libzpool_la_SOURCES = \
+	%D%/abd_os.c \
 	%D%/kernel.c \
 	%D%/taskq.c \
 	%D%/util.c
@@ -39,7 +40,6 @@ nodist_libzpool_la_SOURCES = \
 	module/lua/lvm.c \
 	module/lua/lzio.c \
 	\
-	module/os/linux/zfs/abd_os.c \
 	module/os/linux/zfs/arc_os.c \
 	module/os/linux/zfs/trace.c \
 	module/os/linux/zfs/vdev_file.c \

lib/libzpool/abd_os.c

@@ -0,0 +1,492 @@
+/*
+ * 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>
+
+
+#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 will be allocated with a zero'ed PAGESIZE buffer, which is
+ * assigned to each of the pages of abd_zero_scatter.
+ */
+static struct page *abd_zero_page = NULL;
+
+static kmem_cache_t *abd_cache = NULL;
+
+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);
+
+	return (abd);
+}
+
+void
+abd_free_struct_impl(abd_t *abd)
+{
+	kmem_cache_free(abd_cache, abd);
+}
+
+#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) {
+		struct page *p = nth_page(sg_page(sg), 0);
+		umem_free_aligned(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_SCATTER(abd_zero_scatter).abd_offset = 0;
+	ABD_SCATTER(abd_zero_scatter).abd_nents = nr_pages;
+	abd_zero_scatter->abd_size = SPA_MAXBLOCKSIZE;
+	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);
+	}
+}
+
+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) {
+		arc_space_consume(waste, ARC_SPACE_ABD_CHUNK_WASTE);
+	} else {
+		arc_space_return(waste, ARC_SPACE_ABD_CHUNK_WASTE);
+	}
+}
+
+void
+abd_update_linear_stats(abd_t *abd, abd_stats_op_t op)
+{
+	(void) abd;
+	(void) op;
+	ASSERT(op == ABDSTAT_INCR || op == ABDSTAT_DECR);
+}
+
+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)
+{
+	abd_free_sg_table(abd_zero_scatter);
+	abd_free_struct(abd_zero_scatter);
+	abd_zero_scatter = NULL;
+	ASSERT3P(abd_zero_page, !=, NULL);
+	umem_free_aligned(abd_zero_page, PAGESIZE);
+}
+
+void
+abd_init(void)
+{
+	abd_cache = kmem_cache_create("abd_t", sizeof (abd_t),
+	    0, NULL, NULL, NULL, NULL, NULL, 0);
+
+	abd_alloc_zero_scatter();
+}
+
+void
+abd_fini(void)
+{
+	abd_free_zero_scatter();
+
+	if (abd_cache) {
+		kmem_cache_destroy(abd_cache);
+		abd_cache = NULL;
+	}
+}
+
+void
+abd_free_linear_page(abd_t *abd)
+{
+	(void) abd;
+	__builtin_unreachable();
+}
+
+/*
+ * 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;
+
+	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 = 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;
+
+	ASSERT3P(aiter->iter_mapaddr, !=, NULL);
+	ASSERT3U(aiter->iter_mapsize, >, 0);
+
+	aiter->iter_mapaddr = NULL;
+	aiter->iter_mapsize = 0;
+}
+
+void
+abd_cache_reap_now(void)
+{
+}

module/os/linux/zfs/abd_os.c

@@ -58,22 +58,16 @@
 #include <sys/arc.h>
 #include <sys/zfs_context.h>
 #include <sys/zfs_znode.h>
-#ifdef _KERNEL
 #include <linux/kmap_compat.h>
 #include <linux/mm_compat.h>
 #include <linux/scatterlist.h>
 #include <linux/version.h>
-#endif
 
-#ifdef _KERNEL
 #if defined(MAX_ORDER)
 #define	ABD_MAX_ORDER	(MAX_ORDER)
 #elif defined(MAX_PAGE_ORDER)
 #define	ABD_MAX_ORDER	(MAX_PAGE_ORDER)
 #endif
-#else
-#define	ABD_MAX_ORDER	(1)
-#endif
 
 typedef struct abd_stats {
 	kstat_named_t abdstat_struct_size;
@@ -193,11 +187,9 @@ abd_t *abd_zero_scatter = NULL;
 
 struct page;
 /*
- * _KERNEL   - Will point to ZERO_PAGE if it is available or it will be
+ * abd_zero_page is assigned to each of the pages of abd_zero_scatter. It will
- *             an allocated zero'd PAGESIZE buffer.
+ * point to ZERO_PAGE if it is available or it will be an allocated zero'd
- * Userspace - Will be an allocated zero'ed PAGESIZE buffer.
+ * PAGESIZE buffer.
- *
- * abd_zero_page is assigned to each of the pages of abd_zero_scatter.
  */
 static struct page *abd_zero_page = NULL;
 
@@ -232,7 +224,6 @@ abd_free_struct_impl(abd_t *abd)
 	ABDSTAT_INCR(abdstat_struct_size, -(int)sizeof (abd_t));
 }
 
-#ifdef _KERNEL
 static unsigned zfs_abd_scatter_max_order = ABD_MAX_ORDER - 1;
 
 /*
@@ -520,134 +511,6 @@ abd_alloc_zero_scatter(void)
 	ABDSTAT_BUMP(abdstat_scatter_page_multi_chunk);
 }
 
-#else /* _KERNEL */
-
-#ifndef PAGE_SHIFT
-#define	PAGE_SHIFT (highbit64(PAGESIZE)-1)
-#endif
-
-#define	zfs_kmap_local(chunk)		((void *)chunk)
-#define	zfs_kunmap_local(addr)		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) {
-		struct page *p = nth_page(sg_page(sg), 0);
-		umem_free_aligned(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_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)
 {
@@ -712,14 +575,10 @@ abd_free_zero_scatter(void)
 	abd_free_struct(abd_zero_scatter);
 	abd_zero_scatter = NULL;
 	ASSERT3P(abd_zero_page, !=, NULL);
-#if defined(_KERNEL)
 #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 */
-#else
-	umem_free_aligned(abd_zero_page, PAGESIZE);
-#endif /* _KERNEL */
 }
 
 static int
@@ -1014,8 +873,6 @@ abd_cache_reap_now(void)
 {
 }
 
-#if defined(_KERNEL)
-
 /*
  * 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
@@ -1297,5 +1154,3 @@ MODULE_PARM_DESC(zfs_abd_scatter_min_size,
 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 /* _KERNEL */