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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
This commit is contained in:
Rob Norris 2023-12-25 22:25:48 +11:00 committed by Brian Behlendorf
parent 2b7d9a7863
commit 7a5b4355e2
4 changed files with 498 additions and 149 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
include/sys/abd.h
View File

@ -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;

2
lib/libzpool/Makefile.am
View File

@ -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 \

492
lib/libzpool/abd_os.c Normal file
View File

@ -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)
{
}

151
module/os/linux/zfs/abd_os.c
View File

@ -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
* an allocated zero'd PAGESIZE buffer.
* Userspace - Will be an allocated zero'ed PAGESIZE buffer.
*
* abd_zero_page is assigned to each of the pages of abd_zero_scatter.
* 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;
@ -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 */