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libzpool/abd_os: iovec-based scatter abd

Browse Source 
This is intended to be a simple userspace scatter abd based on struct
iovec. It's not very sophisticated as-is, but sets a base for something
much more interesting.

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 2024-04-21 16:37:06 +10:00 committed by Brian Behlendorf
parent 5b9e695392
commit b69bebb535
2 changed files with 174 additions and 301 deletions
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471
lib/libzpool/abd_os.c
View File

@ -24,34 +24,6 @@
* 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>
@ -59,199 +31,112 @@
#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)
/*
* We're simulating scatter/gather with 4K allocations, since that's more like
* what a typical kernel does.
*/
#define ABD_PAGESIZE (4096)
#define ABD_PAGESHIFT (12)
#define ABD_PAGEMASK (ABD_PAGESIZE-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).
* See rationale in module/os/linux/zfs/abd_os.c, but in userspace this is
* mostly useful to get a mix of linear and scatter ABDs for testing.
*/
static int zfs_abd_scatter_min_size = 512 * 3;
#define 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)
abd_iovcnt_for_bytes(size_t size)
{
return (P2ROUNDUP(size, PAGESIZE) / PAGESIZE);
/*
* Each iovec points to a 4K page. There's no real reason to do this
* in userspace, but our whole point here is to make it feel a bit
* more like a real paged memory model.
*/
return (P2ROUNDUP(size, ABD_PAGESIZE) / ABD_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.
* Zero-sized means it will be used for a linear or gang abd, so just
* allocate the abd itself and return.
*/
(void) size;
abd_t *abd = kmem_cache_alloc(abd_cache, KM_PUSHPAGE);
ASSERT3P(abd, !=, NULL);
if (size == 0)
return (umem_alloc(sizeof (abd_t), UMEM_NOFAIL));
/*
* Allocating for a scatter abd, so compute how many ABD_PAGESIZE
* iovecs we will need to hold this size. Append that allocation to the
* end. Note that struct abd_scatter has includes abd_iov[1], so we
* allocate one less iovec than we need.
*
* Note we're not allocating the pages proper, just the iovec pointers.
* That's down in abd_alloc_chunks. We _could_ do it here in a single
* allocation, but it's fiddly and harder to read for no real gain.
*/
uint_t n = abd_iovcnt_for_bytes(size);
abd_t *abd = umem_alloc(sizeof (abd_t) + (n-1) * sizeof (struct iovec),
UMEM_NOFAIL);
ABD_SCATTER(abd).abd_offset = 0;
ABD_SCATTER(abd).abd_iovcnt = n;
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);
/* For scatter, compute the extra amount we need to free */
uint_t iovcnt =
abd_is_linear(abd) || abd_is_gang(abd) ?
0 : (ABD_SCATTER(abd).abd_iovcnt - 1);
umem_free(abd, sizeof (abd_t) + iovcnt * sizeof (struct iovec));
}
void
abd_alloc_chunks(abd_t *abd, size_t size)
{
unsigned nr_pages = abd_chunkcnt_for_bytes(size);
struct scatterlist *sg;
int i;
/*
* We've already allocated the iovec array; ensure that the wanted size
* actually matches, otherwise the caller has made a mistake somewhere.
*/
uint_t n = ABD_SCATTER(abd).abd_iovcnt;
ASSERT3U(n, ==, abd_iovcnt_for_bytes(size));
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);
/*
* Allocate a ABD_PAGESIZE region for each iovec.
*/
struct iovec *iov = ABD_SCATTER(abd).abd_iov;
for (int i = 0; i < n; i++) {
iov[i].iov_base =
umem_alloc_aligned(ABD_PAGESIZE, ABD_PAGESIZE, UMEM_NOFAIL);
iov[i].iov_len = ABD_PAGESIZE;
}
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);
}
uint_t n = ABD_SCATTER(abd).abd_iovcnt;
struct iovec *iov = ABD_SCATTER(abd).abd_iov;
for (int i = 0; i < n; i++)
umem_free_aligned(iov[i].iov_base, ABD_PAGESIZE);
}
boolean_t
abd_size_alloc_linear(size_t size)
{
return (!zfs_abd_scatter_enabled || size < zfs_abd_scatter_min_size);
return (size < 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;
int waste = P2ROUNDUP(abd->abd_size, ABD_PAGESIZE) - abd->abd_size;
if (op == ABDSTAT_INCR) {
arc_space_consume(waste, ARC_SPACE_ABD_CHUNK_WASTE);
} else {
@ -270,67 +155,72 @@ abd_update_linear_stats(abd_t *abd, abd_stats_op_t op)
void
abd_verify_scatter(abd_t *abd)
{
size_t n;
int i = 0;
struct scatterlist *sg = NULL;
#ifdef ZFS_DEBUG
/*
* scatter abds shall have:
* - at least one iovec
* - all iov_base point somewhere
* - all iov_len are ABD_PAGESIZE
* - offset set within the abd pages somewhere
*/
uint_t n = ABD_SCATTER(abd).abd_iovcnt;
ASSERT3U(n, >, 0);
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);
uint_t len = 0;
for (int i = 0; i < n; i++) {
ASSERT3P(ABD_SCATTER(abd).abd_iov[i].iov_base, !=, NULL);
ASSERT3U(ABD_SCATTER(abd).abd_iov[i].iov_len, ==, ABD_PAGESIZE);
len += ABD_PAGESIZE;
}
}
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);
ASSERT3U(ABD_SCATTER(abd).abd_offset, <, len);
#endif
}
void
abd_init(void)
{
abd_cache = kmem_cache_create("abd_t", sizeof (abd_t),
0, NULL, NULL, NULL, NULL, NULL, 0);
/*
* Create the "zero" scatter abd. This is always the size of the
* largest possible block, but only actually has a single allocated
* page, which all iovecs in the abd point to.
*/
abd_zero_scatter = abd_alloc_struct(SPA_MAXBLOCKSIZE);
abd_zero_scatter->abd_flags |= ABD_FLAG_OWNER;
abd_zero_scatter->abd_size = SPA_MAXBLOCKSIZE;
abd_alloc_zero_scatter();
void *zero =
umem_alloc_aligned(ABD_PAGESIZE, ABD_PAGESIZE, UMEM_NOFAIL);
memset(zero, 0, ABD_PAGESIZE);
uint_t n = abd_iovcnt_for_bytes(SPA_MAXBLOCKSIZE);
struct iovec *iov = ABD_SCATTER(abd_zero_scatter).abd_iov;
for (int i = 0; i < n; i++) {
iov[i].iov_base = zero;
iov[i].iov_len = ABD_PAGESIZE;
}
}
void
abd_fini(void)
{
abd_free_zero_scatter();
if (abd_cache) {
kmem_cache_destroy(abd_cache);
abd_cache = NULL;
}
umem_free_aligned(
ABD_SCATTER(abd_zero_scatter).abd_iov[0].iov_base, ABD_PAGESIZE);
abd_free_struct(abd_zero_scatter);
abd_zero_scatter = NULL;
}
void
abd_free_linear_page(abd_t *abd)
{
/*
* LINEAR_PAGE is specific to the Linux kernel; we never set this
* flag, so this will never be called.
*/
(void) abd;
__builtin_unreachable();
PANIC("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)
{
@ -338,43 +228,60 @@ abd_alloc_for_io(size_t size, boolean_t is_metadata)
}
abd_t *
abd_get_offset_scatter(abd_t *abd, abd_t *sabd, size_t off,
size_t size)
abd_get_offset_scatter(abd_t *dabd, 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.
* Create a new scatter dabd by borrowing data pages from sabd to cover
* off+size.
*
* sabd is an existing scatter abd with a set of iovecs, each covering
* an ABD_PAGESIZE (4K) allocation. It's "zero" is at abd_offset.
*
* [........][........][........][........]
* ^- sabd_offset
*
* We want to produce a new abd, referencing those allocations at the
* given offset.
*
* [........][........][........][........]
* ^- dabd_offset = sabd_offset + off
* ^- dabd_offset + size
*
* In this example, dabd needs three iovecs. The first iovec is offset
* 0, so the final dabd_offset is masked back into the first iovec.
*
* [........][........][........]
* ^- dabd_offset
*/
size_t soff = ABD_SCATTER(sabd).abd_offset + off;
size_t doff = soff & ABD_PAGEMASK;
size_t iovcnt = abd_iovcnt_for_bytes(doff + size);
abd_for_each_sg(sabd, sg, ABD_SCATTER(sabd).abd_nents, i) {
if (new_offset < sg->length)
break;
new_offset -= sg->length;
}
/*
* If the passed-in abd has enough allocated iovecs already, reuse it.
* Otherwise, make a new one. The caller will free the original if the
* one it gets back is not the same.
*
* Note that it's ok if we reuse an abd with more iovecs than we need.
* abd_size has the usable amount of data, and the abd does not own the
* pages referenced by the iovecs. At worst, they're holding dangling
* pointers that we'll never use anyway.
*/
if (dabd == NULL || ABD_SCATTER(dabd).abd_iovcnt < iovcnt)
dabd = abd_alloc_struct(iovcnt << ABD_PAGESHIFT);
ABD_SCATTER(abd).abd_sgl = sg;
ABD_SCATTER(abd).abd_offset = new_offset;
ABD_SCATTER(abd).abd_nents = ABD_SCATTER(sabd).abd_nents - i;
/* Set offset into first page in view */
ABD_SCATTER(dabd).abd_offset = doff;
return (abd);
/* Copy the wanted iovecs from the source to the dest */
memcpy(&ABD_SCATTER(dabd).abd_iov[0],
&ABD_SCATTER(sabd).abd_iov[soff >> ABD_PAGESHIFT],
iovcnt * sizeof (struct iovec));
return (dabd);
}
/*
* Initialize the abd_iter.
*/
void
abd_iter_init(struct abd_iter *aiter, abd_t *abd)
{
@ -382,16 +289,8 @@ abd_iter_init(struct abd_iter *aiter, abd_t *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)
{
@ -399,83 +298,57 @@ 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)
{
/*
* 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;
}
}
}
ASSERT3U(aiter->iter_pos, <=, aiter->iter_abd->abd_size);
}
/*
* 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 =
ABD_LINEAR_BUF(aiter->iter_abd) + aiter->iter_pos;
aiter->iter_mapsize =
aiter->iter_abd->abd_size - aiter->iter_pos;
return;
}
aiter->iter_mapaddr = (char *)paddr + offset;
/*
* For scatter, we index into the appropriate iovec, and return the
* smaller of the amount requested, or up to the end of the page.
*/
size_t poff = aiter->iter_pos + ABD_SCATTER(aiter->iter_abd).abd_offset;
ASSERT3U(poff >> ABD_PAGESHIFT, <=,
ABD_SCATTER(aiter->iter_abd).abd_iovcnt);
struct iovec *iov = &ABD_SCATTER(aiter->iter_abd).
abd_iov[poff >> ABD_PAGESHIFT];
aiter->iter_mapsize = MIN(ABD_PAGESIZE - (poff & ABD_PAGEMASK),
aiter->iter_abd->abd_size - aiter->iter_pos);
ASSERT3U(aiter->iter_mapsize, <=, ABD_PAGESIZE);
aiter->iter_mapaddr = iov->iov_base + (poff & ABD_PAGEMASK);
}
/*
* 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;

4
lib/libzpool/include/sys/abd_os.h
View File

@ -32,8 +32,8 @@ extern "C" {
struct abd_scatter {
uint_t abd_offset;
uint_t abd_nents;
struct scatterlist *abd_sgl;
uint_t abd_iovcnt;
struct iovec abd_iov[1]; /* actually variable-length */
};
struct abd_linear {