On Fri, Jun 21, 2024 at 10:05:30AM +0000, John Garry wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
>
> We currently align extent allocation to stripe unit or stripe width.
> That is specified by an external parameter to the allocation code,
> which then manipulates the xfs_alloc_args alignment configuration in
> interesting ways.
Is this the "stripe_align" wrangling? That has confused me in the
past...
> The args->alignment field specifies extent start alignment, but
> because we may be attempting non-aligned allocation first there are
> also slop variables that allow for those allocation attempts to
> account for aligned allocation if they fail.
>
> This gets much more complex as we introduce forced allocation
> alignment, where extent size hints are used to generate the extent
> start alignment. extent size hints currently only affect extent
> lengths (via args->prod and args->mod) and so with this change we
> will have two different start alignment conditions.
>
> Avoid this complexity by always using args->alignment to indicate
> extent start alignment, and always using args->prod/mod to indicate
> extent length adjustment.
>
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> [jpg: fixup alignslop references in xfs_trace.h and xfs_ialloc.c]
> Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx>
> ---
> fs/xfs/libxfs/xfs_alloc.c | 4 +-
> fs/xfs/libxfs/xfs_alloc.h | 2 +-
> fs/xfs/libxfs/xfs_bmap.c | 96 +++++++++++++++++---------------------
> fs/xfs/libxfs/xfs_ialloc.c | 10 ++--
> fs/xfs/xfs_trace.h | 8 ++--
> 5 files changed, 54 insertions(+), 66 deletions(-)
>
> diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
> index 32f72217c126..35fbd6b19682 100644
> --- a/fs/xfs/libxfs/xfs_alloc.c
> +++ b/fs/xfs/libxfs/xfs_alloc.c
> @@ -2391,7 +2391,7 @@ xfs_alloc_space_available(
> reservation = xfs_ag_resv_needed(pag, args->resv);
>
> /* do we have enough contiguous free space for the allocation? */
> - alloc_len = args->minlen + (args->alignment - 1) + args->minalignslop;
> + alloc_len = args->minlen + (args->alignment - 1) + args->alignslop;
> longest = xfs_alloc_longest_free_extent(pag, min_free, reservation);
> if (longest < alloc_len)
> return false;
> @@ -2420,7 +2420,7 @@ xfs_alloc_space_available(
> * allocation as we know that will definitely succeed and match the
> * callers alignment constraints.
> */
> - alloc_len = args->maxlen + (args->alignment - 1) + args->minalignslop;
> + alloc_len = args->maxlen + (args->alignment - 1) + args->alignslop;
> if (longest < alloc_len) {
> args->maxlen = args->minlen;
> ASSERT(args->maxlen > 0);
> diff --git a/fs/xfs/libxfs/xfs_alloc.h b/fs/xfs/libxfs/xfs_alloc.h
> index 0b956f8b9d5a..aa2c103d98f0 100644
> --- a/fs/xfs/libxfs/xfs_alloc.h
> +++ b/fs/xfs/libxfs/xfs_alloc.h
> @@ -46,7 +46,7 @@ typedef struct xfs_alloc_arg {
> xfs_extlen_t minleft; /* min blocks must be left after us */
> xfs_extlen_t total; /* total blocks needed in xaction */
> xfs_extlen_t alignment; /* align answer to multiple of this */
> - xfs_extlen_t minalignslop; /* slop for minlen+alignment calcs */
> + xfs_extlen_t alignslop; /* slop for alignment calcs */
Er... what is "slop", exactly? I've never felt like I understood how
the aligned allocation code works. AFAICT, the rough idea is that we'll
search the free space for an extent that is at least (len + alignment)
blocks long, and then pick whatever subset of that extent satisfies the
alignment requirements?
But that's the 30,000ft version -- sometimes for the sake of contiguity,
we want to try for an allocation that starts at an *exact* location.
Contiguity is always important for storage allocation, so we're willing
to forego the alignment of the start of the free space, but we should
still try to align the end of the allocation. Therefore, we add this
"alignslop" parameter and look for an AG with a free extent that is at
least (len + alignment - 1 + alignslop) blocks long? And then we pick
the subset that satisfies us?
> xfs_agblock_t min_agbno; /* set an agbno range for NEAR allocs */
> xfs_agblock_t max_agbno; /* ... */
> xfs_extlen_t len; /* output: actual size of extent */
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index c101cf266bc4..7f8c8e4dd244 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -3285,6 +3285,10 @@ xfs_bmap_select_minlen(
> xfs_extlen_t blen)
> {
>
> + /* Adjust best length for extent start alignment. */
> + if (blen > args->alignment)
> + blen -= args->alignment;
But why? I guess this is artifact of moving stripe_align into
args->alignment? So now we actually need to subtract out the alignment
when figuring out how to set minlen?
<confused>
> +
> /*
> * Since we used XFS_ALLOC_FLAG_TRYLOCK in _longest_free_extent(), it is
> * possible that there is enough contiguous free space for this request.
> @@ -3300,6 +3304,7 @@ xfs_bmap_select_minlen(
> if (blen < args->maxlen)
> return blen;
> return args->maxlen;
> +
Nit: unnecessary whitespace.
> }
>
> static int
> @@ -3393,35 +3398,43 @@ xfs_bmap_alloc_account(
> xfs_trans_mod_dquot_byino(ap->tp, ap->ip, fld, ap->length);
> }
>
> -static int
> +/*
> + * Calculate the extent start alignment and the extent length adjustments that
> + * constrain this allocation.
> + *
> + * Extent start alignment is currently determined by stripe configuration and is
> + * carried in args->alignment, whilst extent length adjustment is determined by
> + * extent size hints and is carried by args->prod and args->mod.
> + *
> + * Low level allocation code is free to either ignore or override these values
> + * as required.
> + */
> +static void
> xfs_bmap_compute_alignments(
> struct xfs_bmalloca *ap,
> struct xfs_alloc_arg *args)
> {
> struct xfs_mount *mp = args->mp;
> xfs_extlen_t align = 0; /* minimum allocation alignment */
> - int stripe_align = 0;
>
> /* stripe alignment for allocation is determined by mount parameters */
> if (mp->m_swidth && xfs_has_swalloc(mp))
> - stripe_align = mp->m_swidth;
> + args->alignment = mp->m_swidth;
> else if (mp->m_dalign)
> - stripe_align = mp->m_dalign;
> + args->alignment = mp->m_dalign;
>
> if (ap->flags & XFS_BMAPI_COWFORK)
> align = xfs_get_cowextsz_hint(ap->ip);
> else if (ap->datatype & XFS_ALLOC_USERDATA)
> align = xfs_get_extsz_hint(ap->ip);
> +
> if (align) {
> if (xfs_bmap_extsize_align(mp, &ap->got, &ap->prev, align, 0,
> ap->eof, 0, ap->conv, &ap->offset,
> &ap->length))
> ASSERT(0);
> ASSERT(ap->length);
> - }
>
> - /* apply extent size hints if obtained earlier */
> - if (align) {
> args->prod = align;
> div_u64_rem(ap->offset, args->prod, &args->mod);
> if (args->mod)
> @@ -3436,7 +3449,6 @@ xfs_bmap_compute_alignments(
> args->mod = args->prod - args->mod;
> }
>
> - return stripe_align;
> }
>
> static void
> @@ -3508,7 +3520,7 @@ xfs_bmap_exact_minlen_extent_alloc(
> args.total = ap->total;
>
> args.alignment = 1;
> - args.minalignslop = 0;
> + args.alignslop = 0;
>
> args.minleft = ap->minleft;
> args.wasdel = ap->wasdel;
> @@ -3548,7 +3560,6 @@ xfs_bmap_btalloc_at_eof(
> struct xfs_bmalloca *ap,
> struct xfs_alloc_arg *args,
> xfs_extlen_t blen,
> - int stripe_align,
> bool ag_only)
> {
> struct xfs_mount *mp = args->mp;
> @@ -3562,23 +3573,15 @@ xfs_bmap_btalloc_at_eof(
> * allocation.
> */
> if (ap->offset) {
> - xfs_extlen_t nextminlen = 0;
> + xfs_extlen_t alignment = args->alignment;
>
> /*
> - * Compute the minlen+alignment for the next case. Set slop so
> - * that the value of minlen+alignment+slop doesn't go up between
> - * the calls.
> + * Compute the alignment slop for the fallback path so we ensure
> + * we account for the potential alignemnt space required by the
Spelling: alignment
> + * fallback paths before we modify the AGF and AGFL here.
> */
> args->alignment = 1;
> - if (blen > stripe_align && blen <= args->maxlen)
> - nextminlen = blen - stripe_align;
> - else
> - nextminlen = args->minlen;
> - if (nextminlen + stripe_align > args->minlen + 1)
> - args->minalignslop = nextminlen + stripe_align -
> - args->minlen - 1;
> - else
> - args->minalignslop = 0;
> + args->alignslop = alignment - args->alignment;
I think this is easier to follow, assuming I'm not just completely
confused. :/
--D
>
> if (!caller_pag)
> args->pag = xfs_perag_get(mp, XFS_FSB_TO_AGNO(mp, ap->blkno));
> @@ -3596,19 +3599,8 @@ xfs_bmap_btalloc_at_eof(
> * Exact allocation failed. Reset to try an aligned allocation
> * according to the original allocation specification.
> */
> - args->alignment = stripe_align;
> - args->minlen = nextminlen;
> - args->minalignslop = 0;
> - } else {
> - /*
> - * Adjust minlen to try and preserve alignment if we
> - * can't guarantee an aligned maxlen extent.
> - */
> - args->alignment = stripe_align;
> - if (blen > args->alignment &&
> - blen <= args->maxlen + args->alignment)
> - args->minlen = blen - args->alignment;
> - args->minalignslop = 0;
> + args->alignment = alignment;
> + args->alignslop = 0;
> }
>
> if (ag_only) {
> @@ -3626,9 +3618,8 @@ xfs_bmap_btalloc_at_eof(
> return 0;
>
> /*
> - * Allocation failed, so turn return the allocation args to their
> - * original non-aligned state so the caller can proceed on allocation
> - * failure as if this function was never called.
> + * Aligned allocation failed, so all fallback paths from here drop the
> + * start alignment requirement as we know it will not succeed.
> */
> args->alignment = 1;
> return 0;
> @@ -3636,7 +3627,9 @@ xfs_bmap_btalloc_at_eof(
>
> /*
> * We have failed multiple allocation attempts so now are in a low space
> - * allocation situation. Try a locality first full filesystem minimum length
> + * allocation situation. We give up on any attempt at aligned allocation here.
> + *
> + * Try a locality first full filesystem minimum length
> * allocation whilst still maintaining necessary total block reservation
> * requirements.
> *
> @@ -3653,6 +3646,7 @@ xfs_bmap_btalloc_low_space(
> {
> int error;
>
> + args->alignment = 1;
> if (args->minlen > ap->minlen) {
> args->minlen = ap->minlen;
> error = xfs_alloc_vextent_start_ag(args, ap->blkno);
> @@ -3672,13 +3666,11 @@ xfs_bmap_btalloc_low_space(
> static int
> xfs_bmap_btalloc_filestreams(
> struct xfs_bmalloca *ap,
> - struct xfs_alloc_arg *args,
> - int stripe_align)
> + struct xfs_alloc_arg *args)
> {
> xfs_extlen_t blen = 0;
> int error = 0;
>
> -
> error = xfs_filestream_select_ag(ap, args, &blen);
> if (error)
> return error;
> @@ -3697,8 +3689,7 @@ xfs_bmap_btalloc_filestreams(
>
> args->minlen = xfs_bmap_select_minlen(ap, args, blen);
> if (ap->aeof)
> - error = xfs_bmap_btalloc_at_eof(ap, args, blen, stripe_align,
> - true);
> + error = xfs_bmap_btalloc_at_eof(ap, args, blen, true);
>
> if (!error && args->fsbno == NULLFSBLOCK)
> error = xfs_alloc_vextent_near_bno(args, ap->blkno);
> @@ -3722,8 +3713,7 @@ xfs_bmap_btalloc_filestreams(
> static int
> xfs_bmap_btalloc_best_length(
> struct xfs_bmalloca *ap,
> - struct xfs_alloc_arg *args,
> - int stripe_align)
> + struct xfs_alloc_arg *args)
> {
> xfs_extlen_t blen = 0;
> int error;
> @@ -3747,8 +3737,7 @@ xfs_bmap_btalloc_best_length(
> * trying.
> */
> if (ap->aeof && !(ap->tp->t_flags & XFS_TRANS_LOWMODE)) {
> - error = xfs_bmap_btalloc_at_eof(ap, args, blen, stripe_align,
> - false);
> + error = xfs_bmap_btalloc_at_eof(ap, args, blen, false);
> if (error || args->fsbno != NULLFSBLOCK)
> return error;
> }
> @@ -3775,27 +3764,26 @@ xfs_bmap_btalloc(
> .resv = XFS_AG_RESV_NONE,
> .datatype = ap->datatype,
> .alignment = 1,
> - .minalignslop = 0,
> + .alignslop = 0,
> };
> xfs_fileoff_t orig_offset;
> xfs_extlen_t orig_length;
> int error;
> - int stripe_align;
>
> ASSERT(ap->length);
> orig_offset = ap->offset;
> orig_length = ap->length;
>
> - stripe_align = xfs_bmap_compute_alignments(ap, &args);
> + xfs_bmap_compute_alignments(ap, &args);
>
> /* Trim the allocation back to the maximum an AG can fit. */
> args.maxlen = min(ap->length, mp->m_ag_max_usable);
>
> if ((ap->datatype & XFS_ALLOC_USERDATA) &&
> xfs_inode_is_filestream(ap->ip))
> - error = xfs_bmap_btalloc_filestreams(ap, &args, stripe_align);
> + error = xfs_bmap_btalloc_filestreams(ap, &args);
> else
> - error = xfs_bmap_btalloc_best_length(ap, &args, stripe_align);
> + error = xfs_bmap_btalloc_best_length(ap, &args);
> if (error)
> return error;
>
> diff --git a/fs/xfs/libxfs/xfs_ialloc.c b/fs/xfs/libxfs/xfs_ialloc.c
> index 14c81f227c5b..9f71a9a3a65e 100644
> --- a/fs/xfs/libxfs/xfs_ialloc.c
> +++ b/fs/xfs/libxfs/xfs_ialloc.c
> @@ -758,12 +758,12 @@ xfs_ialloc_ag_alloc(
> *
> * For an exact allocation, alignment must be 1,
> * however we need to take cluster alignment into account when
> - * fixing up the freelist. Use the minalignslop field to
> - * indicate that extra blocks might be required for alignment,
> - * but not to use them in the actual exact allocation.
> + * fixing up the freelist. Use the alignslop field to indicate
> + * that extra blocks might be required for alignment, but not
> + * to use them in the actual exact allocation.
> */
> args.alignment = 1;
> - args.minalignslop = igeo->cluster_align - 1;
> + args.alignslop = igeo->cluster_align - 1;
>
> /* Allow space for the inode btree to split. */
> args.minleft = igeo->inobt_maxlevels;
> @@ -783,7 +783,7 @@ xfs_ialloc_ag_alloc(
> * on, so reset minalignslop to ensure it is not included in
> * subsequent requests.
> */
> - args.minalignslop = 0;
> + args.alignslop = 0;
> }
>
> if (unlikely(args.fsbno == NULLFSBLOCK)) {
> diff --git a/fs/xfs/xfs_trace.h b/fs/xfs/xfs_trace.h
> index 25ff6fe1eb6c..0b2a2a1379bd 100644
> --- a/fs/xfs/xfs_trace.h
> +++ b/fs/xfs/xfs_trace.h
> @@ -1808,7 +1808,7 @@ DECLARE_EVENT_CLASS(xfs_alloc_class,
> __field(xfs_extlen_t, minleft)
> __field(xfs_extlen_t, total)
> __field(xfs_extlen_t, alignment)
> - __field(xfs_extlen_t, minalignslop)
> + __field(xfs_extlen_t, alignslop)
> __field(xfs_extlen_t, len)
> __field(char, wasdel)
> __field(char, wasfromfl)
> @@ -1827,7 +1827,7 @@ DECLARE_EVENT_CLASS(xfs_alloc_class,
> __entry->minleft = args->minleft;
> __entry->total = args->total;
> __entry->alignment = args->alignment;
> - __entry->minalignslop = args->minalignslop;
> + __entry->alignslop = args->alignslop;
> __entry->len = args->len;
> __entry->wasdel = args->wasdel;
> __entry->wasfromfl = args->wasfromfl;
> @@ -1836,7 +1836,7 @@ DECLARE_EVENT_CLASS(xfs_alloc_class,
> __entry->highest_agno = args->tp->t_highest_agno;
> ),
> TP_printk("dev %d:%d agno 0x%x agbno 0x%x minlen %u maxlen %u mod %u "
> - "prod %u minleft %u total %u alignment %u minalignslop %u "
> + "prod %u minleft %u total %u alignment %u alignslop %u "
> "len %u wasdel %d wasfromfl %d resv %d "
> "datatype 0x%x highest_agno 0x%x",
> MAJOR(__entry->dev), MINOR(__entry->dev),
> @@ -1849,7 +1849,7 @@ DECLARE_EVENT_CLASS(xfs_alloc_class,
> __entry->minleft,
> __entry->total,
> __entry->alignment,
> - __entry->minalignslop,
> + __entry->alignslop,
> __entry->len,
> __entry->wasdel,
> __entry->wasfromfl,
> --
> 2.31.1
>
>
