Re: [PATCH 41/42] xfs: return a referenced perag from filestreams allocator

Darrick J. Wong
2023-02-02T00:01:32Z

On Thu, Jan 19, 2023 at 09:45:04AM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> Now that the filestreams AG selection tracks active perags, we need
> to return an active perag to the core allocator code. This is
> because the file allocation the filestreams code will run are AG
> specific allocations and so need to pin the AG until the allocations
> complete.
>
> We cannot rely on the filestreams item reference to do this - the
> filestreams association can be torn down at any time, hence we
> need to have a separate reference for the allocation process to pin
> the AG after it has been selected.
>
> This means there is some perag juggling in allocation failure
> fallback paths as they will do all AG scans in the case the AG
> specific allocation fails. Hence we need to track the perag
> reference that the filestream allocator returned to make sure we
> don't leak it on repeated allocation failure.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> fs/xfs/libxfs/xfs_bmap.c | 38 +++++++++++-----
> fs/xfs/xfs_filestream.c | 93 ++++++++++++++++++++++++----------------
> 2 files changed, 84 insertions(+), 47 deletions(-)
>
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index 098b46f3f3e3..7f56002b545d 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -3427,6 +3427,7 @@ xfs_bmap_btalloc_at_eof(
> bool ag_only)
> {
> struct xfs_mount *mp = args->mp;
> + struct xfs_perag *caller_pag = args->pag;
> int error;
>
> /*
> @@ -3454,9 +3455,11 @@ xfs_bmap_btalloc_at_eof(
> else
> args->minalignslop = 0;
>
> - args->pag = xfs_perag_get(mp, XFS_FSB_TO_AGNO(mp, ap->blkno));
> + if (!caller_pag)
> + args->pag = xfs_perag_get(mp, XFS_FSB_TO_AGNO(mp, ap->blkno));
> error = xfs_alloc_vextent_exact_bno(args, ap->blkno);
> - xfs_perag_put(args->pag);
> + if (!caller_pag)
> + xfs_perag_put(args->pag);
> if (error)
> return error;
>
> @@ -3482,10 +3485,13 @@ xfs_bmap_btalloc_at_eof(
> args->minalignslop = 0;
> }
>
> - if (ag_only)
> + if (ag_only) {
> error = xfs_alloc_vextent_near_bno(args, ap->blkno);
> - else
> + } else {
> + args->pag = NULL;
> error = xfs_alloc_vextent_start_ag(args, ap->blkno);
> + args->pag = caller_pag;

At first glance I wondered if we end up leaking any args->pag set by the
_iterate_ags function, but I think it's the case that _finish will
release args->pag and set it back to NULL? So in effect we're
preserving the caller's args->pag here, and nothing leaks. In that
case, I think we should check that assumption:

ASSERT(args->pag == NULL);
args->pag = caller_pag;

If the answer to the above is yes, then with the above fixed,
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> + }
> if (error)
> return error;
>
> @@ -3544,12 +3550,13 @@ xfs_bmap_btalloc_filestreams(
> int stripe_align)
> {
> xfs_extlen_t blen = 0;
> - int error;
> + int error = 0;
>
>
> error = xfs_filestream_select_ag(ap, args, &blen);
> if (error)
> return error;
> + ASSERT(args->pag);
>
> /*
> * If we are in low space mode, then optimal allocation will fail so
> @@ -3558,22 +3565,31 @@ xfs_bmap_btalloc_filestreams(
> */
> if (ap->tp->t_flags & XFS_TRANS_LOWMODE) {
> args->minlen = ap->minlen;
> + ASSERT(args->fsbno == NULLFSBLOCK);
> goto out_low_space;
> }
>
> args->minlen = xfs_bmap_select_minlen(ap, args, blen);
> - if (ap->aeof) {
> + if (ap->aeof)
> error = xfs_bmap_btalloc_at_eof(ap, args, blen, stripe_align,
> true);
> - if (error || args->fsbno != NULLFSBLOCK)
> - return error;
> - }
>
> - error = xfs_alloc_vextent_near_bno(args, ap->blkno);
> + if (!error && args->fsbno == NULLFSBLOCK)
> + error = xfs_alloc_vextent_near_bno(args, ap->blkno);
> +
> +out_low_space:
> + /*
> + * We are now done with the perag reference for the filestreams
> + * association provided by xfs_filestream_select_ag(). Release it now as
> + * we've either succeeded, had a fatal error or we are out of space and
> + * need to do a full filesystem scan for free space which will take it's
> + * own references.
> + */
> + xfs_perag_rele(args->pag);
> + args->pag = NULL;
> if (error || args->fsbno != NULLFSBLOCK)
> return error;
>
> -out_low_space:
> return xfs_bmap_btalloc_low_space(ap, args);
> }
>
> diff --git a/fs/xfs/xfs_filestream.c b/fs/xfs/xfs_filestream.c
> index 81aebe3e09ba..523a3b8b5754 100644
> --- a/fs/xfs/xfs_filestream.c
> +++ b/fs/xfs/xfs_filestream.c
> @@ -53,8 +53,9 @@ xfs_fstrm_free_func(
> */
> static int
> xfs_filestream_pick_ag(
> + struct xfs_alloc_arg *args,
> struct xfs_inode *ip,
> - xfs_agnumber_t *agp,
> + xfs_agnumber_t start_agno,
> int flags,
> xfs_extlen_t *longest)
> {
> @@ -64,7 +65,6 @@ xfs_filestream_pick_ag(
> struct xfs_perag *max_pag = NULL;
> xfs_extlen_t minlen = *longest;
> xfs_extlen_t free = 0, minfree, maxfree = 0;
> - xfs_agnumber_t start_agno = *agp;
> xfs_agnumber_t agno;
> int err, trylock;
>
> @@ -73,8 +73,6 @@ xfs_filestream_pick_ag(
> /* 2% of an AG's blocks must be free for it to be chosen. */
> minfree = mp->m_sb.sb_agblocks / 50;
>
> - *agp = NULLAGNUMBER;
> -
> /* For the first pass, don't sleep trying to init the per-AG. */
> trylock = XFS_ALLOC_FLAG_TRYLOCK;
>
> @@ -89,7 +87,7 @@ xfs_filestream_pick_ag(
> break;
> /* Couldn't lock the AGF, skip this AG. */
> err = 0;
> - goto next_ag;
> + continue;
> }
>
> /* Keep track of the AG with the most free blocks. */
> @@ -146,16 +144,19 @@ xfs_filestream_pick_ag(
> /*
> * No unassociated AGs are available, so select the AG with the
> * most free space, regardless of whether it's already in use by
> - * another filestream. It none suit, return NULLAGNUMBER.
> + * another filestream. It none suit, just use whatever AG we can
> + * grab.
> */
> if (!max_pag) {
> - *agp = NULLAGNUMBER;
> - trace_xfs_filestream_pick(ip, NULL, free);
> - return 0;
> + for_each_perag_wrap(mp, start_agno, agno, pag)
> + break;
> + atomic_inc(&pag->pagf_fstrms);
> + *longest = 0;
> + } else {
> + pag = max_pag;
> + free = maxfree;
> + atomic_inc(&pag->pagf_fstrms);
> }
> - pag = max_pag;
> - free = maxfree;
> - atomic_inc(&pag->pagf_fstrms);
> } else if (max_pag) {
> xfs_perag_rele(max_pag);
> }
> @@ -167,16 +168,29 @@ xfs_filestream_pick_ag(
> if (!item)
> goto out_put_ag;
>
> +
> + /*
> + * We are going to use this perag now, so take another ref to it for the
> + * allocation context returned to the caller. If we raced to create and
> + * insert the filestreams item into the MRU (-EEXIST), then we still
> + * keep this reference but free the item reference we gained above. On
> + * any other failure, we have to drop both.
> + */
> + atomic_inc(&pag->pag_active_ref);
> item->pag = pag;
> + args->pag = pag;
>
> err = xfs_mru_cache_insert(mp->m_filestream, ip->i_ino, &item->mru);
> if (err) {
> - if (err == -EEXIST)
> + if (err == -EEXIST) {
> err = 0;
> + } else {
> + xfs_perag_rele(args->pag);
> + args->pag = NULL;
> + }
> goto out_free_item;
> }
>
> - *agp = pag->pag_agno;
> return 0;
>
> out_free_item:
> @@ -236,7 +250,14 @@ xfs_filestream_select_ag_mru(
> if (!mru)
> goto out_default_agno;
>
> + /*
> + * Grab the pag and take an extra active reference for the caller whilst
> + * the mru item cannot go away. This means we'll pin the perag with
> + * the reference we get here even if the filestreams association is torn
> + * down immediately after we mark the lookup as done.
> + */
> pag = container_of(mru, struct xfs_fstrm_item, mru)->pag;
> + atomic_inc(&pag->pag_active_ref);
> xfs_mru_cache_done(mp->m_filestream);
>
> trace_xfs_filestream_lookup(pag, ap->ip->i_ino);
> @@ -246,6 +267,8 @@ xfs_filestream_select_ag_mru(
>
> error = xfs_bmap_longest_free_extent(pag, args->tp, blen);
> if (error) {
> + /* We aren't going to use this perag */
> + xfs_perag_rele(pag);
> if (error != -EAGAIN)
> return error;
> *blen = 0;
> @@ -253,12 +276,18 @@ xfs_filestream_select_ag_mru(
>
> /*
> * We are done if there's still enough contiguous free space to succeed.
> + * If there is very little free space before we start a filestreams
> + * allocation, we're almost guaranteed to fail to find a better AG with
> + * larger free space available so we don't even try.
> */
> *agno = pag->pag_agno;
> - if (*blen >= args->maxlen)
> + if (*blen >= args->maxlen || (ap->tp->t_flags & XFS_TRANS_LOWMODE)) {
> + args->pag = pag;
> return 0;
> + }
>
> /* Changing parent AG association now, so remove the existing one. */
> + xfs_perag_rele(pag);
> mru = xfs_mru_cache_remove(mp->m_filestream, pip->i_ino);
> if (mru) {
> struct xfs_fstrm_item *item =
> @@ -297,46 +326,38 @@ xfs_filestream_select_ag(
> struct xfs_inode *pip = NULL;
> xfs_agnumber_t agno;
> int flags = 0;
> - int error;
> + int error = 0;
>
> args->total = ap->total;
> *blen = 0;
>
> pip = xfs_filestream_get_parent(ap->ip);
> if (!pip) {
> - agno = 0;
> - goto out_select;
> + ap->blkno = XFS_AGB_TO_FSB(mp, 0, 0);
> + return 0;
> }
>
> error = xfs_filestream_select_ag_mru(ap, args, pip, &agno, blen);
> - if (error || *blen >= args->maxlen)
> + if (error)
> goto out_rele;
> -
> - ap->blkno = XFS_AGB_TO_FSB(args->mp, agno, 0);
> - xfs_bmap_adjacent(ap);
> -
> - /*
> - * If there is very little free space before we start a filestreams
> - * allocation, we're almost guaranteed to fail to find a better AG with
> - * larger free space available so we don't even try.
> - */
> + if (*blen >= args->maxlen)
> + goto out_select;
> if (ap->tp->t_flags & XFS_TRANS_LOWMODE)
> goto out_select;
>
> + ap->blkno = XFS_AGB_TO_FSB(args->mp, agno, 0);
> + xfs_bmap_adjacent(ap);
> + *blen = ap->length;
> if (ap->datatype & XFS_ALLOC_USERDATA)
> flags |= XFS_PICK_USERDATA;
> if (ap->tp->t_flags & XFS_TRANS_LOWMODE)
> flags |= XFS_PICK_LOWSPACE;
>
> - *blen = ap->length;
> - error = xfs_filestream_pick_ag(pip, &agno, flags, blen);
> - if (agno == NULLAGNUMBER) {
> - agno = 0;
> - *blen = 0;
> - }
> -
> + error = xfs_filestream_pick_ag(args, pip, agno, flags, blen);
> + if (error)
> + goto out_rele;
> out_select:
> - ap->blkno = XFS_AGB_TO_FSB(mp, agno, 0);
> + ap->blkno = XFS_AGB_TO_FSB(mp, args->pag->pag_agno, 0);
> out_rele:
> xfs_irele(pip);
> return error;
> --
> 2.39.0
>