Re: [PATCH] xfs: account extra freespace btree splits for multiple allocations

Dave Chinner
2022-11-11T20:39:15Z

On Wed, Nov 09, 2022 at 11:48:02AM +0800, Gao Xiang wrote:
> There is a long standing issue which could cause fs shutdown due to
> inode extent-to-btree conversion failure right after an extent
> allocation in the same AG, which is absolutely unexpected due to the
> proper minleft reservation in the previous allocation. Brian once
> addressed one of the root cause [1], however, such symptom can still
> occur after the commit is merged as reported [2], and our cloud
> environment is also suffering from this issue.
>
> From the description of the commit [1], I found that Zirong has an
> in-house stress test reproducer for this issue, therefore I asked him
> to reproduce again and he confirmed that such issue can still be
> reproduced on RHEL 9 in several days.
>
> Thanks to him, after adding some debugging code to dump the current
> transaction log items, I think the root cause is as below:
>
> 1. xfs_bmapi_allocate() with the following condition:
> freeblks: 18304 pagf_flcount: 6
> reservation: 18276 need (min_free): 6
> args->minleft: 1
> available = freeblks + agflcount - reservation - need - minleft
> = 18304 + min(6, 6) - 18276 - 6 - 1 = 27
> The first allocation check itself is ok, and args->maxlen = 27
> here
>
> At this time, AG 3 also has the following state:
> 1st:64 last:69 cnt:6 longest:6395
>
> AGFL has the following state:
> 64:547 65:167 66:1651 67:2040807 68:783 69:604
>
> 2. Tried to get 27 blocks from this AG, but in order to finish such
> allocation, it had to need a new btree block for cntbt (so take
> another free block from agfl). It can be seen with a new AGF
> recorded in the transaction:
> blkno 62914177, len 1, map_size 1
> 00000000: 58 41 47 46 00 00 00 01 00 00 00 03 00 27 ff f0 XAGF.........'..
> 00000010: 00 00 00 09 00 00 00 07 00 00 00 00 00 00 00 02 ................
> 00000020: 00 00 00 02 00 00 00 00 00 00 00 41 00 00 00 45 ...........A...E
> 00000030: 00 00 00 05 00 00 47 65 00 00 18 fb 00 00 00 09 ......Ge........
> 00000040: 75 dc c1 b5 1a 45 40 2a 80 50 72 f0 59 6e 62 66 u....E@*.Pr.Ynbf
>
> It can be parsed as:
> agf 3 flfirst: 65 (0x41) fllast: 69 (0x45) cnt: 5
> freeblks 18277
>
> 3. agfl 64 (agbno 547, daddr 62918552) was then written as a cntbt
> block, which can also be seen in a log item as below:
> type#011= 0x123c
> flags#011= 0x8
> blkno 62918552, len 8, map_size 1
> 00000000: 41 42 33 43 00 00 00 fd 00 1f 23 e4 ff ff ff ff AB3C......#.....
> 00000010: 00 00 00 00 03 c0 0f 98 00 00 00 00 00 00 00 00 ................
> 00000020: 75 dc c1 b5 1a 45 40 2a 80 50 72 f0 59 6e 62 66 u....E@*.Pr.Ynbf
> ...
>
> 4. Finally, the following inode extent to btree allocation fails
> as below:
> kernel: ------------[ cut here ]------------
> WARNING: CPU: 15 PID: 49290 at fs/xfs/libxfs/xfs_bmap.c:717 xfs_bmap_extents_to_btree+0xc51/0x1050 [xfs]
> ...
> XFS (sda2): agno 3 agflcount 5 freeblks 18277 reservation 18276 6
>
> since freeblks = 18304 - 27 = 18277, but with another agfl
> block allocated (pagf_flcount from 6 to 5), the inequality will
> not be satisfied:
>
> available = freeblks + agflcount - reservation - need - minleft
> = 18277 + min(5, 6) - 18276 - 6 - 0 = 0 < 1
>
> Full current transaction log item dump can be fetched from [3].
>
> As a short-term solution, the following allocations (e.g. allocation
> for inode extent-to-btree conversion) can be recorded in order to count
> more blocks to reserve for safely freespace btree splits so that it
> will shorten available and args->maxlen to
> available = freeblks + agflcount - reservation - need - minleft
> = 18304 + min(6, 6) - 18276 - 6*2 - 1 = 21
> args->maxlen = 21
> in the first allocation, and the following conversion should then
> succeed. At least, it's easy to be backported and do hotfix.
>
> In the long term, args->total and args->minleft have be revisited
> although it could cause more refactoring.
>
> [1] commit 1ca89fbc48e1 ("xfs: don't account extra agfl blocks as available")
> https://lore.kernel.org/r/20190327145000.10756-1-bfoster@redhat.com
> [2] https://lore.kernel.org/r/20220105071052.GD20464@templeofstupid.com
> [3] https://lore.kernel.org/linux-xfs/Y2RevDyoeJZSpiat@B-P7TQMD6M-0146.local/2-dmesg.log.xz
> Reported-by: Zirong Lang <zlang@redhat.com>
> Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
> ---
> Previous discussion is at:
> https://lore.kernel.org/linux-xfs/202211040048.FeUQMLE6-lkp@intel.com/T/#mfcfac181079ddaa5a22eecb74db56534fc4ff918
>
> fs/xfs/libxfs/xfs_alloc.c | 9 +++++++--
> fs/xfs/libxfs/xfs_alloc.h | 1 +
> fs/xfs/libxfs/xfs_bmap.c | 2 ++
> 3 files changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
> index 6261599bb389..684c67310175 100644
> --- a/fs/xfs/libxfs/xfs_alloc.c
> +++ b/fs/xfs/libxfs/xfs_alloc.c
> @@ -2630,7 +2630,12 @@ xfs_alloc_fix_freelist(
> goto out_agbp_relse;
> }
>
> - need = xfs_alloc_min_freelist(mp, pag);
> + /*
> + * Also need to fulfill freespace btree splits by reservaing more
> + * blocks to perform multiple allocations from a single AG and
> + * transaction if needed.
> + */
> + need = xfs_alloc_min_freelist(mp, pag) * (1 + args->postallocs);
> if (!xfs_alloc_space_available(args, need, flags |
> XFS_ALLOC_FLAG_CHECK))
> goto out_agbp_relse;
> @@ -2654,7 +2659,7 @@ xfs_alloc_fix_freelist(
> xfs_agfl_reset(tp, agbp, pag);
>
> /* If there isn't enough total space or single-extent, reject it. */
> - need = xfs_alloc_min_freelist(mp, pag);
> + need = xfs_alloc_min_freelist(mp, pag) * (1 + args->postallocs);
> if (!xfs_alloc_space_available(args, need, flags))
> goto out_agbp_relse;
>
> diff --git a/fs/xfs/libxfs/xfs_alloc.h b/fs/xfs/libxfs/xfs_alloc.h
> index 2c3f762dfb58..be7f15d6a40d 100644
> --- a/fs/xfs/libxfs/xfs_alloc.h
> +++ b/fs/xfs/libxfs/xfs_alloc.h
> @@ -73,6 +73,7 @@ typedef struct xfs_alloc_arg {
> int datatype; /* mask defining data type treatment */
> char wasdel; /* set if allocation was prev delayed */
> char wasfromfl; /* set if allocation is from freelist */
> + bool postallocs; /* number of post-allocations */
> struct xfs_owner_info oinfo; /* owner of blocks being allocated */
> enum xfs_ag_resv_type resv; /* block reservation to use */
> #ifdef DEBUG
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index 49d0d4ea63fc..ed92c6a314b6 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -3497,6 +3497,7 @@ xfs_bmap_exact_minlen_extent_alloc(
> args.alignment = 1;
> args.minalignslop = 0;
>
> + args.postallocs = 1;
> args.minleft = ap->minleft;
> args.wasdel = ap->wasdel;
> args.resv = XFS_AG_RESV_NONE;
> @@ -3658,6 +3659,7 @@ xfs_bmap_btalloc(
> args.alignment = 1;
> args.minalignslop = 0;
> }
> + args.postallocs = 1;
> args.minleft = ap->minleft;
> args.wasdel = ap->wasdel;
> args.resv = XFS_AG_RESV_NONE;

That's not going to work. What happens when we do a full bno
split? Or we do both a bno and a cnt split in the same allocation?

Regardless, I don't see anything wrong with the allocation setup -
it's telling the allocation code exactly what it needs for
subsequent BMBT block allocations to succeed (i.e. args->minleft).
The problem here is that the internal allocation code is failing to
handle the corner case where space is just about gone correctly.

As I pointed out previously - we have a huge amount of reserve space
available in the AG here, so why not use some of the reserve space
to get out of this temporary deficit corner case? We can argue that
it's not really a deficit, either, because moving free blocks to the
free list still accounts them as unused and free, so could still
make up part of the unused reservation....

i.e. is the problem here simply that we don't allow AGFL blocks to
be considered part of the reserved free space?

-Dave.
--
Dave Chinner
david@fromorbit.com