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

Zorro Lang
2022-11-11T06:06:02Z

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>
> ---

FYI, I've tested this bug, with the original reproducer I hit this bug. Although
the reproducer is a big long time (3days) stress test, and can't reproduce
this bug 100%, but generally with about 1/5 chance.

So I submitted 6 testing jobs, all test passed today, didn't this bug, so I
think this patch works for me. (This testing can't replace regression test)

Anway, for more convince, I just submitted more 6 testing jobs again. They'll
be done on next Monday (11-14). I'll feedback if there're any problems, or
nothing wrong.

Thanks,
Zorro

> 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;
> --
> 2.24.4
>