linux-kernel - Re: [PATCH v2] xfs: extend the freelist before available space check

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Message-ID: <Y2PIZ6AFJoSk+9SQ@B-P7TQMD6M-0146.local>
Date:   Thu, 3 Nov 2022 21:55:51 +0800
From:   Gao Xiang <hsiangkao@...ux.alibaba.com>
To:     linux-xfs@...r.kernel.org, "Darrick J. Wong" <djwong@...nel.org>,
        Dave Chinner <dchinner@...hat.com>,
        Brian Foster <bfoster@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, Zirong Lang <zlang@...hat.com>
Subject: Re: [PATCH v2] xfs: extend the freelist before available space check

Hi,

On Thu, Nov 03, 2022 at 09:10:25PM +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
> reproducable on RHEL 9.
> 
> Thanks to him, after dumping the transaction log items, I think
> the root cause is as below:
>  1. Allocate space 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;
> 
>  2. At that time, the AG state is
>     AGF Buffer: (XAGF)
>         ver:1  seq#:3  len:2621424
>         root BNO:9  CNT:7
>         level BNO:2  CNT:2
>         1st:64  last:69  cnt:6  freeblks:18277  longest:6395
> 
>     agfl (flfirst = 64, fllast = 69, flcount = 6)
>     64:547 65:167 66:1651 67:2040807 68:783 69:604
> 
>  3. Then, cntbt needs a new btree block (so take one block
>     from agfl), and then the log records a new AGF:
>     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@...r.Ynbf
>     agf 3  flfirst: 65 (0x41) fllast: 69 (0x45) cnt: 5
>     freeblks 18277
> 
>  4. agfl 64 (daddr 62918552) was then written as a cntbt block
>     log item:
>       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@...r.Ynbf
> 
>  5. Finally, the following inode extent to btree allocation fails:
>     Nov  1 07:56:09 dell-per750-08 kernel: ------------[ cut here ]------------
>     Nov  1 07:56:09 dell-per750-08 kernel: WARNING: CPU: 15 PID: 49290 at fs/xfs/libxfs/xfs_bmap.c:717 xfs_bmap_extents_to_btree+0xc51/0x1050 [xfs]
>     ...
>     Nov  1 07:56:10 dell-per750-08 kernel: XFS (sda2): agno 3 agflcount 5 freeblks 18277 reservation 18276 6
> 
>     since
> 
>     available = freeblks + agflcount - reservation - need - minleft
>               = 18277 + min(5, 6) - 18276 - 6 - 0 = 0   < 1
>     kaboom.
>

Perhaps it's still not a correct fix since the second conversion
allocation will fail as:

      available = freeblks + agflcount - reservation - need - minleft
                = 18276 + min(6, 6) - 18276 - 6 - 0 = 0   < 1

If we don't want to use the last blocks of the AG, we should shorten
args->maxlen to avoid touch these agfl blocks, thoughts?

2300 static bool
2301 xfs_alloc_space_available(
2302         struct xfs_alloc_arg    *args,
2303         xfs_extlen_t            min_free,
2304         int                     flags)
2305 {

...

2329         available = (int)(pag->pagf_freeblks + agflcount -
2330                           reservation - min_free - args->minleft);

             ^ here available = 27

2331         if (available < (int)max(args->total, alloc_len))
2332                 return false;
2333
2334         /*
2335          * Clamp maxlen to the amount of free space available for the actual
2336          * extent allocation.
2337          */
2338         if (available < (int)args->maxlen && !(flags & XFS_ALLOC_FLAG_CHECK)) {
2339                 args->maxlen = available;

             ^ so args->maxlen = 27 here

and then freeblks from 18304 - 27 = 18277, but with another agfl block
allocated (pagf_flcount from 6 to 5), the inequality will not satisfy:

     available = freeblks + agflcount - reservation - need - minleft
               = 18277 + min(5, 6) - 18276 - 6 - 0 = 0   < 1

I think one of the correct fix is to fix args->maxlen above though, or
some better preferred idea to fix this?

Thanks,
Gao Xiang