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

[Zorro Lang]

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@xxxxxxxxxx
> [2] https://lore.kernel.org/r/20220105071052.GD20464@xxxxxxxxxxxxxxxxxx
> [3] https://lore.kernel.org/linux-xfs/Y2RevDyoeJZSpiat@B-P7TQMD6M-0146.local/2-dmesg.log.xz
> Reported-by: Zirong Lang <zlang@xxxxxxxxxx>
> Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>
> ---

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@xxxxxxxxx/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
>