Re: [PATCH] xfs: shrink failure needs to hold AGI buffer

[Gao Xiang <hsiangkao@linux.alibaba.com>]

On 2024/3/6 09:12, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> Chandan reported a AGI/AGF lock order hang on xfs/168 during recent
> testing. The cause of the problem was the task running xfs_growfs
> to shrink the filesystem. A failure occurred trying to remove the
> free space from the btrees that the shrink would make disappear,
> and that meant it ran the error handling for a partial failure.
> 
> This error path involves restoring the per-ag block reservations,
> and that requires calculating the amount of space needed to be
> reserved for the free inode btree. The growfs operation hung here:
> 
> [18679.536829]  down+0x71/0xa0
> [18679.537657]  xfs_buf_lock+0xa4/0x290 [xfs]
> [18679.538731]  xfs_buf_find_lock+0xf7/0x4d0 [xfs]
> [18679.539920]  xfs_buf_lookup.constprop.0+0x289/0x500 [xfs]
> [18679.542628]  xfs_buf_get_map+0x2b3/0xe40 [xfs]
> [18679.547076]  xfs_buf_read_map+0xbb/0x900 [xfs]
> [18679.562616]  xfs_trans_read_buf_map+0x449/0xb10 [xfs]
> [18679.569778]  xfs_read_agi+0x1cd/0x500 [xfs]
> [18679.573126]  xfs_ialloc_read_agi+0xc2/0x5b0 [xfs]
> [18679.578708]  xfs_finobt_calc_reserves+0xe7/0x4d0 [xfs]
> [18679.582480]  xfs_ag_resv_init+0x2c5/0x490 [xfs]
> [18679.586023]  xfs_ag_shrink_space+0x736/0xd30 [xfs]
> [18679.590730]  xfs_growfs_data_private.isra.0+0x55e/0x990 [xfs]
> [18679.599764]  xfs_growfs_data+0x2f1/0x410 [xfs]
> [18679.602212]  xfs_file_ioctl+0xd1e/0x1370 [xfs]
> 
> trying to get the AGI lock. The AGI lock was held by a fstress task
> trying to do an inode allocation, and it was waiting on the AGF
> lock to allocate a new inode chunk on disk. Hence deadlock.
> 
> The fix for this is for the growfs code to hold the AGI over the
> transaction roll it does in the error path. It already holds the AGF
> locked across this, and that is what causes the lock order inversion
> in the xfs_ag_resv_init() call.
> 
> Reported-by: Chandan Babu R <chandanbabu@kernel.org>
> Fixes: 46141dc891f7 ("xfs: introduce xfs_ag_shrink_space()")
> Signed-off-by: Dave Chinner <dchinner@redhat.com>

Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>

Thanks,
Gao Xiang