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
^^^^^^^^^^^^^^
Hold on - pag->pagf_freeblks != agf->freeblks, and if we start with
the agf freeblocks:
available = 18277 + 6 - 18276 - 6 - 1 = 0
IOWs, the allocation should never selected this AG in the first
place.
So why is pag->pagf_freeblks not equal to agf->freeblks when this
allocation was first checked? It's clearly not because the AGFL is
unpopulated - both the perag and the agf indicate it has the minimum
6 blocks already allocated....
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
