On 9/16/20 7:19 AM, Brian Foster wrote: > On Tue, Sep 15, 2020 at 07:33:27AM -0400, Brian Foster wrote: >> Hi Jens, >> >> I'm seeing an occasional metadata (read) I/O error (EOPNOTSUPP) when >> running Zorro's recent io_uring enabled fsstress on XFS (fsstress -d >> <mnt> -n 99999999 -p 8). The storage is a 50GB dm-linear device on a >> virtio disk (within a KVM guest). The full callstack of the I/O >> submission path is appended below [2], acquired via inserting a >> WARN_ON() in my local tree. >> >> From tracing around a bit, it looks like what happens is that fsstress >> calls into io_uring, the latter starts a plug and sets plug.nowait = >> true (via io_submit_sqes() -> io_submit_state_start()) and eventually >> XFS needs to read an inode cluster buffer in the context of this task. >> That buffer read ultimately fails due to submit_bio_checks() setting >> REQ_NOWAIT on the bio and the following logic in the same function >> causing a BLK_STS_NOTSUPP status: >> >> if ((bio->bi_opf & REQ_NOWAIT) && !queue_is_mq(q)) >> goto not_supported; >> >> In turn, this leads to the following behavior in XFS: >> >> [ 3839.273519] XFS (dm-2): metadata I/O error in "xfs_imap_to_bp+0x116/0x2c0 [xfs]" at daddr 0x323a5a0 len 32 error 95 >> [ 3839.303283] XFS (dm-2): log I/O error -95 >> [ 3839.321437] XFS (dm-2): xfs_do_force_shutdown(0x2) called from line 1196 of file fs/xfs/xfs_log.c. Return address = ffffffffc12dea8a >> [ 3839.323554] XFS (dm-2): Log I/O Error Detected. Shutting down filesystem >> [ 3839.324773] XFS (dm-2): Please unmount the filesystem and rectify the problem(s) >> >> I suppose it's possible fsstress is making an invalid request based on >> my setup, but I find it a little strange that this state appears to leak >> into filesystem I/O requests. What's more concerning is that this also >> seems to impact an immediately subsequent log write submission, which is >> a fatal error and causes the filesystem to shutdown. >> >> Finally, note that I've seen your patch associated with Zorro's recent >> bug report [1] and that does seem to prevent the problem. I'm still >> sending this report because the connection between the plug and that >> change is not obvious to me, so I wanted to 1.) confirm this is intended >> to fix this problem and 2.) try to understand whether this plugging >> behavior introduces any constraints on the fs when invoked in io_uring >> context. Thoughts? Thanks. >> > > To expand on this a bit, I was playing more with the aforementioned fix > yesterday while waiting for this email's several hour trip to the > mailing list to complete and eventually realized that I don't think the > plug.nowait thing properly accommodates XFS' use of multiple devices. A > simple example is XFS on a data device with mq support and an external > log device without mq support. Presumably io_uring requests could thus > enter XFS with plug.nowait set to true, and then any log bio submission > that happens to occur in that context is doomed to fail and shutdown the > fs. Do we ever read from the logdev? It'll only be a concern on the read side. And even from there, you'd need nested reads from the log device. In general, the 'can async' check should be advisory, the -EAGAIN or -EOPNOTSUPP should be caught and reissued. The failure path was just related to this happening off the retry path on arming for the async buffered callback. -- Jens Axboe
