Re: [PATCH v2] xfs: ensure submit buffers on LSN boundaries in error handlers

[Long Li <leo.lilong@huawei.com>]

On Wed, Feb 14, 2024 at 12:23:40PM +0530, Chandan Babu R wrote:
> On Wed, Jan 17, 2024 at 08:31:26 PM +0800, Long Li wrote:
> > While performing the IO fault injection test, I caught the following data
> > corruption report:
> >
> >  XFS (dm-0): Internal error ltbno + ltlen > bno at line 1957 of file fs/xfs/libxfs/xfs_alloc.c.  Caller xfs_free_ag_extent+0x79c/0x1130
> >  CPU: 3 PID: 33 Comm: kworker/3:0 Not tainted 6.5.0-rc7-next-20230825-00001-g7f8666926889 #214
> >  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20190727_073836-buildvm-ppc64le-16.ppc.fedoraproject.org-3.fc31 04/01/2014
> >  Workqueue: xfs-inodegc/dm-0 xfs_inodegc_worker
> >  Call Trace:
> >   <TASK>
> >   dump_stack_lvl+0x50/0x70
> >   xfs_corruption_error+0x134/0x150
> >   xfs_free_ag_extent+0x7d3/0x1130
> >   __xfs_free_extent+0x201/0x3c0
> >   xfs_trans_free_extent+0x29b/0xa10
> >   xfs_extent_free_finish_item+0x2a/0xb0
> >   xfs_defer_finish_noroll+0x8d1/0x1b40
> >   xfs_defer_finish+0x21/0x200
> >   xfs_itruncate_extents_flags+0x1cb/0x650
> >   xfs_free_eofblocks+0x18f/0x250
> >   xfs_inactive+0x485/0x570
> >   xfs_inodegc_worker+0x207/0x530
> >   process_scheduled_works+0x24a/0xe10
> >   worker_thread+0x5ac/0xc60
> >   kthread+0x2cd/0x3c0
> >   ret_from_fork+0x4a/0x80
> >   ret_from_fork_asm+0x11/0x20
> >   </TASK>
> >  XFS (dm-0): Corruption detected. Unmount and run xfs_repair
> >
> > After analyzing the disk image, it was found that the corruption was
> > triggered by the fact that extent was recorded in both inode datafork
> > and AGF btree blocks. After a long time of reproduction and analysis,
> > we found that the reason of free sapce btree corruption was that the
> > AGF btree was not recovered correctly.
> >
> > Consider the following situation, Checkpoint A and Checkpoint B are in
> > the same record and share the same start LSN1, buf items of same object
> > (AGF btree block) is included in both Checkpoint A and Checkpoint B. If
> > the buf item in Checkpoint A has been recovered and updates metadata LSN
> > permanently, then the buf item in Checkpoint B cannot be recovered,
> > because log recovery skips items with a metadata LSN >= the current LSN
> > of the recovery item. If there is still an inode item in Checkpoint B
> > that records the Extent X, the Extent X will be recorded in both inode
> > datafork and AGF btree block after Checkpoint B is recovered. Such
> > transaction can be seen when allocing enxtent for inode bmap, it record
> > both the addition of extent to the inode extent list and the removing
> > extent from the AGF.
> >
> >   |------------Record (LSN1)------------------|---Record (LSN2)---|
> >   |-------Checkpoint A----------|----------Checkpoint B-----------|
> >   |     Buf Item(Extent X)      | Buf Item / Inode item(Extent X) |
> >   |     Extent X is freed       |     Extent X is allocated       |
> >
> > After commit 12818d24db8a ("xfs: rework log recovery to submit buffers
> > on LSN boundaries") was introduced, we submit buffers on lsn boundaries
> > during log recovery. The above problem can be avoided under normal paths,
> > but it's not guaranteed under abnormal paths. Consider the following
> > process, if an error was encountered after recover buf item in Checkpoint
> > A and before recover buf item in Checkpoint B, buffers that have been
> > added to the buffer_list will still be submitted, this violates the
> > submits rule on lsn boundaries. So buf item in Checkpoint B cannot be
> > recovered on the next mount due to current lsn of transaction equal to
> > metadata lsn on disk. The detailed process of the problem is as follows.
> >
> > First Mount:
> >
> >   xlog_do_recovery_pass
> >     error = xlog_recover_process
> >       xlog_recover_process_data
> >         xlog_recover_process_ophdr
> >           xlog_recovery_process_trans
> >             ...
> >               /* recover buf item in Checkpoint A */
> >               xlog_recover_buf_commit_pass2
> >                 xlog_recover_do_reg_buffer
> >                 /* add buffer of agf btree block to buffer_list */
> >                 xfs_buf_delwri_queue(bp, buffer_list)
> >             ...
> >             ==> Encounter read IO error and return
> >     /* submit buffers regardless of error */
> >     if (!list_empty(&buffer_list))
> >       xfs_buf_delwri_submit(&buffer_list);
> >
> >     <buf items of agf btree block in Checkpoint A recovery success>
> >
> > Second Mount:
> >
> >   xlog_do_recovery_pass
> >     error = xlog_recover_process
> >       xlog_recover_process_data
> >         xlog_recover_process_ophdr
> >           xlog_recovery_process_trans
> >             ...
> >               /* recover buf item in Checkpoint B */
> >               xlog_recover_buf_commit_pass2
> >                 /* buffer of agf btree block wouldn't added to
> >                    buffer_list due to lsn equal to current_lsn */
> >                 if (XFS_LSN_CMP(lsn, current_lsn) >= 0)
> >                   goto out_release
> >
> >     <buf items of agf btree block in Checkpoint B wouldn't recovery>
> >
> > In order to make sure that submits buffers on lsn boundaries in the
> > abnormal paths, we need to check error status before submit buffers that
> > have been added from the last record processed. If error status exist,
> > buffers in the bufffer_list should not be writen to disk.
> >
> > Canceling the buffers in the buffer_list directly isn't correct, unlike
> > any other place where write list was canceled, these buffers has been
> > initialized by xfs_buf_item_init() during recovery and held by buf item,
> > buf items will not be released in xfs_buf_delwri_cancel(), it's not easy
> > to solve.
> >
> > If the filesystem has been shut down, then delwri list submission will
> > error out all buffers on the list via IO submission/completion and do
> > all the correct cleanup automatically. So shutting down the filesystem
> > could prevents buffers in the bufffer_list from being written to disk.
> >
> 
> Hi,
> 
> The patch did not apply cleanly on the collection of XFS patches due for
> Linux-v6.9. This was due to a trivial conflict with one of the patches that
> was included earlier in the collection.
> 
> Can you please review your patch at
> https://git.kernel.org/pub/scm/fs/xfs/xfs-linux.git/commit/?h=for-next&id=9ee85f235efeebd5146c4436fb255d636e8063d6
> and let me know if you have any disagreements.

It looks ok for me. 

Thanks,
Long Li