Re: [PATCH v2] xfs: fix mount hang during primary superblock recovery failure

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

On Wed, Jan 08, 2025 at 08:41:42PM -0800, Darrick J. Wong wrote:
> On Thu, Jan 09, 2025 at 10:13:20AM +0800, Long Li wrote:
> > When mounting an image containing a log with sb modifications that require
> > log replay, the mount process hang all the time and stack as follows:
> > 
> >   [root@localhost ~]# cat /proc/557/stack
> >   [<0>] xfs_buftarg_wait+0x31/0x70
> >   [<0>] xfs_buftarg_drain+0x54/0x350
> >   [<0>] xfs_mountfs+0x66e/0xe80
> >   [<0>] xfs_fs_fill_super+0x7f1/0xec0
> >   [<0>] get_tree_bdev_flags+0x186/0x280
> >   [<0>] get_tree_bdev+0x18/0x30
> >   [<0>] xfs_fs_get_tree+0x1d/0x30
> >   [<0>] vfs_get_tree+0x2d/0x110
> >   [<0>] path_mount+0xb59/0xfc0
> >   [<0>] do_mount+0x92/0xc0
> >   [<0>] __x64_sys_mount+0xc2/0x160
> >   [<0>] x64_sys_call+0x2de4/0x45c0
> >   [<0>] do_syscall_64+0xa7/0x240
> >   [<0>] entry_SYSCALL_64_after_hwframe+0x76/0x7e
> > 
> > During log recovery, while updating the in-memory superblock from the
> > primary SB buffer, if an error is encountered, such as superblock
> > corruption occurs or some other reasons, we will proceed to out_release
> > and release the xfs_buf. However, this is insufficient because the
> > xfs_buf's log item has already been initialized and the xfs_buf is held
> > by the buffer log item as follows, the xfs_buf will not be released,
> > causing the mount thread to hang.
> 
> Can you post a regression test for us, pretty please? :)
> 

I performed regression testing by mounting specific images that can be
obtained through fault injection on kernels without metadir feature support.
I can provide it if anyone needs it.The image is big and inconvenient to send
in the mail. The detailed steps are as follows:

1) Kernel Build
  - The latest realtime AG update bug [1] remains unfixed
  - Build kernel without CONFIG_XFS_RT
  
2) Mount XFS Image (superblock needs replay, incompatible with metadir and
   no realtime subvolume)

3) Mount Result Verification
  - Without the current patch: mount thread hangs indefinitely
  - With the current patch: mount thread does not hang, but XFS is shut down

The xfstests already have the fault injection test, and this test requires
mounting specific images on specifically-compiled kernels, making it impractical
to incorporate into xfstests.

> >   xlog_recover_do_primary_sb_buffer
> >     xlog_recover_do_reg_buffer
> >       xlog_recover_validate_buf_type
> >         xfs_buf_item_init(bp, mp)
> > 
> > The solution is straightforward, we simply need to allow it to be
> > handled by the normal buffer write process. The filesystem will be
> > shutdown before the submission of buffer_list in xlog_do_recovery_pass(),
> > ensuring the correct release of the xfs_buf as follows:
> > 
> >   xlog_do_recovery_pass
> >     error = xlog_recover_process
> >       xlog_recover_process_data
> >         xlog_recover_process_ophdr
> >           xlog_recovery_process_trans
> >             ...
> >               xlog_recover_buf_commit_pass2
> >                 error = xlog_recover_do_primary_sb_buffer
> >                   //Encounter error and return
> >                 if (error)
> >                   goto out_writebuf
> >                 ...
> >               out_writebuf:
> >                 xfs_buf_delwri_queue(bp, buffer_list) //add bp to list
> >                 return  error
> >             ...
> >     if (!list_empty(&buffer_list))
> >       if (error)
> >         xlog_force_shutdown(log, SHUTDOWN_LOG_IO_ERROR); //shutdown first
> >       xfs_buf_delwri_submit(&buffer_list); //write buffer in list
> >         __xfs_buf_submit
> >           if (bp->b_mount->m_log && xlog_is_shutdown(bp->b_mount->m_log))
> >             xfs_buf_ioend_fail(bp)  //release bp correctly
> > 
> 
> Please add:
> Cc: <stable@vger.kernel.org> # v6.12
> 
> > Fixes: 6a18765b54e2 ("xfs: update the file system geometry after recoverying superblock buffers")
> > Signed-off-by: Long Li <leo.lilong@huawei.com>
> > ---
> > v1->v2: Add code comments and add the fixed stack description to the 
> >         commit message.
> >  fs/xfs/xfs_buf_item_recover.c | 11 ++++++++++-
> >  1 file changed, 10 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/xfs/xfs_buf_item_recover.c b/fs/xfs/xfs_buf_item_recover.c
> > index 3d0c6402cb36..04122bbdd5f3 100644
> > --- a/fs/xfs/xfs_buf_item_recover.c
> > +++ b/fs/xfs/xfs_buf_item_recover.c
> > @@ -1079,7 +1079,7 @@ xlog_recover_buf_commit_pass2(
> >  		error = xlog_recover_do_primary_sb_buffer(mp, item, bp, buf_f,
> >  				current_lsn);
> >  		if (error)
> > -			goto out_release;
> > +			goto out_writebuf;
> >  
> >  		/* Update the rt superblock if we have one. */
> >  		if (xfs_has_rtsb(mp) && mp->m_rtsb_bp) {
> > @@ -1096,6 +1096,15 @@ xlog_recover_buf_commit_pass2(
> >  		xlog_recover_do_reg_buffer(mp, item, bp, buf_f, current_lsn);
> >  	}
> >  
> > +	/*
> > +	 * Buffer held by buf log item during 'normal' buffer recovery must
> > +	 * be committed through buffer I/O submission path to ensure proper
> > +	 * release. When error occurs during do sb buffer recovery, log
> 
> "...during sb buffer recovery..."
> 
> and with those two things amended,
> Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
> 
> --D

Ok, thanks for your review.

Long Li.