Re: [PATCH 2/2] xfs: only run COW extent recovery when there are no live extents

[Dave Chinner <david@fromorbit.com>]

On Wed, Dec 08, 2021 at 03:15:16PM -0800, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> As part of multiple customer escalations due to file data corruption
> after copy on write operations, I wrote some fstests that use fsstress
> to hammer on COW to shake things loose.  Regrettably, I caught some
> filesystem shutdowns due to incorrect rmap operations with the following
> loop:
> 
> mount <filesystem>				# (0)
> fsstress <run only readonly ops> &		# (1)
> while true; do
> 	fsstress <run all ops>
> 	mount -o remount,ro			# (2)
> 	fsstress <run only readonly ops>
> 	mount -o remount,rw			# (3)
> done
> 
> When (2) happens, notice that (1) is still running.  xfs_remount_ro will
> call xfs_blockgc_stop to walk the inode cache to free all the COW
> extents, but the blockgc mechanism races with (1)'s reader threads to
> take IOLOCKs and loses, which means that it doesn't clean them all out.
> Call such a file (A).
> 
> When (3) happens, xfs_remount_rw calls xfs_reflink_recover_cow, which
> walks the ondisk refcount btree and frees any COW extent that it finds.
> This function does not check the inode cache, which means that incore
> COW forks of inode (A) is now inconsistent with the ondisk metadata.  If
> one of those former COW extents are allocated and mapped into another
> file (B) and someone triggers a COW to the stale reservation in (A), A's
> dirty data will be written into (B) and once that's done, those blocks
> will be transferred to (A)'s data fork without bumping the refcount.
> 
> The results are catastrophic -- file (B) and the refcount btree are now
> corrupt.  In the first patch, we fixed the race condition in (2) so that
> (A) will always flush the COW fork.  In this second patch, we move the
> _recover_cow call to the initial mount call in (0) for safety.
> 
> As mentioned previously, xfs_reflink_recover_cow walks the refcount
> btree looking for COW staging extents, and frees them.  This was
> intended to be run at mount time (when we know there are no live inodes)
> to clean up any leftover staging events that may have been left behind
> during an unclean shutdown.  As a time "optimization" for readonly
> mounts, we deferred this to the ro->rw transition, not realizing that
> any failure to clean all COW forks during a rw->ro transition would
> result in catastrophic corruption.
> 
> Therefore, remove this optimization and only run the recovery routine
> when we're guaranteed not to have any COW staging extents anywhere,
> which means we always run this at mount time.  While we're at it, move
> the callsite to xfs_log_mount_finish because any refcount btree
> expansion (however unlikely given that we're removing records from the
> right side of the index) must be fed by a per-AG reservation, which
> doesn't exist in its current location.
> 
> Fixes: 174edb0e46e5 ("xfs: store in-progress CoW allocations in the refcount btree")
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> ---
>  fs/xfs/xfs_log.c     |   23 ++++++++++++++++++++++-
>  fs/xfs/xfs_mount.c   |   10 ----------
>  fs/xfs/xfs_reflink.c |    5 ++++-
>  fs/xfs/xfs_super.c   |    9 ---------
>  4 files changed, 26 insertions(+), 21 deletions(-)
> 
> 
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index 89fec9a18c34..c17344fc1275 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -10,6 +10,7 @@
>  #include "xfs_log_format.h"
>  #include "xfs_trans_resv.h"
>  #include "xfs_mount.h"
> +#include "xfs_inode.h"
>  #include "xfs_errortag.h"
>  #include "xfs_error.h"
>  #include "xfs_trans.h"
> @@ -20,6 +21,7 @@
>  #include "xfs_sysfs.h"
>  #include "xfs_sb.h"
>  #include "xfs_health.h"
> +#include "xfs_reflink.h"
>  
>  struct kmem_cache	*xfs_log_ticket_cache;
>  
> @@ -847,9 +849,28 @@ xfs_log_mount_finish(
>  	/* Make sure the log is dead if we're returning failure. */
>  	ASSERT(!error || xlog_is_shutdown(log));
>  
> -	return error;
> +	if (error)
> +		return error;
> +
> +	/*
> +	 * Recover any CoW staging blocks that are still referenced by the
> +	 * ondisk refcount metadata.  During mount there cannot be any live
> +	 * staging extents as we have not permitted any user modifications.
> +	 * Therefore, it is safe to free them all right now, even on a
> +	 * read-only mount.
> +	 */
> +	error = xfs_reflink_recover_cow(mp);
> +	if (error) {
> +		xfs_err(mp, "Error %d recovering leftover CoW allocations.",
> +				error);
> +		xfs_force_shutdown(mp, SHUTDOWN_CORRUPT_INCORE);
> +		return error;
> +	}
> +
> +	return 0;
>  }

THis is after we've emitted an "Ending recovery ...." log message.
I kinda expected you to move this up to just after the
evict_inodes() call before we force the log, push the AIL, drain
the buffers used during recovery and potentially turn the
"filesystem is read-only" flag back on.

i.e. if this is a recovery operation, it should be done before we
finish recovery....

Other than that, the change is fine.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com