On 09 Dec 2021 at 04:45, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@xxxxxxxxxx>
>
> 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.
>
With Dave's review comments addressed,
Reviewed-by: Chandan Babu R <chandan.babu@xxxxxxxxxx>
> Fixes: 174edb0e46e5 ("xfs: store in-progress CoW allocations in the refcount btree")
> Signed-off-by: Darrick J. Wong <djwong@xxxxxxxxxx>
> ---
> 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;
> }
>
> +
> /*
> * The mount has failed. Cancel the recovery if it hasn't completed and destroy
> * the log.
> diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
> index 359109b6f0d3..bed73e8002a5 100644
> --- a/fs/xfs/xfs_mount.c
> +++ b/fs/xfs/xfs_mount.c
> @@ -936,15 +936,6 @@ xfs_mountfs(
> xfs_warn(mp,
> "Unable to allocate reserve blocks. Continuing without reserve pool.");
>
> - /* Recover any CoW blocks that never got remapped. */
> - 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);
> - goto out_quota;
> - }
> -
> /* Reserve AG blocks for future btree expansion. */
> error = xfs_fs_reserve_ag_blocks(mp);
> if (error && error != -ENOSPC)
> @@ -955,7 +946,6 @@ xfs_mountfs(
>
> out_agresv:
> xfs_fs_unreserve_ag_blocks(mp);
> - out_quota:
> xfs_qm_unmount_quotas(mp);
> out_rtunmount:
> xfs_rtunmount_inodes(mp);
> diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
> index cb0edb1d68ef..8b6c7163f684 100644
> --- a/fs/xfs/xfs_reflink.c
> +++ b/fs/xfs/xfs_reflink.c
> @@ -749,7 +749,10 @@ xfs_reflink_end_cow(
> }
>
> /*
> - * Free leftover CoW reservations that didn't get cleaned out.
> + * Free all CoW staging blocks that are still referenced by the ondisk refcount
> + * metadata. The ondisk metadata does not track which inode created the
> + * staging extent, so callers must ensure that there are no cached inodes with
> + * live CoW staging extents.
> */
> int
> xfs_reflink_recover_cow(
> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> index 778b57b1f020..c7ac486ca5d3 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -1739,15 +1739,6 @@ xfs_remount_rw(
> */
> xfs_restore_resvblks(mp);
> xfs_log_work_queue(mp);
> -
> - /* Recover any CoW blocks that never got remapped. */
> - 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;
> - }
> xfs_blockgc_start(mp);
>
> /* Create the per-AG metadata reservation pool .*/
--
chandan
