xfs: use a separate frextents counter for rt extent reservations

As mentioned in the previous commit, the kernel misuses sb_frextents in
the incore mount to reflect both incore reservations made by running
transactions as well as the actual count of free rt extents on disk.
This results in the superblock being written to the log with an
underestimate of the number of rt extents that are marked free in the
rtbitmap.

Teaching XFS to recompute frextents after log recovery avoids
operational problems in the current mount, but it doesn't solve the
problem of us writing undercounted frextents which are then recovered by
an older kernel that doesn't have that fix.

Create an incore percpu counter to mirror the ondisk frextents.  This
new counter will track transaction reservations and the only time we
will touch the incore super counter (i.e the one that gets logged) is
when those transactions commit updates to the rt bitmap.  This is in
contrast to the lazysbcount counters (e.g. fdblocks), where we know that
log recovery will always fix any incorrect counter that we log.
As a bonus, we only take m_sb_lock at transaction commit time.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

1 files changed, 16 insertions, 3 deletions

diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
index f6dc19de8322..a6b8efb2df52 100644
--- a/fs/xfs/xfs_mount.h
+++ b/fs/xfs/xfs_mount.h
@@ -183,6 +183,8 @@ typedef struct xfs_mount {
 	struct percpu_counter	m_icount;	/* allocated inodes counter */
 	struct percpu_counter	m_ifree;	/* free inodes counter */
 	struct percpu_counter	m_fdblocks;	/* free block counter */
+	struct percpu_counter	m_frextents;	/* free rt extent counter */
+
 	/*
 	 * Count of data device blocks reserved for delayed allocations,
 	 * including indlen blocks.  Does not include allocated CoW staging
@@ -494,9 +496,20 @@ xfs_fdblocks_unavailable(
 	return mp->m_alloc_set_aside + atomic64_read(&mp->m_allocbt_blks);
 }
 
-extern int	xfs_mod_fdblocks(struct xfs_mount *mp, int64_t delta,
-				 bool reserved);
-extern int	xfs_mod_frextents(struct xfs_mount *mp, int64_t delta);
+int xfs_mod_freecounter(struct xfs_mount *mp, struct percpu_counter *counter,
+		int64_t delta, bool rsvd);
+
+static inline int
+xfs_mod_fdblocks(struct xfs_mount *mp, int64_t delta, bool reserved)
+{
+	return xfs_mod_freecounter(mp, &mp->m_fdblocks, delta, reserved);
+}
+
+static inline int
+xfs_mod_frextents(struct xfs_mount *mp, int64_t delta)
+{
+	return xfs_mod_freecounter(mp, &mp->m_frextents, delta, false);
+}
 
 extern int	xfs_readsb(xfs_mount_t *, int);
 extern void	xfs_freesb(xfs_mount_t *);