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, 21 insertions, 29 deletions

diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
index 53e130f803b1..0c0bcbd4949d 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -1110,24 +1110,33 @@ xfs_fs_writable(
 	return true;
 }
 
+/* Adjust m_fdblocks or m_frextents. */
 int
-xfs_mod_fdblocks(
+xfs_mod_freecounter(
 	struct xfs_mount	*mp,
+	struct percpu_counter	*counter,
 	int64_t			delta,
 	bool			rsvd)
 {
 	int64_t			lcounter;
 	long long		res_used;
+	uint64_t		set_aside = 0;
 	s32			batch;
-	uint64_t		set_aside;
+	bool			has_resv_pool;
+
+	ASSERT(counter == &mp->m_fdblocks || counter == &mp->m_frextents);
+	has_resv_pool = (counter == &mp->m_fdblocks);
+	if (rsvd)
+		ASSERT(has_resv_pool);
 
 	if (delta > 0) {
 		/*
 		 * If the reserve pool is depleted, put blocks back into it
 		 * first. Most of the time the pool is full.
 		 */
-		if (likely(mp->m_resblks == mp->m_resblks_avail)) {
-			percpu_counter_add(&mp->m_fdblocks, delta);
+		if (likely(!has_resv_pool ||
+			   mp->m_resblks == mp->m_resblks_avail)) {
+			percpu_counter_add(counter, delta);
 			return 0;
 		}
 
@@ -1139,7 +1148,7 @@ xfs_mod_fdblocks(
 		} else {
 			delta -= res_used;
 			mp->m_resblks_avail = mp->m_resblks;
-			percpu_counter_add(&mp->m_fdblocks, delta);
+			percpu_counter_add(counter, delta);
 		}
 		spin_unlock(&mp->m_sb_lock);
 		return 0;
@@ -1153,7 +1162,7 @@ xfs_mod_fdblocks(
 	 * then make everything serialise as we are real close to
 	 * ENOSPC.
 	 */
-	if (__percpu_counter_compare(&mp->m_fdblocks, 2 * XFS_FDBLOCKS_BATCH,
+	if (__percpu_counter_compare(counter, 2 * XFS_FDBLOCKS_BATCH,
 				     XFS_FDBLOCKS_BATCH) < 0)
 		batch = 1;
 	else
@@ -1170,9 +1179,10 @@ xfs_mod_fdblocks(
 	 * problems (i.e. transaction abort, pagecache discards, etc.) than
 	 * slightly premature -ENOSPC.
 	 */
-	set_aside = xfs_fdblocks_unavailable(mp);
-	percpu_counter_add_batch(&mp->m_fdblocks, delta, batch);
-	if (__percpu_counter_compare(&mp->m_fdblocks, set_aside,
+	if (has_resv_pool)
+		set_aside = xfs_fdblocks_unavailable(mp);
+	percpu_counter_add_batch(counter, delta, batch);
+	if (__percpu_counter_compare(counter, set_aside,
 				     XFS_FDBLOCKS_BATCH) >= 0) {
 		/* we had space! */
 		return 0;
@@ -1183,8 +1193,8 @@ xfs_mod_fdblocks(
 	 * that took us to ENOSPC.
 	 */
 	spin_lock(&mp->m_sb_lock);
-	percpu_counter_add(&mp->m_fdblocks, -delta);
-	if (!rsvd)
+	percpu_counter_add(counter, -delta);
+	if (!has_resv_pool || !rsvd)
 		goto fdblocks_enospc;
 
 	lcounter = (long long)mp->m_resblks_avail + delta;
@@ -1201,24 +1211,6 @@ fdblocks_enospc:
 	return -ENOSPC;
 }
 
-int
-xfs_mod_frextents(
-	struct xfs_mount	*mp,
-	int64_t			delta)
-{
-	int64_t			lcounter;
-	int			ret = 0;
-
-	spin_lock(&mp->m_sb_lock);
-	lcounter = mp->m_sb.sb_frextents + delta;
-	if (lcounter < 0)
-		ret = -ENOSPC;
-	else
-		mp->m_sb.sb_frextents = lcounter;
-	spin_unlock(&mp->m_sb_lock);
-	return ret;
-}
-
 /*
  * Used to free the superblock along various error paths.
  */