xfs: drop freeze protection when running GETFSMAP

A recent log refactoring patchset from Brian Foster relaxed fsfreeze
behavior with regards to the buffer cache -- now freeze only waits for
pending buffer IO to finish, and does not try to drain the buffer cache
LRU.  As a result, fsfreeze should no longer stall indefinitely while
fsmap runs.  Drop the sb_start_write calls around fsmap invocations.

While we're cleaning things, add a comment to the xfs_trans_alloc_empty
call explaining why we're running around with empty transactions.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

diff --git a/fs/xfs/xfs_fsmap.c b/fs/xfs/xfs_fsmap.c
index 9ce5e7d..34f2b97 100644 (file)
--- a/fs/xfs/xfs_fsmap.c
+++ b/fs/xfs/xfs_fsmap.c
@@ -904,14 +904,6 @@ xfs_getfsmap(
        info.fsmap_recs = fsmap_recs;
        info.head = head;
 
-       /*
-        * If fsmap runs concurrently with a scrub, the freeze can be delayed
-        * indefinitely as we walk the rmapbt and iterate over metadata
-        * buffers.  Freeze quiesces the log (which waits for the buffer LRU to
-        * be emptied) and that won't happen while we're reading buffers.
-        */
-       sb_start_write(mp->m_super);
-
        /* For each device we support... */
        for (i = 0; i < XFS_GETFSMAP_DEVS; i++) {
                /* Is this device within the range the user asked for? */
@@ -934,6 +926,11 @@ xfs_getfsmap(
                if (handlers[i].dev > head->fmh_keys[0].fmr_device)
                        memset(&dkeys[0], 0, sizeof(struct xfs_fsmap));
 
+               /*
+                * Grab an empty transaction so that we can use its recursive
+                * buffer locking abilities to detect cycles in the rmapbt
+                * without deadlocking.
+                */
                error = xfs_trans_alloc_empty(mp, &tp);
                if (error)
                        break;
@@ -951,7 +948,6 @@ xfs_getfsmap(
 
        if (tp)
                xfs_trans_cancel(tp);
-       sb_end_write(mp->m_super);
        head->fmh_oflags = FMH_OF_DEV_T;
        return error;
 }