ext4: flush delalloc blocks when space is low

Creating many small files in rapid succession on a small
filesystem can lead to spurious ENOSPC; on a 104MB filesystem:

for i in `seq 1 22500`; do
    echo -n > $SCRATCH_MNT/$i
    echo XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX > $SCRATCH_MNT/$i
done

leads to ENOSPC even though after a sync, 40% of the fs is free
again.

This is because we reserve worst-case metadata for delalloc writes,
and when data is allocated that worst-case reservation is not
usually needed.

When freespace is low, kicking off an async writeback will start
converting that worst-case space usage into something more realistic,
almost always freeing up space to continue.

This resolves the testcase for me, and survives all 4 generic
ENOSPC tests in xfstests.

We'll still need a hard synchronous sync to squeeze out the last bit,
but this fixes things up to a large degree.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index ab80796..282621f 100644 (file)
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2993,11 +2993,18 @@ static int ext4_nonda_switch(struct super_block *sb)
        if (2 * free_blocks < 3 * dirty_blocks ||
                free_blocks < (dirty_blocks + EXT4_FREEBLOCKS_WATERMARK)) {
                /*
-                * free block count is less that 150% of dirty blocks
-                * or free blocks is less that watermark
+                * free block count is less than 150% of dirty blocks
+                * or free blocks is less than watermark
                 */
                return 1;
        }
+       /*
+        * Even if we don't switch but are nearing capacity,
+        * start pushing delalloc when 1/2 of free blocks are dirty.
+        */
+       if (free_blocks < 2 * dirty_blocks)
+               writeback_inodes_sb_if_idle(sb);
+
        return 0;
 }