[PATCH, RFC V2] ext4: flush delalloc blocks when space is low

[Eric Sandeen <sandeen@redhat.com>]

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 was not
needed.

I've added 2 flushers here:

 * when free space is low compared to dirty blocks, do an async flush
 * when we get a hard ENOSPC, do a sync flush before retry

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

V2: don't try to sync if we're still in a (probably nested) transaction.

Thanks to Josef for pointing out that possibility.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
---

diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
index 1d04189..28bde58 100644
--- a/fs/ext4/balloc.c
+++ b/fs/ext4/balloc.c
@@ -605,11 +605,27 @@ int ext4_claim_free_blocks(struct ext4_sb_info *sbi,
  */
 int ext4_should_retry_alloc(struct super_block *sb, int *retries)
 {
-	if (!ext4_has_free_blocks(EXT4_SB(sb), 1) ||
+	s64 dirtyblocks = 0;
+	struct percpu_counter *dbc = &EXT4_SB(sb)->s_dirtyblocks_counter;
+
+	if (test_opt(sb, DELALLOC))
+		dirtyblocks = percpu_counter_read_positive(dbc);
+
+	if ((!ext4_has_free_blocks(EXT4_SB(sb), 1) && !dirtyblocks) ||
 	    (*retries)++ > 3 ||
 	    !EXT4_SB(sb)->s_journal)
 		return 0;
 
+	/* try a sync to flush delalloc space & free resvd metadata */
+	if (!ext4_has_free_blocks(EXT4_SB(sb), 1) && dirtyblocks) {
+		if (!ext4_journal_current_handle()) {
+			down_read(&sb->s_umount);
+			sync_inodes_sb(sb);
+			up_read(&sb->s_umount);
+			return 1;
+		}
+	}
+
 	jbd_debug(1, "%s: retrying operation after ENOSPC\n", sb->s_id);
 
 	return jbd2_journal_force_commit_nested(EXT4_SB(sb)->s_journal);
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 5c5bc5d..27c8b9b 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3024,11 +3024,18 @@ 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(sb);
+
 	return 0;
 }