[PATCH, RFC] 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.

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

diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
index 1d04189..63519fc 100644
--- a/fs/ext4/balloc.c
+++ b/fs/ext4/balloc.c
@@ -605,6 +605,17 @@ int ext4_claim_free_blocks(struct ext4_sb_info *sbi,
  */
 int ext4_should_retry_alloc(struct super_block *sb, int *retries)
 {
+	/* try a sync to flush delalloc space & free resvd metadata */
+	if (test_opt(sb, DELALLOC) &&
+	    *retries == 0 &&
+	    !ext4_has_free_blocks(EXT4_SB(sb), 1)) {
+		down_read(&sb->s_umount);
+		sync_inodes_sb(sb);
+		up_read(&sb->s_umount);
+		(*retries)++;
+		return 1;
+	}
+
 	if (!ext4_has_free_blocks(EXT4_SB(sb), 1) ||
 	    (*retries)++ > 3 ||
 	    !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;
 }