[RFC PATCH 1/2] ext4: warn on delalloc md block allocation without reservation

[Brian Foster <bfoster@redhat.com>]

From: Brian Foster <bfoster@redhat.com>

If we hit a condition where we have allocated metadata blocks that
were not appropriately reserved, we risk underflow of
ei->i_reserved_meta_blocks. In turn, this can throw
sbi->s_dirtyclusters_counter significantly out of whack and undermine
the nondelalloc fallback logic in ext4_nonda_switch(). Warn if this
occurs and fix up the counter to handle only what we've reserved.
This condition is reproduced by xfstests 270 against ext2 with
delalloc enabled.

Signed-off-by: Brian Foster <bfoster@redhat.com>
---
 fs/ext4/inode.c |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index c77b0bd..b58845c 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -279,6 +279,15 @@ void ext4_da_update_reserve_space(struct inode *inode,
 		used = ei->i_reserved_data_blocks;
 	}
 
+	if (unlikely(ei->i_allocated_meta_blocks > ei->i_reserved_meta_blocks)) {
+		ext4_msg(inode->i_sb, KERN_NOTICE, "%s: ino %lu, allocated %d "
+			 "with only %d reserved metadata blocks\n", __func__,
+			 inode->i_ino, ei->i_allocated_meta_blocks,
+			 ei->i_reserved_meta_blocks);
+		WARN_ON(1);
+		ei->i_allocated_meta_blocks = ei->i_reserved_meta_blocks;
+	}
+
 	/* Update per-inode reservations */
 	ei->i_reserved_data_blocks -= used;
 	ei->i_reserved_meta_blocks -= ei->i_allocated_meta_blocks;
-- 
1.7.7.6