[PATCH] btrfs: Fix a data space underflow warning

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

Even with quota disabled, generic/127 will trigger a kernel warning by
underflow data space info.

The bug is caused by buffered write, which in case of short copy, the
start parameter for btrfs_delalloc_release_space() is wrong, and
round_up/down() in btrfs_delalloc_release() extents the range to page
aligned, decreasing one more page than expected.

This patch will fix it by passing correct start.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
---
 fs/btrfs/file.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index dce93dc..933a723 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1607,12 +1607,17 @@ again:
 				BTRFS_I(inode)->outstanding_extents++;
 				spin_unlock(&BTRFS_I(inode)->lock);
 			}
-			if (only_release_metadata)
+			if (only_release_metadata) {
 				btrfs_delalloc_release_metadata(inode,
 								release_bytes);
-			else
-				btrfs_delalloc_release_space(inode, pos,
+			} else {
+				u64 __pos;
+
+				__pos = round_down(pos, root->sectorsize) +
+					(dirty_pages << PAGE_CACHE_SHIFT);
+				btrfs_delalloc_release_space(inode, __pos,
 							     release_bytes);
+			}
 		}
 
 		release_bytes = dirty_pages << PAGE_CACHE_SHIFT;
-- 
2.6.2