[PATCH v2] btrfs: Fix error handling in btrfs_cleanup_ordered_extents

[Nikolay Borisov <nborisov@suse.com>]

Running btrfs/124 in a loop hung up on me sporadically with the
following call trace:
	btrfs           D    0  5760   5324 0x00000000
	Call Trace:
	 ? __schedule+0x243/0x800
	 schedule+0x33/0x90
	 btrfs_start_ordered_extent+0x10c/0x1b0 [btrfs]
	 ? wait_woken+0xa0/0xa0
	 btrfs_wait_ordered_range+0xbb/0x100 [btrfs]
	 btrfs_relocate_block_group+0x1ff/0x230 [btrfs]
	 btrfs_relocate_chunk+0x49/0x100 [btrfs]
	 btrfs_balance+0xbeb/0x1740 [btrfs]
	 btrfs_ioctl_balance+0x2ee/0x380 [btrfs]
	 btrfs_ioctl+0x1691/0x3110 [btrfs]
	 ? lockdep_hardirqs_on+0xed/0x180
	 ? __handle_mm_fault+0x8e7/0xfb0
	 ? _raw_spin_unlock+0x24/0x30
	 ? __handle_mm_fault+0x8e7/0xfb0
	 ? do_vfs_ioctl+0xa5/0x6e0
	 ? btrfs_ioctl_get_supported_features+0x30/0x30 [btrfs]
	 do_vfs_ioctl+0xa5/0x6e0
	 ? entry_SYSCALL_64_after_hwframe+0x3e/0xbe
	 ksys_ioctl+0x3a/0x70
	 __x64_sys_ioctl+0x16/0x20
	 do_syscall_64+0x60/0x1b0
	 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Turns out during page writeback it's possible that the code in
writepage_delalloc can instantiate a delalloc range which doesn't
belong to the page currently being written back. This happens since
find_lock_delalloc_range returns up to BTRFS_MAX_EXTENT_SIZE delalloc
range when asked and doens't really consider the range of the passed
page. When such a foregin range is found the code proceeds to
run_delalloc_range and calls the appropriate function to fill the
delalloc and create ordered extents. If, however, a failure occurs
while this operation is in effect then the clean up code in
btrfs_cleanup_ordered_extents will be called. This function has the
wrong assumption that caller of run_delalloc_range always properly
cleans the first page of the range hence when it calls
__endio_write_update_ordered it explicitly ommits the first page of
the delalloc range. This is wrong because this function could be
cleaning a delalloc range that doesn't belong to the current page. This
in turn means that the page cleanup code in __extent_writepage will
not really free the initial page for the range, leaving a hanging
ordered extent with bytes_left set to 4k. This bug has been present
ever since the original introduction of the cleanup code in 524272607e88.

Fix this by correctly checking whether the current page belongs to the
range being instantiated and if so correctly adjust the range parameters
passed for cleaning up. If it doesn't, then just clean the whole OE
range directly.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Fixes: 524272607e88 ("btrfs: Handle delalloc error correctly to avoid ordered extent hang")
---

V2: 
 * Fix compilation failure due to missing parentheses
 * Fixed the "Fixes" tag. 

 fs/btrfs/inode.c | 29 ++++++++++++++++++++---------
 1 file changed, 20 insertions(+), 9 deletions(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index e1f00d8c24ce..5906564ae2e9 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -109,17 +109,17 @@ static void __endio_write_update_ordered(struct inode *inode,
  * extent_clear_unlock_delalloc() to clear both the bits EXTENT_DO_ACCOUNTING
  * and EXTENT_DELALLOC simultaneously, because that causes the reserved metadata
  * to be released, which we want to happen only when finishing the ordered
- * extent (btrfs_finish_ordered_io()). Also note that the caller of the
- * fill_delalloc() callback already does proper cleanup for the first page of
- * the range, that is, it invokes the callback writepage_end_io_hook() for the
- * range of the first page.
+ * extent (btrfs_finish_ordered_io()).
  */
 static inline void btrfs_cleanup_ordered_extents(struct inode *inode,
-						 const u64 offset,
-						 const u64 bytes)
+						 struct page *locked_page,
+						 u64 offset, u64 bytes)
 {
 	unsigned long index = offset >> PAGE_SHIFT;
 	unsigned long end_index = (offset + bytes - 1) >> PAGE_SHIFT;
+	u64 page_start = page_offset(locked_page);
+	u64 page_end = page_start + PAGE_SIZE - 1;
+
 	struct page *page;
 
 	while (index <= end_index) {
@@ -130,8 +130,18 @@ static inline void btrfs_cleanup_ordered_extents(struct inode *inode,
 		ClearPagePrivate2(page);
 		put_page(page);
 	}
-	return __endio_write_update_ordered(inode, offset + PAGE_SIZE,
-					    bytes - PAGE_SIZE, false);
+
+	/*
+	 * In case this page belongs to the delalloc range being instantiated
+	 * then skip it, since the first page of a range is going to be
+	 * properly cleaned up by the caller of run_delalloc_range
+	 */
+	if (page_start >= offset && page_end <= (offset + bytes - 1)) {
+		offset += PAGE_SIZE;
+		bytes -= PAGE_SIZE;
+	}
+
+	return __endio_write_update_ordered(inode, offset, bytes, false);
 }
 
 static int btrfs_dirty_inode(struct inode *inode);
@@ -1606,7 +1616,8 @@ static int run_delalloc_range(void *private_data, struct page *locked_page,
 					   write_flags);
 	}
 	if (ret)
-		btrfs_cleanup_ordered_extents(inode, start, end - start + 1);
+		btrfs_cleanup_ordered_extents(inode, locked_page, start,
+					      end - start + 1);
 	return ret;
 }
 
-- 
2.7.4