btrfs: ensure pages are unlocked on cow_file_range() failure

There is a hung_task report on zoned btrfs like below.

https://github.com/naota/linux/issues/59

  [726.328648] INFO: task rocksdb:high0:11085 blocked for more than 241 seconds.
  [726.329839]       Not tainted 5.16.0-rc1+ #1
  [726.330484] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
  [726.331603] task:rocksdb:high0   state:D stack:    0 pid:11085 ppid: 11082 flags:0x00000000
  [726.331608] Call Trace:
  [726.331611]  <TASK>
  [726.331614]  __schedule+0x2e5/0x9d0
  [726.331622]  schedule+0x58/0xd0
  [726.331626]  io_schedule+0x3f/0x70
  [726.331629]  __folio_lock+0x125/0x200
  [726.331634]  ? find_get_entries+0x1bc/0x240
  [726.331638]  ? filemap_invalidate_unlock_two+0x40/0x40
  [726.331642]  truncate_inode_pages_range+0x5b2/0x770
  [726.331649]  truncate_inode_pages_final+0x44/0x50
  [726.331653]  btrfs_evict_inode+0x67/0x480
  [726.331658]  evict+0xd0/0x180
  [726.331661]  iput+0x13f/0x200
  [726.331664]  do_unlinkat+0x1c0/0x2b0
  [726.331668]  __x64_sys_unlink+0x23/0x30
  [726.331670]  do_syscall_64+0x3b/0xc0
  [726.331674]  entry_SYSCALL_64_after_hwframe+0x44/0xae
  [726.331677] RIP: 0033:0x7fb9490a171b
  [726.331681] RSP: 002b:00007fb943ffac68 EFLAGS: 00000246 ORIG_RAX: 0000000000000057
  [726.331684] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fb9490a171b
  [726.331686] RDX: 00007fb943ffb040 RSI: 000055a6bbe6ec20 RDI: 00007fb94400d300
  [726.331687] RBP: 00007fb943ffad00 R08: 0000000000000000 R09: 0000000000000000
  [726.331688] R10: 0000000000000031 R11: 0000000000000246 R12: 00007fb943ffb000
  [726.331690] R13: 00007fb943ffb040 R14: 0000000000000000 R15: 00007fb943ffd260
  [726.331693]  </TASK>

While we debug the issue, we found running fstests generic/551 on 5GB
non-zoned null_blk device in the emulated zoned mode also had a
similar hung issue.

Also, we can reproduce the same symptom with an error injected
cow_file_range() setup.

The hang occurs when cow_file_range() fails in the middle of
allocation. cow_file_range() called from do_allocation_zoned() can
split the give region ([start, end]) for allocation depending on
current block group usages. When btrfs can allocate bytes for one part
of the split regions but fails for the other region (e.g. because of
-ENOSPC), we return the error leaving the pages in the succeeded regions
locked. Technically, this occurs only when @unlock == 0. Otherwise, we
unlock the pages in an allocated region after creating an ordered
extent.

Considering the callers of cow_file_range(unlock=0) won't write out
the pages, we can unlock the pages on error exit from
cow_file_range(). So, we can ensure all the pages except @locked_page
are unlocked on error case.

In summary, cow_file_range now behaves like this:

- page_started == 1 (return value)
  - All the pages are unlocked. IO is started.
- unlock == 1
  - All the pages except @locked_page are unlocked in any case
- unlock == 0
  - On success, all the pages are locked for writing out them
  - On failure, all the pages except @locked_page are unlocked

Fixes: 42c011000963 ("btrfs: zoned: introduce dedicated data write path for zoned filesystems")
CC: stable@vger.kernel.org # 5.12+
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 64 insertions, 8 deletions

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index a00052bb194d..5a58042a404b 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -1128,6 +1128,28 @@ static u64 get_extent_allocation_hint(struct btrfs_inode *inode, u64 start,
  * *page_started is set to one if we unlock locked_page and do everything
  * required to start IO on it.  It may be clean and already done with
  * IO when we return.
+ *
+ * When unlock == 1, we unlock the pages in successfully allocated regions.
+ * When unlock == 0, we leave them locked for writing them out.
+ *
+ * However, we unlock all the pages except @locked_page in case of failure.
+ *
+ * In summary, page locking state will be as follow:
+ *
+ * - page_started == 1 (return value)
+ *     - All the pages are unlocked. IO is started.
+ *     - Note that this can happen only on success
+ * - unlock == 1
+ *     - All the pages except @locked_page are unlocked in any case
+ * - unlock == 0
+ *     - On success, all the pages are locked for writing out them
+ *     - On failure, all the pages except @locked_page are unlocked
+ *
+ * When a failure happens in the second or later iteration of the
+ * while-loop, the ordered extents created in previous iterations are kept
+ * intact. So, the caller must clean them up by calling
+ * btrfs_cleanup_ordered_extents(). See btrfs_run_delalloc_range() for
+ * example.
  */
 static noinline int cow_file_range(struct btrfs_inode *inode,
 				   struct page *locked_page,
@@ -1137,6 +1159,7 @@ static noinline int cow_file_range(struct btrfs_inode *inode,
 	struct btrfs_root *root = inode->root;
 	struct btrfs_fs_info *fs_info = root->fs_info;
 	u64 alloc_hint = 0;
+	u64 orig_start = start;
 	u64 num_bytes;
 	unsigned long ram_size;
 	u64 cur_alloc_size = 0;
@@ -1324,18 +1347,44 @@ out_reserve:
 	btrfs_dec_block_group_reservations(fs_info, ins.objectid);
 	btrfs_free_reserved_extent(fs_info, ins.objectid, ins.offset, 1);
 out_unlock:
+	/*
+	 * Now, we have three regions to clean up:
+	 *
+	 * |-------(1)----|---(2)---|-------------(3)----------|
+	 * `- orig_start  `- start  `- start + cur_alloc_size  `- end
+	 *
+	 * We process each region below.
+	 */
+
 	clear_bits = EXTENT_LOCKED | EXTENT_DELALLOC | EXTENT_DELALLOC_NEW |
 		EXTENT_DEFRAG | EXTENT_CLEAR_META_RESV;
 	page_ops = PAGE_UNLOCK | PAGE_START_WRITEBACK | PAGE_END_WRITEBACK;
+
 	/*
-	 * If we reserved an extent for our delalloc range (or a subrange) and
-	 * failed to create the respective ordered extent, then it means that
-	 * when we reserved the extent we decremented the extent's size from
-	 * the data space_info's bytes_may_use counter and incremented the
-	 * space_info's bytes_reserved counter by the same amount. We must make
-	 * sure extent_clear_unlock_delalloc() does not try to decrement again
-	 * the data space_info's bytes_may_use counter, therefore we do not pass
-	 * it the flag EXTENT_CLEAR_DATA_RESV.
+	 * For the range (1). We have already instantiated the ordered extents
+	 * for this region. They are cleaned up by
+	 * btrfs_cleanup_ordered_extents() in e.g,
+	 * btrfs_run_delalloc_range(). EXTENT_LOCKED | EXTENT_DELALLOC are
+	 * already cleared in the above loop. And, EXTENT_DELALLOC_NEW |
+	 * EXTENT_DEFRAG | EXTENT_CLEAR_META_RESV are handled by the cleanup
+	 * function.
+	 *
+	 * However, in case of unlock == 0, we still need to unlock the pages
+	 * (except @locked_page) to ensure all the pages are unlocked.
+	 */
+	if (!unlock && orig_start < start)
+		extent_clear_unlock_delalloc(inode, orig_start, start - 1,
+					     locked_page, 0, page_ops);
+
+	/*
+	 * For the range (2). If we reserved an extent for our delalloc range
+	 * (or a subrange) and failed to create the respective ordered extent,
+	 * then it means that when we reserved the extent we decremented the
+	 * extent's size from the data space_info's bytes_may_use counter and
+	 * incremented the space_info's bytes_reserved counter by the same
+	 * amount. We must make sure extent_clear_unlock_delalloc() does not try
+	 * to decrement again the data space_info's bytes_may_use counter,
+	 * therefore we do not pass it the flag EXTENT_CLEAR_DATA_RESV.
 	 */
 	if (extent_reserved) {
 		extent_clear_unlock_delalloc(inode, start,
@@ -1347,6 +1396,13 @@ out_unlock:
 		if (start >= end)
 			goto out;
 	}
+
+	/*
+	 * For the range (3). We never touched the region. In addition to the
+	 * clear_bits above, we add EXTENT_CLEAR_DATA_RESV to release the data
+	 * space_info's bytes_may_use counter, reserved in
+	 * btrfs_check_data_free_space().
+	 */
 	extent_clear_unlock_delalloc(inode, start, end, locked_page,
 				     clear_bits | EXTENT_CLEAR_DATA_RESV,
 				     page_ops);