Re: [PATCH] btrfs: ensure pages are unlocked on cow_file_range() failure

[Josef Bacik <josef@toxicpanda.com>]

On Mon, Dec 13, 2021 at 12:43:38PM +0900, Naohiro Aota wrote:
> There is a hung_task report regarding page lock 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.
> 
> 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.
> 
> Theoretically, the same issue can happen on
> submit_uncompressed_range(). However, I could not make it happen even
> if I modified the code to go always through
> submit_uncompressed_range().
> 
> 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 == 0
>   - All the pages except @locked_page are unlocked in any case
> - unlock == 1
>   - 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+
> Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
> ---
> Theoretically, this can fix a potential issue in
> submit_uncompressed_range(). However, I set the stable target only
> related to zoned code, because I cannot make compress writes fail on
> regular btrfs.
> ---

I spent a good deal of time going through this code, and it needs some cleaning
up that can be done separately from this patch.  However this patch isn't quite
right either.

For the unlock == 1 case we'll just leave the ordered extent sitting around,
never submitting it for IO.  So eventually we'll come around and do
btrfs_wait_ordered_extent() and hang.

We need the extent_clear_unlock_delalloc() with START_WRITEBACK and
END_WRITEBACK so the ordered extent cleanup for the one we created gets run, we
just need to not do the PAGE_UNLOCK for that range.

Also you are doing CLEAR_META_RESV here, which isn't correct because that'll be
handled at ordered extent cleanup time.  I'm sort of surprised you didn't get
leak errors with this fix.

There's like 3 different error conditions we want to handle here, all with
subtly different error handling.

1. We didn't do anything, we can just clean everything up.  We can just do

        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;   
        extent_clear_unlock_delalloc(inode, start, end, locked_page,
                                     clear_bits | EXTENT_CLEAR_DATA_RESV,                                  
                                     page_ops);

  This is because we need to clear the META and DATA reservations, and we need
  to unlock all pages (except lock_page) and be done.

2. start == orig_start, but we couldn't create our ordered_extent.  The above
   needs to be called on [start, start+ins.offset-1] without DATA_RESV because
   that was handled by the btrfs_reserve_extent().  Then we need to do the above
   for [start+ins.offset, end] if they aren't equal.

3. Finally your case, start != orig_start, and btrfs_reserve_extent() failed.
   In both the unlock == 1 and unlock == 0 case we have to make sure that we get
   START_WRITEBACK | END_WRITEBACK for the [orig_start, start - 1] range,
   without DATA_RESV or META_RESV as that'll be handled by the ordered extent
   cleanup.  Then we need to do the above with the range [start, end].

I'd like to see the error handling cleaned up, but at the very least your fix is
incomplete.  If you tackle the cleanup that can be separate.  Thanks,

Josef