Re: FAILED: patch "[PATCH] btrfs: flush write bio if we loop in extent_write_cache_pages" failed to apply to 4.9-stable tree

[Sasha Levin <sashal@kernel.org>]

On Sun, Feb 09, 2020 at 01:03:49PM +0100, gregkh@linuxfoundation.org wrote:
>
>The patch below does not apply to the 4.9-stable tree.
>If someone wants it applied there, or to any other stable or longterm
>tree, then please email the backport, including the original git commit
>id to <stable@vger.kernel.org>.
>
>thanks,
>
>greg k-h
>
>------------------ original commit in Linus's tree ------------------
>
>From 42ffb0bf584ae5b6b38f72259af1e0ee417ac77f Mon Sep 17 00:00:00 2001
>From: Josef Bacik <josef@toxicpanda.com>
>Date: Thu, 23 Jan 2020 15:33:02 -0500
>Subject: [PATCH] btrfs: flush write bio if we loop in extent_write_cache_pages
>
>There exists a deadlock with range_cyclic that has existed forever.  If
>we loop around with a bio already built we could deadlock with a writer
>who has the page locked that we're attempting to write but is waiting on
>a page in our bio to be written out.  The task traces are as follows
>
>  PID: 1329874  TASK: ffff889ebcdf3800  CPU: 33  COMMAND: "kworker/u113:5"
>   #0 [ffffc900297bb658] __schedule at ffffffff81a4c33f
>   #1 [ffffc900297bb6e0] schedule at ffffffff81a4c6e3
>   #2 [ffffc900297bb6f8] io_schedule at ffffffff81a4ca42
>   #3 [ffffc900297bb708] __lock_page at ffffffff811f145b
>   #4 [ffffc900297bb798] __process_pages_contig at ffffffff814bc502
>   #5 [ffffc900297bb8c8] lock_delalloc_pages at ffffffff814bc684
>   #6 [ffffc900297bb900] find_lock_delalloc_range at ffffffff814be9ff
>   #7 [ffffc900297bb9a0] writepage_delalloc at ffffffff814bebd0
>   #8 [ffffc900297bba18] __extent_writepage at ffffffff814bfbf2
>   #9 [ffffc900297bba98] extent_write_cache_pages at ffffffff814bffbd
>
>  PID: 2167901  TASK: ffff889dc6a59c00  CPU: 14  COMMAND:
>  "aio-dio-invalid"
>   #0 [ffffc9003b50bb18] __schedule at ffffffff81a4c33f
>   #1 [ffffc9003b50bba0] schedule at ffffffff81a4c6e3
>   #2 [ffffc9003b50bbb8] io_schedule at ffffffff81a4ca42
>   #3 [ffffc9003b50bbc8] wait_on_page_bit at ffffffff811f24d6
>   #4 [ffffc9003b50bc60] prepare_pages at ffffffff814b05a7
>   #5 [ffffc9003b50bcd8] btrfs_buffered_write at ffffffff814b1359
>   #6 [ffffc9003b50bdb0] btrfs_file_write_iter at ffffffff814b5933
>   #7 [ffffc9003b50be38] new_sync_write at ffffffff8128f6a8
>   #8 [ffffc9003b50bec8] vfs_write at ffffffff81292b9d
>   #9 [ffffc9003b50bf00] ksys_pwrite64 at ffffffff81293032
>
>I used drgn to find the respective pages we were stuck on
>
>page_entry.page 0xffffea00fbfc7500 index 8148 bit 15 pid 2167901
>page_entry.page 0xffffea00f9bb7400 index 7680 bit 0 pid 1329874
>
>As you can see the kworker is waiting for bit 0 (PG_locked) on index
>7680, and aio-dio-invalid is waiting for bit 15 (PG_writeback) on index
>8148.  aio-dio-invalid has 7680, and the kworker epd looks like the
>following
>
>  crash> struct extent_page_data ffffc900297bbbb0
>  struct extent_page_data {
>    bio = 0xffff889f747ed830,
>    tree = 0xffff889eed6ba448,
>    extent_locked = 0,
>    sync_io = 0
>  }
>
>Probably worth mentioning as well that it waits for writeback of the
>page to complete while holding a lock on it (at prepare_pages()).
>
>Using drgn I walked the bio pages looking for page
>0xffffea00fbfc7500 which is the one we're waiting for writeback on
>
>  bio = Object(prog, 'struct bio', address=0xffff889f747ed830)
>  for i in range(0, bio.bi_vcnt.value_()):
>      bv = bio.bi_io_vec[i]
>      if bv.bv_page.value_() == 0xffffea00fbfc7500:
>	  print("FOUND IT")
>
>which validated what I suspected.
>
>The fix for this is simple, flush the epd before we loop back around to
>the beginning of the file during writeout.
>
>Fixes: b293f02e1423 ("Btrfs: Add writepages support")
>CC: stable@vger.kernel.org # 4.4+
>Reviewed-by: Filipe Manana <fdmanana@suse.com>
>Signed-off-by: Josef Bacik <josef@toxicpanda.com>
>Signed-off-by: David Sterba <dsterba@suse.com>

There was a build error here because we didn't have aab6e9edf07f
("btrfs: unify extent_page_data type passed as void") on 4.9 and older.

Fixed and queued for 4.9-4.4.

-- 
Thanks,
Sasha