[BUG]
When running btrfs/160 in a loop for subpage with experimental
compression support, it has a high chance to crash (~20%):

 BTRFS critical (device dm-7): panic in __btrfs_add_ordered_extent:238: inconsistency in ordered tree at offset 0 (errno=-17 Object already exists)
 ------------[ cut here ]------------
 kernel BUG at fs/btrfs/ordered-data.c:238!
 Internal error: Oops - BUG: 0 [#1] SMP
 pc : __btrfs_add_ordered_extent+0x550/0x670 [btrfs]
 lr : __btrfs_add_ordered_extent+0x550/0x670 [btrfs]
 Call trace:
  __btrfs_add_ordered_extent+0x550/0x670 [btrfs]
  btrfs_add_ordered_extent+0x2c/0x50 [btrfs]
  run_delalloc_nocow+0x81c/0x8fc [btrfs]
  btrfs_run_delalloc_range+0xa4/0x390 [btrfs]
  writepage_delalloc+0xc0/0x1ac [btrfs]
  __extent_writepage+0xf4/0x370 [btrfs]
  extent_write_cache_pages+0x288/0x4f4 [btrfs]
  extent_writepages+0x58/0xe0 [btrfs]
  btrfs_writepages+0x1c/0x30 [btrfs]
  do_writepages+0x60/0x110
  __filemap_fdatawrite_range+0x108/0x170
  filemap_fdatawrite_range+0x20/0x30
  btrfs_fdatawrite_range+0x34/0x4dc [btrfs]
  __btrfs_write_out_cache+0x34c/0x480 [btrfs]
  btrfs_write_out_cache+0x144/0x220 [btrfs]
  btrfs_start_dirty_block_groups+0x3ac/0x6b0 [btrfs]
  btrfs_commit_transaction+0xd0/0xbb4 [btrfs]
  btrfs_sync_fs+0x64/0x1cc [btrfs]
  sync_fs_one_sb+0x3c/0x50
  iterate_supers+0xcc/0x1d4
  ksys_sync+0x6c/0xd0
  __arm64_sys_sync+0x1c/0x30
  invoke_syscall+0x50/0x120
  el0_svc_common.constprop.0+0x4c/0xd4
  do_el0_svc+0x30/0x9c
  el0_svc+0x2c/0x54
  el0_sync_handler+0x1a8/0x1b0
  el0_sync+0x198/0x1c0
 ---[ end trace 336f67369ae6e0af ]---

[CAUSE]
For subpage case, we can have multiple sectors inside a page, this makes
it possible for __extent_writepage() to have part of its page submitted
before returning.

In btrfs/160, we are using dm-dust to emulate write error, this means
for certain pages, we could have everything running fine, but at the end
of __extent_writepage(), one of the submitted bios fails due to dm-dust.

Then the page is marked Error, and we change @ret from 0 to -EIO.

This makes the caller extent_write_cache_pages() to error out, without
submitting the remaining pages.

Furthermore, since we're erroring out for free space cache, it doesn't
really care about the error and will update the inode and retry the
writeback.

Then we re-run the delalloc range, and will try to insert the same
delalloc range while previous delalloc range is still hanging there,
triggering the above error.

[FIX]
The proper fix is to handle errors from __extent_writepage() properly,
by ending the remaining ordered extent.

But that fix needs the following changes:

- Know at exactly which sector the error happened
  Currently __extent_writepage_io() works for the full page, can't
  return at which sector we hit the error.

- Grab the ordered extent covering the failed sector

As a hotfix for subpage case, here we unify the error paths in
__extent_writepage().

In fact, the "if (PageError(page))" branch never get executed if @ret is
still 0 for non-subpage cases.

As for non-subpage case, we never submit current page in
__extent_writepage(), but only add current page into bio.
The bio can only get submitted in next page.

Thus we never get PageError() set due to IO failure, thus when we hit
the branch, @ret is never 0.

By simply removing that @ret assignment, we let subpage case ignore the
IO failure, thus only error out for fatal errors just like regular
sectorsize.

So that IO error won't be treated as fatal error not trigger the hanging
OE problem.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>