[PATCH RFC v2] btrfs: disable direct reads to avoid dirty folios without fs knowing

There is a bug report that a reproducer which doing the following
workloads in two threads:

- Direct read into memory mapped from page cache
- Sync the above range

This can lead to dirty folios without fs knowing, this can be a huge
problem for btrfs, as even on the very basic bs == ps cases without
large folios, such reproducer can screw up the ordered extent accounting
already:

 ------------[ cut here ]------------
 WARNING: fs/btrfs/ordered-data.c:390 at can_finish_ordered_extent.isra.0+0x56/0x1f0 [btrfs], CPU#1: kworker/u42:0/68
 CPU: 1 UID: 0 PID: 68 Comm: kworker/u42:0 Tainted: G            E       7.2.0-rc4-custom+ #415 PREEMPT(full)  74dbeafab12c410178747d5bec9fb200ae56949f
 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS unknown 02/02/2022
 Workqueue: btrfs-endio simple_end_io_work [btrfs]
 RIP: 0010:can_finish_ordered_extent.isra.0+0x56/0x1f0 [btrfs]
 Call Trace:
  <TASK>
  btrfs_finish_ordered_extent+0x39/0xd0 [btrfs 7d1ffd99bb9696179883f257e5fd838e2d1a0ca3]
  end_bbio_data_write+0x1ff/0x280 [btrfs 7d1ffd99bb9696179883f257e5fd838e2d1a0ca3]
  btrfs_bio_end_io+0x76/0xf0 [btrfs 7d1ffd99bb9696179883f257e5fd838e2d1a0ca3]
  process_one_work+0x198/0x380
  worker_thread+0x1c8/0x330
  kthread+0xee/0x120
  ret_from_fork+0x28f/0x310
  ret_from_fork_asm+0x11/0x20
  </TASK>
 ---[ end trace 0000000000000000 ]---
 BTRFS critical (device dm-3): bad ordered extent accounting, root=5 ino=257 OE offset=3465216 OE len=2826240 to_dec=319488 left=135168

Unfortunately we have removed cow fixup mechanism, which is to work
around such dirty folios by re-dirtying them and reserve space for them,
across several kernel releases, meaning we can not easily revert a
single commit to bring it back.
And without doubt, that old cow fixup mechanism is not support larger
folios.

As a hot fix, disable btrfs direct reads for non-experimental builds for
now, so this can buy some time before we find out a proper way to address
this.

Reported-by: Christian Borntraeger <[email protected]>
Link: https://lore.kernel.org/linux-btrfs/[email protected]/
Signed-off-by: Qu Wenruo <[email protected]>
---
Changelog:
v2:
- Fallback to buffered IO to avoid failing existing direct IO users

- Allow direct writes

Reason for RFC:
I'm not 100% sure if disabling direct IOs can fill all the holes.

We still allow mmapping page cache into user spaces, thus I'm not sure
if this is the only hole.
---
 fs/btrfs/direct-io.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/fs/btrfs/direct-io.c b/fs/btrfs/direct-io.c
index ed1779ccb4de..32b29a4faaf2 100644
--- a/fs/btrfs/direct-io.c
+++ b/fs/btrfs/direct-io.c
@@ -1093,6 +1093,11 @@ ssize_t btrfs_direct_read(struct kiocb *iocb, struct iov_iter *to)
 	if (check_direct_read(inode_to_fs_info(inode), to, iocb->ki_pos))
 		return 0;
 
+#ifndef CONFIG_BTRFS_EXPERIMENTAL
+	/* To avoid dirty folios without fs knowing through */
+	return 0;
+#endif
+
 	btrfs_inode_lock(BTRFS_I(inode), BTRFS_ILOCK_SHARED);
 again:
 	/*
-- 
2.54.0