[PATCH RFC] generic/730: ensure EIO after device delete

[Boris Burkov <boris@bur.io>]

This test removes a SCSI debug device out from under a mounted
filesystem with a (probably) dirty file. This assumes that page cache
cannot save us from EIO, for a reason that I can't quite explain. In
fact, this test fails for exactly that reason, at least on btrfs.

The original patches:

      https://lore.kernel.org/fstests/20230807112100.GB15405@lst.de/

refer to this passing on xfs and not btrfs, so I suspect I am missing
something. With that said, on my machine this actually fails on xfs with
and without my patch, so this is clearly not enough.

High level, I am trying to understand what is really the expected
behavior from a filesystem under this condition and what this test is
getting at. Of btrfs, ext4, and xfs, only ext4 passes it, while btrfs
does pass with this additional syncing/cache dropping to nudge it to an
error.

Signed-off-by: Boris Burkov <boris@bur.io>
---
 tests/generic/730 | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tests/generic/730 b/tests/generic/730
index 11308cdaa..ca5037c57 100755
--- a/tests/generic/730
+++ b/tests/generic/730
@@ -47,6 +47,9 @@ exec 3< $SCSI_DEBUG_MNT/testfile
 # delete the scsi debug device while it still has dirty data
 echo 1 > /sys/block/$(_short_dev $SCSI_DEBUG_DEV)/device/delete
 
+sync
+echo 3 > /proc/sys/vm/drop_caches
+
 # try to read from the file, which should give us -EIO
 cat <&3 > /dev/null
 
-- 
2.43.0