[PATCH 1/1] common/rc: re-fix detection of device-mapper/persistent memory incompatibility

["Darrick J. Wong" <djwong@kernel.org>]

From: Darrick J. Wong <djwong@kernel.org>

In commit e05491b3, I tried to resolve false test failures that were a
result of device mapper refusing to change access modes on a block
device that supports the FSDAX access mode.  Unfortunately, I did not
realize that there are two ways that fsdax support can be detected via
sysfs: /sys/block/XXX/queue/dax and /sys/block/XXX/dax/, so I only added
a test for the latter.

As of 5.15-rc1 this doesn't seem to work anymore for some reason.  I
don't know enough about the byzantine world of pmem device driver
initialization, but fsdax mode actually does work even though the
/sys/block/XXX/dax/ path went away.  So clearly we have to detect it
via the other sysfs path.

Fixes: e05491b3 ("common/rc: fix detection of device-mapper/persistent memory incompatibility")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
---
 common/rc |   18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)


diff --git a/common/rc b/common/rc
index 154bc2dd..275b1f24 100644
--- a/common/rc
+++ b/common/rc
@@ -1964,6 +1964,20 @@ _require_sane_bdev_flush()
 	fi
 }
 
+# Decide if the scratch filesystem is likely to be mounted in fsdax mode.
+# If there's a dax clause in the mount options we assume the test runner
+# wants us to test DAX; or if the scratch device itself advertises dax mode
+# in sysfs.
+__detect_scratch_fsdax()
+{
+	_normalize_mount_options | egrep -q "dax(=always| |$)" && return 0
+
+	local sysfs="/sys/block/$(_short_dev $SCRATCH_DEV)"
+	test -e "${sysfs}/dax" && return 0
+	test "$(cat "${sysfs}/queue/dax" 2>/dev/null)" = "1" && return 0
+	return 1
+}
+
 # this test requires a specific device mapper target
 _require_dm_target()
 {
@@ -1975,9 +1989,7 @@ _require_dm_target()
 	_require_sane_bdev_flush $SCRATCH_DEV
 	_require_command "$DMSETUP_PROG" dmsetup
 
-	_normalize_mount_options | egrep -q "dax(=always| |$)" || \
-			test -e "/sys/block/$(_short_dev $SCRATCH_DEV)/dax"
-	if [ $? -eq 0 ]; then
+	if __detect_scratch_fsdax; then
 		case $target in
 		stripe|linear|log-writes)
 			;;