[PATCH 4/4] generic/081: don't run on DAX capable devices

Dave Chinner
2022-05-16T08:59:38Z

From: Dave Chinner <dchinner@redhat.com>

LVM/DM has conniptions when you try to use snapshots on a device
that has DAX capability. It first sets up the underlying device as a
DAX capable mapping (type 3 or DM_TYPE_DAX_BIO_BASED) but because
snapshots require COW and shared mappings, it isn't supported on DAX
capable devices. Hence creating the snapshot device fails because it
requires a type 1 (DM_TYPE_BIO_BASED) device and DM can't change
types on a loaded mapping.

Hence we get this obscure error message in the log:

device-mapper: ioctl: can't change device type (old=3 vs new=1) after initial table load.

and these obscure, unhelpful error messages from the LVM command
outputs:

device-mapper: reload ioctl on (251:0) failed: Invalid argument
Failed to suspend logical volume vg_081/base_081.
Device vg_081-base_081-real (251:1) is used by another device.
Failed to revert logical volume vg_081/base_081.
Aborting. Manual intervention required.
Failed to create snapshot

How to turn off DAX capability is not documented in dmsetup or LVM
man pages, nor is dax mentioned anywhere in
Documentation/admin/device-mapper/ so I have no idea how to tell
LVM/DM "don't try to enable DAX support!".

As such, if the uderlying block device is dax capable, skip this
test.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
---
common/rc | 33 +++++++++++++++++++++++----------
1 file changed, 23 insertions(+), 10 deletions(-)

diff --git a/common/rc b/common/rc
index 4201a059..f5ead044 100644
--- a/common/rc
+++ b/common/rc
@@ -2167,7 +2167,7 @@ _require_sane_bdev_flush()
# 3. "dax=inode" or nothing means "use scratch dev capability" to
# determine whether DAX is going to be used.
#
-# Returns 0 if DAX will be used, 1 if DAX is not going to be used.
+# Returns 0 if the filesytem will use DAX, 1 if it won't.
__scratch_uses_fsdax()
{
local ops=$(_normalize_mount_options "$MOUNT_OPTIONS")
@@ -2175,9 +2175,19 @@ __scratch_uses_fsdax()
echo $ops | egrep -qw "dax(=always| |$)" && return 0
echo $ops | grep -qw "dax=never" && return 1

+ return 0
+}
+
+# Determine if the scratch device is DAX capable. Every if the fs is not
+# using DAX, we still can't use certain device mapper targets if the block
+# device is DAX capable. hence the check needs to be separat from the FS
+# capability.
+__scratch_dev_has_dax()
+{
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
}

@@ -2194,15 +2204,18 @@ _require_dm_target()
_require_sane_bdev_flush $SCRATCH_DEV
_require_command "$DMSETUP_PROG" dmsetup

- if __scratch_uses_fsdax; then
- case $target in
- stripe|linear|log-writes)
- ;;
- *)
- _notrun "Cannot run tests with DAX on $target devices."
- ;;
- esac
- fi
+ case $target in
+ stripe|linear|log-writes)
+ ;;
+ *)
+ if __scratch_uses_fsdax; then
+ _notrun "Cannot run tests with fsdax on $target devices."
+ fi
+ if __scratch_dev_has_dax; then
+ _notrun "Cannot use $target devices on DAX capable block devices."
+ fi
+ ;;
+ esac

modprobe dm-$target >/dev/null 2>&1

--
2.35.1