[PATCH v2] dm: avoid filesystem lookup in dm_get_dev_t()

From: Hannes Reinecke <hare@suse.de>

This reverts commit
644bda6f3460 ("dm table: fall back to getting device using name_to_dev_t()")

dm_get_dev_t() is just used to convert an arbitrary 'path' string
into a dev_t. It doesn't presume that the device is present; that
check will be done later, as the only caller is dm_get_device(),
which does a dm_get_table_device() later on, which will properly
open the device.
So if the path string already _is_ in major:minor representation
we can convert it directly, avoiding a recursion into the filesystem
to lookup the block device.
This avoids a hang in multipath_message() when the filesystem is
inaccessible.

Fixes: 644bda6f3460 ("dm table: fall back to getting device using name_to_dev_t()")

Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin Wilck <redacted>
---
 drivers/md/dm-table.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index 188f41287f18..4acf2342f7ad 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c

@@ -363,14 +363,23 @@ int dm_get_device(struct dm_target *ti, const char *path, fmode_t mode,
 {
 	int r;
 	dev_t dev;
+	unsigned int major, minor;
+	char dummy;
 	struct dm_dev_internal *dd;
 	struct dm_table *t = ti->table;
 
 	BUG_ON(!t);
 
-	dev = dm_get_dev_t(path);
-	if (!dev)
-		return -ENODEV;
+	if (sscanf(path, "%u:%u%c", &major, &minor, &dummy) == 2) {
+		/* Extract the major/minor numbers */
+		dev = MKDEV(major, minor);
+		if (MAJOR(dev) != major || MINOR(dev) != minor)
+			return -EOVERFLOW;
+	} else {
+		dev = dm_get_dev_t(path);
+		if (!dev)
+			return -ENODEV;
+	}
 
 	dd = find_device(&t->devices, dev);
 	if (!dd) {

-- 
2.29.2

Looks good,

Reviewed-by: Christoph Hellwig <hch@lst.de>

Mike, Jens - can we make sure this goes in before branching off the
block branch for 5.12?  I have some work pending that would otherwise
conflict.

On Thu, Jan 21 2021 at 12:53pm -0500,
Christoph Hellwig [off-list ref] wrote:

Looks good,

Reviewed-by: Christoph Hellwig <hch@lst.de>

Mike, Jens - can we make sure this goes in before branching off the
block branch for 5.12?  I have some work pending that would otherwise
conflict.

Sure, I'll do my part to get this fix staged now and sent to Linus
(likely tomorrow) for 5.11-rc5.

Thanks,
Mike