linux-kernel - [PATCH 4.19 08/58] dm: avoid filesystem lookup in dm_get_dev

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Date:   Mon, 25 Jan 2021 19:39:09 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Hannes Reinecke <hare@...e.de>,
        Martin Wilck <mwilck@...e.com>, Christoph Hellwig <hch@....de>,
        Mike Snitzer <snitzer@...hat.com>
Subject: [PATCH 4.19 08/58] dm: avoid filesystem lookup in dm_get_dev_t()

From: Hannes Reinecke <hare@...e.de>

commit 809b1e4945774c9ec5619a8f4e2189b7b3833c0c upstream.

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()")
Cc: stable@...r.kernel.org
Signed-off-by: Hannes Reinecke <hare@...e.de>
Signed-off-by: Martin Wilck <mwilck@...e.com>
Reviewed-by: Christoph Hellwig <hch@....de>
Signed-off-by: Mike Snitzer <snitzer@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 drivers/md/dm-table.c |   15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -431,14 +431,23 @@ int dm_get_device(struct dm_target *ti,
 {
 	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) {