[dm-devel] [PATCH 4/6] block: propagate BLKROSET on the whole device to all partitions

[Christoph Hellwig]

Change the policy so that a BLKROSET on the whole device also affects
partitions.  To quote Martin K. Petersen:

It's very common for database folks to twiddle the read-only state of
block devices and partitions. I know that our users will find it very
counter-intuitive that setting /dev/sda read-only won't prevent writes
to /dev/sda1.

The existing behavior is inconsistent in the sense that doing:

permits writes. But:

<something triggers revalidate>

doesn't.

And a subsequent:

doesn't work either since sda1's read-only policy has been inherited
from the whole-disk device.

You need to do:

after setting the whole-disk device rw to effectuate the same change on
the partitions, otherwise they are stuck being read-only indefinitely.

However, setting the read-only policy on a partition does *not* require
the revalidate step. As a matter of fact, doing the revalidate will blow
away the policy setting you just made.

So the user needs to take different actions depending on whether they
are trying to read-protect a whole-disk device or a partition. Despite
using the same ioctl. That is really confusing.

I have lost count how many times our customers have had data clobbered
because of ambiguity of the existing whole-disk device policy. The
current behavior violates the principle of least surprise by letting the
user think they write protected the whole disk when they actually
didn't.

Suggested-by: Martin K. Petersen <martin.petersen at oracle.com>
Signed-off-by: Christoph Hellwig <hch at lst.de>
---
 block/genhd.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/block/genhd.c b/block/genhd.c
index 878f94727aaa96..c214fcd25a05c9 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -1449,8 +1449,7 @@ EXPORT_SYMBOL(set_disk_ro);
 
 int bdev_read_only(struct block_device *bdev)
 {
-	return bdev->bd_read_only ||
-		test_bit(GD_READ_ONLY, &bdev->bd_disk->state);
+	return bdev->bd_read_only || get_disk_ro(bdev->bd_disk);
 }
 EXPORT_SYMBOL(bdev_read_only);
 
-- 
2.29.2