[Devel] [PATCH hci-8.1 v6 5/6] dm-qcow2: keep the dirty bit of an image opened dirty

[Alexander Ivanov <alexander.ivanov@virtuozzo.com>]

With kernel_sets_dirty_bit, postsuspend clears the dirty bit
unconditionally. An image that is dirty at open, e.g. after a crash,
has unrepaired refcounts, which may be below real usage, yet it is
marked clean on disk after the first suspend. A userspace check before
the next open then has no reason to repair it.

Clear the bit only if it isn't set in the parsed header. The kernel
sets the bit in the raw header page only, so hdr.incompatible_features
keeps the state read from disk, which is reread on every preresume.
Clearing the bit of an image opened dirty is left to userspace check.

This only preserves the on-disk marker. The driver still accepts a
dirty image read-write and allocates from its refcounts, so
find_unused_cluster() may hand out clusters that L2 still references
until the image is checked.

https://virtuozzo.atlassian.net/browse/VSTOR-148171
Feature: dm-qcow2: block device over QCOW2 files driver
Signed-off-by: Alexander Ivanov <alexander.ivanov@virtuozzo.com>
---
 drivers/md/dm-qcow2-target.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/md/dm-qcow2-target.c b/drivers/md/dm-qcow2-target.c
index b667adb5472a..eb56c8de65c3 100644
--- a/drivers/md/dm-qcow2-target.c
+++ b/drivers/md/dm-qcow2-target.c
@@ -676,7 +676,8 @@ int qcow2_set_image_file_features(struct qcow2 *qcow2, bool dirty)
 	if (kernel_sets_dirty_bit) {
 		if (dirty)
 			raw_hdr->incompatible_features |= dirty_mask;
-		else
+		else if (!(qcow2->hdr.incompatible_features &
+			   INCOMPATIBLE_FEATURES_DIRTY_BIT))
 			raw_hdr->incompatible_features &= ~dirty_mask;
 	}
 	kunmap(md->page);
-- 
2.53.0

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