[Devel] [PATCH hci-8.1 v7 5/6] dm-qcow2: keep the dirty bit of an image opened dirty

[no-reply at sashiko.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 the image was clean before the kernel set it.
> The parsed header can't tell this: it is reread on every preresume and
> may carry the kernel's own bit, e.g. on the lower image of a backward
> merge cancelled by suspend in waiting_completion or of a merge with
> depth > 0. So track two flags per image: kernel_dirty_bit is set only
> while the last header write was a successful set and the reread header
> still has the bit, opened_dirty is set when the header has the bit and
> kernel_dirty_bit is not set. Clearing the bit of an image opened dirty
> is left to userspace check.
> 
> After a failed write the bit found on disk is taken as opened dirty,
> which costs an extra check, rather than as the kernel's, which could
> clear a bit set by userspace while the device was suspended.
> 
> 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 at virtuozzo.com>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.vzint.dev/#/patchset/20261022145032.2128679-1-alexander.ivanov@virtuozzo.com?part=5