Re: [Devel] [PATCH hci-8.1 v9 09/10] dm-qcow2: flag the image for check after a reverted L1/L2 writeback

[Konstantin Khorenko <khorenko@virtuozzo.com>]

I won't apply this version, please send v10 with the changes below.

On 10/28/26 16:06, Alexander Ivanov wrote:
>  		if (cleared)
>  			continue; /* no cluster was allocated */
>
> +		wbd->md->status |= MD_LX_REVERTED;

- Leak the clusters on a failed L1/L2 writeback instead of freeing
  them.
- If you keep freeing them, set MD_LX_REVERTED in the pe_page branch
  above as well.

The pe_page branch restores the old entry and continues before this
line. It is taken for a COW over a compressed, snapshot-shared or
preallocated cluster. The new cluster is still freed: the qio waiting
on the page ends with the error, and handle_cleanup_mask() frees it
through FREE_ALLOCATED_CLU. So L2 on disk may point to a free cluster,
lx_writeback_error stays clear, resume is allowed, and patch 10 can
cut that cluster off.

Freeing also lets the allocator give the cluster to another offset
before suspend, while L2 on disk still maps the old offset to it. Two
guest offsets then share one cluster, and the check that wants_check
asks for cannot undo that. A leaked cluster has none of this: the
check just frees it.

Leaking also needs no resume refusal. ploop does not look at
wants_check, so with this patch a transient L2 write error leaves a
live check or a snapshot with the device suspended.

-- 
Konstantin Khorenko,
Virtuozzo Linux Kernel Team

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