Re: [PATCH for-4.2? v3 0/8] block: Fix resize (extending) of short overlays

[Max Reitz]

On 10.12.19 18:46, Kevin Wolf wrote:
> Am 22.11.2019 um 17:05 hat Kevin Wolf geschrieben:
>> See patch 4 for the description of the bug fixed.
> 
> I'm applying patches 3 and 5-7 to the block branch because they make
> sense on their own.
> 
> The real fix will need another approach because the error handling is
> broken in this one: If zeroing out fails (either because of NO_FALLBACK
> or because of some other I/O error), bdrv_co_truncate() will return
> failure, but the image size has already been increased, with potentially
> incorrect data in the new area.
> 
> To fix this, we need to make sure that zeros will be read before we
> commit the new image size to the image file (e.g. qcow2 header) and to
> bs->total_sectors. In other words, it must become the responsibility of
> the block driver.
> 
> To this effect, I'm planning to introduce a PREALLOC_MODE_ZERO_INIT flag
> that can be or'ed to the preallocation mode. This will fail by default
> because it looks like just another unimplemented preallocation mode to
> block drivers. It will be requested explicitly by commit jobs and
> automatically added by bdrv_co_truncate() if the backing file would
> become visible (like in this series, but now for all preallocation
> modes). I'm planning to implement it for qcow2 and file-posix for now,
> which should cover most interesting cases.
> 
> Does this make sense to you?

Sounds good to me.

Max

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