Re: [PATCH 2/6] block: truncate: Don't make backing file data visible

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

20.11.2019 17:03, Kevin Wolf wrote:
> When extending the size of an image that has a backing file larger than
> its old size, make sure that the backing file data doesn't become
> visible in the guest, but the added area is properly zeroed out.
> 
> The old behaviour made a difference in 'block_resize' (where showing the
> backing file data from an old snapshot rather than zeros is
> questionable) as well as in commit block jobs (both from active and
> intermediate nodes) and HMP 'commit', where committing to a short
> backing file would incorrectly omit writing zeroes for unallocated
> blocks on the top layer after the EOF of the short backing file.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>   block/io.c | 25 +++++++++++++++++++++++++
>   1 file changed, 25 insertions(+)
> 
> diff --git a/block/io.c b/block/io.c
> index 003f4ea38c..8683f7a4bd 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -3382,6 +3382,31 @@ int coroutine_fn bdrv_co_truncate(BdrvChild *child, int64_t offset, bool exact,
>           goto out;
>       }
>   
> +    /*
> +     * If the image has a backing file that is large enough that it would
> +     * provide data for the new area, we cannot leave it unallocated because
> +     * then the backing file content would become visible. Instead, zero-fill
> +     * the area where backing file and new area overlap.
> +     */

Should we mention that, still, we don't care if user for some reason will change
backing file in future?

> +    if (new_bytes && bs->backing && prealloc == PREALLOC_MODE_OFF) {
> +        int64_t backing_len;
> +
> +        backing_len = bdrv_getlength(backing_bs(bs));
> +        if (backing_len < 0) {
> +            ret = backing_len;
> +            goto out;
> +        }
> +
> +        if (backing_len > old_size) {
> +            ret = bdrv_co_do_pwrite_zeroes(bs, old_size,
> +                                           MIN(new_bytes, backing_len - old_size),
> +                                           BDRV_REQ_ZERO_WRITE | BDRV_REQ_MAY_UNMAP);

two over-80 lines

> +            if (ret < 0) {
> +                goto out;
> +            }
> +        }
> +    }

should we improve "off" mode specification in qapi?

> +
>       ret = refresh_total_sectors(bs, offset >> BDRV_SECTOR_BITS);
>       if (ret < 0) {
>           error_setg_errno(errp, -ret, "Could not refresh total sector count");
> 

Hmm. is it correct to call write_zeroes before refresh_total_sectors?
Note that qcow2_co_pwrite_zeroes rely on bs->total_sectors...

-- 
Best regards,
Vladimir