31.01.2020 22:38, Eric Blake wrote:
On 1/31/20 12:48 PM, Vladimir Sementsov-Ogievskiy wrote:
backup-top "supports" write-unchanged, by skipping CBW operation in
backup_top_co_pwritev. But it forgets to do the same in
backup_top_co_pwrite_zeroes, as well as declare support for
BDRV_REQ_WRITE_UNCHANGED.
Fix this, and, while being here, declare also support for flags
supported by source child.
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com>
---
block/backup-top.c | 28 ++++++++++++++++++----------
1 file changed, 18 insertions(+), 10 deletions(-)
@@ -197,6 +201,10 @@ BlockDriverState *bdrv_backup_top_append(BlockDriverState
*source,
top->total_sectors = source->total_sectors;
state = top->opaque;
+ top->supported_write_flags =
+ BDRV_REQ_WRITE_UNCHANGED | source->supported_write_flags;
+ top->supported_zero_flags =
+ BDRV_REQ_WRITE_UNCHANGED | source->supported_zero_flags;
Elsewhere, in block/filter-compress.c we do:
bs->supported_write_flags = BDRV_REQ_WRITE_UNCHANGED |
(BDRV_REQ_FUA & bs->file->bs->supported_write_flags);
bs->supported_zero_flags = BDRV_REQ_WRITE_UNCHANGED |
((BDRV_REQ_FUA | BDRV_REQ_MAY_UNMAP | BDRV_REQ_NO_FALLBACK) &
bs->file->bs->supported_zero_flags);
I've looked at this too, but didn't understand, why we need it. Now with your
description, I see that this make sense. I'll resend with same pattern, thanks.
That's slightly more robust (if the block layer adds new BDRV_REQ_ bits, we
don't have to revisit filter-compress.c to decide if blindly exposing those
bits breaks for some reason, but rahter DO have to amend the line to opt-in to
supporting the new bits). Whereas your code does NOT need editing if passing
on the new bit is safe, but risks a subtle breakage if we forget to filter out
the new bit when passing it on would be unsafe. I tend to lean towards safety
and opt-in over blind pass-through that works with the current set of defined
bits, but not enough to withhold:
Reviewed-by: Eric Blake <ebl...@redhat.com>
--
Best regards,
Vladimir
