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Re: [PATCH] block/backup-top: fix flags handling
From: |
Eric Blake |
Subject: |
Re: [PATCH] block/backup-top: fix flags handling |
Date: |
Fri, 31 Jan 2020 13:38:39 -0600 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 |
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 <address@hidden>
---
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);
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 <address@hidden>
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org