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TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id DE36530860DC; Fri, 20 Sep 2019 01:13:24 +0000 (UTC) Received: from [10.18.17.38] (dhcp-17-38.bos.redhat.com [10.18.17.38]) by smtp.corp.redhat.com (Postfix) with ESMTP id D209E60BF1; Fri, 20 Sep 2019 01:13:23 +0000 (UTC) Subject: Re: [Qemu-devel] [PATCH v12 1/2] block/backup: fix max_transfer handling for copy_range To: Vladimir Sementsov-Ogievskiy , "qemu-block@nongnu.org" References: <20190917160731.10895-1-vsementsov@virtuozzo.com> <20190917160731.10895-2-vsementsov@virtuozzo.com> <4122264a-f7db-8b76-6930-87e2287c49b8@redhat.com> From: John Snow Autocrypt: addr=jsnow@redhat.com; prefer-encrypt=mutual; keydata= mQINBFTKefwBEAChvwqYC6saTzawbih87LqBYq0d5A8jXYXaiFMV/EvMSDqqY4EY6whXliNO IYzhgrPEe7ZmPxbCSe4iMykjhwMh5byIHDoPGDU+FsQty2KXuoxto+ZdrP9gymAgmyqdk3aV vzzmCa3cOppcqKvA0Kqr10UeX/z4OMVV390V+DVWUvzXpda45/Sxup57pk+hyY52wxxjIqef 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X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "kwolf@redhat.com" , Denis Lunev , "mreitz@redhat.com" , "qemu-devel@nongnu.org" , "qemu-stable@nongnu.org" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 9/19/19 2:50 AM, Vladimir Sementsov-Ogievskiy wrote: > 18.09.2019 22:57, John Snow wrote: >> >> >> On 9/17/19 12:07 PM, Vladimir Sementsov-Ogievskiy wrote: >>> Of course, QEMU_ALIGN_UP is a typo, it should be QEMU_ALIGN_DOWN, as = we >>> are trying to find aligned size which satisfy both source and target. >>> Also, don't ignore too small max_transfer. In this case seems safer t= o >>> disable copy_range. >>> >>> Fixes: 9ded4a0114968e >>> Signed-off-by: Vladimir Sementsov-Ogievskiy >>> --- >>> =C2=A0 block/backup.c | 12 ++++++++---- >>> =C2=A0 1 file changed, 8 insertions(+), 4 deletions(-) >>> >>> diff --git a/block/backup.c b/block/backup.c >>> index 763f0d7ff6..d8fdbfadfe 100644 >>> --- a/block/backup.c >>> +++ b/block/backup.c >>> @@ -741,12 +741,16 @@ BlockJob *backup_job_create(const char *job_id,= BlockDriverState *bs, >>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 job->cluster_size =3D cluster_size; >>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 job->copy_bitmap =3D copy_bitmap; >>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 copy_bitmap =3D NULL; >>> -=C2=A0=C2=A0=C2=A0 job->use_copy_range =3D !compress; /* compression= isn't supported for it */ >>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 job->copy_range_size =3D MIN_NON_ZERO(= blk_get_max_transfer(job->common.blk), >>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0 blk_get_max_transfer(job->target)); >>> -=C2=A0=C2=A0=C2=A0 job->copy_range_size =3D MAX(job->cluster_size, >>> -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 QEMU_ALIGN_UP(job->copy_range_size, >>> -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 job->cluster_size)); >>> +=C2=A0=C2=A0=C2=A0 job->copy_range_size =3D QEMU_ALIGN_DOWN(job->cop= y_range_size, >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 job->cluster_size); >>> +=C2=A0=C2=A0=C2=A0 /* >>> +=C2=A0=C2=A0=C2=A0=C2=A0 * Compression is not supported for copy_ran= ge. Also, we don't want to >>> +=C2=A0=C2=A0=C2=A0=C2=A0 * handle small max_transfer for copy_range = (which currently don't >>> +=C2=A0=C2=A0=C2=A0=C2=A0 * handle max_transfer at all). >>> +=C2=A0=C2=A0=C2=A0=C2=A0 */ >>> +=C2=A0=C2=A0=C2=A0 job->use_copy_range =3D !compress && job->copy_ra= nge_size > 0; >>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 /* Required permissions are already ta= ken with target's blk_new() */ >>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 block_job_add_bdrv(&job->common, "targ= et", target, 0, BLK_PERM_ALL, >>> >> >> I'm clear on the alignment fix, I'm not clear on the comment about max= _transfer and how it relates to copy_range_size being non-zero. >> >> "small max transfer" -- what happens when it's zero? we're apparently = OK with a single cluster, but when it's zero, what happens? >=20 > if it zero it means that source or target requires max_transfer less th= an cluster_size. It seems not valid to call copy_range in this case. > Still it's OK to use normal read/write, as they handle max_transfer int= ernally in a loop (copy_range doesn't do it). >=20 oh, I'm ... sorry, I just didn't quite understand the comment. You're just making sure copy_range after all of our checks is non-zero, plain and simple. If max_transfer was *smaller than a job cluster*, we might end up with a copy_range size that's zero, which is obviously... not useful. So, I might phrase "Also, we don't want to..." as: "copy_range does not respect max_transfer, so we factor that in here. If it's smaller than the job->cluster_size, we are unable to use copy_range.= " Just a suggestion, though, so: Reviewed-by: John Snow (SHOULD copy_range respect max_transfer? I guess it would be quite different -- it would only count things it had to fall back and actually *transfer*, right? I suppose that because it can have that fallback we need to accommodate it here in backup.c, hence this workaround clamp.)