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From: Kevin Wolf <kwolf@redhat.com>
To: Alberto Garcia <berto@igalia.com>
Cc: Nir Soffer <nsoffer@redhat.com>,
	Anton Nefedov <anton.nefedov@virtuozzo.com>,
	qemu-devel@nongnu.org, qemu-block@nongnu.org,
	Max Reitz <mreitz@redhat.com>
Subject: Re: [PATCH v2] block: Reject misaligned write requests with BDRV_REQ_NO_FALLBACK
Date: Mon, 14 Oct 2019 11:26:01 +0200	[thread overview]
Message-ID: <20191014092601.GA7173@localhost.localdomain> (raw)
In-Reply-To: <20191014081545.29694-1-berto@igalia.com>

Am 14.10.2019 um 10:15 hat Alberto Garcia geschrieben:
> The BDRV_REQ_NO_FALLBACK flag means that an operation should only be
> performed if it can be offloaded or otherwise performed efficiently.
> 
> However a misaligned write request requires a RMW so we should return
> an error and let the caller decide how to proceed.
> 
> This hits an assertion since commit c8bb23cbdb if the required
> alignment is larger than the cluster size:
> 
> qemu-img create -f qcow2 -o cluster_size=2k img.qcow2 4G
> qemu-io -c "open -o driver=qcow2,file.align=4k blkdebug::img.qcow2" \
>         -c 'write 0 512'
> qemu-io: block/io.c:1127: bdrv_driver_pwritev: Assertion `!(flags & BDRV_REQ_NO_FALLBACK)' failed.
> Aborted
> 
> The reason is that when writing to an unallocated cluster we try to
> skip the copy-on-write part and zeroize it using BDRV_REQ_NO_FALLBACK
> instead, resulting in a write request that is too small (2KB cluster
> size vs 4KB required alignment).
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>

Thanks, applied to the block branch.

Kevin


  parent reply	other threads:[~2019-10-14  9:26 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-14  8:15 [PATCH v2] block: Reject misaligned write requests with BDRV_REQ_NO_FALLBACK Alberto Garcia
2019-10-14  8:41 ` no-reply
2019-10-14  9:26 ` Kevin Wolf [this message]
2019-10-14 11:22   ` Alberto Garcia
2019-10-14 14:21     ` Kevin Wolf

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