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Re: [PATCH for-5.1 1/2] block: Require aligned image size to avoid asser

From: Max Reitz
Subject: Re: [PATCH for-5.1 1/2] block: Require aligned image size to avoid assertion failure
Date: Mon, 13 Jul 2020 13:19:21 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0

On 10.07.20 16:21, Kevin Wolf wrote:
> Unaligned requests will automatically be aligned to bl.request_alignment
> and we don't want to extend requests to access space beyond the end of
> the image, so it's required that the image size is aligned.
> With write requests, this could cause assertion failures like this if
> RESIZE permissions weren't requested:
> qemu-img: block/io.c:1910: bdrv_co_write_req_prepare: Assertion `end_sector 
> <= bs->total_sectors || child->perm & BLK_PERM_RESIZE' failed.
> This was e.g. triggered by qemu-img converting to a target image with 4k
> request alignment when the image was only aligned to 512 bytes, but not
> to 4k.
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  block.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)

(I think we had some proposal like this before, but I can’t find it,

I can’t see how with this patch you could create qcow2 images and then
use them with direct I/O, because AFAICS, qemu-img create doesn’t allow
specifying caching options, so AFAIU you’re stuck with:

$ ./qemu-img create -f qcow2 /mnt/tmp/foo.qcow2 1M
Formatting '/mnt/tmp/foo.qcow2', fmt=qcow2 cluster_size=65536
compression_type=zlib size=1048576 lazy_refcounts=off refcount_bits=16

$ sudo ./qemu-io -t none /mnt/tmp/foo.qcow2
qemu-io: can't open device /mnt/tmp/foo.qcow2: Image size is not a
multiple of request alignment

(/mnt/tmp is a filesystem on a “losetup -b 4096” device.)

Or you use blockdev-create, that seems to work (because of course you
can set the cache mode on the protocol node when you open it for
formatting).  But, well, I think there should be a working qemu-img
create case.

Also, I’m afraid of breaking existing use cases with this patch (just
qemu-img create + using the image with cache=none).


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