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Re: [Qemu-devel] [Qemu-block] [PATCH v2 0/3] block: Warn about usage of

From: Max Reitz
Subject: Re: [Qemu-devel] [Qemu-block] [PATCH v2 0/3] block: Warn about usage of growing formats over non-growable protocols
Date: Wed, 06 May 2015 19:23:57 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0

On 06.05.2015 18:47, Paolo Bonzini wrote:

On 06/05/2015 18:37, Max Reitz wrote:
Because qcow2 tries to write beyond the end of the file; the NBD client
implementation passes that on to the server, and the server simply
reports an error (which the NBD client turns into EIO).

     *reply = s->reply;
     if (reply->handle != request->handle) {
         reply->error = EIO;
     } else {
         if (qiov && reply->error == 0) {
             ret = qemu_co_recvv(s->sock, qiov->iov, qiov->niov,
                                 offset, request->len);
             if (ret != request->len) {
                 reply->error = EIO;

         /* Tell the read handler to read another header.  */
         s->reply.handle = 0;

It should get into the "else" and then see reply->error != 0.  So the
guest should see ENOSPC.

The guest sees whatever has been written into reply->error, and that field hasn't been written by this function in that case. It has been written by nbd_receive_reply() in nbd.c, and that value comes directly from the server. In case of qemu-nbd being the server, a write beyond the EOF should be caught by blk_check_byte_request() in block/block-backend.c, which returns -EIO. So that's where the EIO comes from.

I don't know whether this EIO is subsequently converted to ENOSPC because of werror=enospc, but considering that https://bugzilla.redhat.com/show_bug.cgi?id=1090713 did not override werror, it doesn't look like it.

Can you check if virtio-scsi
gives ENOSPC?
In which configuration? Using virtio-scsi on top of qcow2 on top of some
SCSI passthrough block driver?  Sounds like we ought to make NBD return
ENOSPC no matter the fate of this series.
virtio-scsi on top of qcow2 on top of NBD.  No passthrough: "-device
scsi-disk" should work.  It seems to me that NBD _should_ be getting
ENOSPC, unless it's nbd-server that throws away the error.

Well, nbd-server will return whatever it feels like, I don't know about that. qemu-nbd on the other hand I do know about, and as written above, it will return EIO because of the check performed from blk_write(). So that error is sent to the client, where the nbd BDS will return it, too, and then qcow2 will propagate it up to virtio-scsi. It looks to me like scsi_handle_rw_error() will then call scsi_check_condition(r, SENSE_CODE(IO_ERROR)).

(Note that I think werror is only used for block jobs, but not for normal guest operations, so EIO coming from a BDS is not converted to ENOSPC for guest requests)

I can test these assumptions, if you'd like me to, but that will have to wait until Friday. :-)


The problem with only warning for a certain non-default configuration is
that people who don't know what they are doing are more likely to use
the default configuration, so I'd like the warning to appear then.
That's entirely true.  Perhaps the werror default should be changed from
enospc to report after all!


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