linux-kernel - Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Date:   Thu, 15 Sep 2016 23:10:52 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Alex Bligh <alex@...x.org.uk>
Cc:     Eric Blake <eblake@...hat.com>, Wouter Verhelst <w@...r.be>,
        Josef Bacik <jbacik@...com>, linux-block@...r.kernel.org,
        Markus Pargmann <mpa@...gutronix.de>, kernel-team@...com,
        "nbd-general@...ts.sourceforge.net" 
        <nbd-general@...ts.sourceforge.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [Nbd] [RESEND][PATCH 0/5] nbd improvements



On 15/09/2016 17:23, Alex Bligh wrote:
> Paolo,
> 
>> On 15 Sep 2016, at 15:07, Paolo Bonzini <pbonzini@...hat.com> wrote:
>>
>> I don't think QEMU forbids multiple clients to the single server, and
>> guarantees consistency as long as there is no overlap between writes and
>> reads.  These are the same guarantees you have for multiple commands on
>> a single connection.
>>
>> In other words, from the POV of QEMU there's no difference whether
>> multiple commands come from one or more connections.
> 
> This isn't really about ordering, it's about cache coherency
> and persisting things to disk.
> 
> What you say is correct as far as it goes in terms of ordering. However
> consider the scenario with read and writes on two channels as follows
> of the same block:
> 
>      Channel1     Channel2
> 
>      R                      Block read, and cached in user space in
>                             channel 1's cache
>                             Reply sent
> 
>                   W         New value written, channel 2's cache updated
>                             channel 1's cache not
> 
>      R                      Value returned from channel 1's cache.
> 
> 
> In the above scenario, there is a problem if the server(s) handling the
> two channels each use a read cache which is not coherent between the
> two channels. An example would be a read-through cache on a server that
> did fork() and shared no state between connections.

qemu-nbd does not fork(), so there is no coherency issue if W has replied.

However, if W hasn't replied, channel1 can get garbage.  Typically the
VM will be the one during writes, everyone else must be ready to handle
whatever mess the VM throws at them.

Paolo

> Similarly, if there is a write on channel 1 that has completed, and
> the flush goes to channel 2, it may not (if state is not shared) guarantee
> that the write on channel 1 (which has completed) is persisted to non-volatile
> media. Obviously if the 'state' is OS block cache/buffers/whatever, it
> will, but if it's (e.g.) a user-space per process write-through cache,
> it won't.
> 
> I don't know whether qemu-nbd is likely to suffer from either of these.

It can't happen.  On the other hand, channel1 must be ready to handle
garbage, it's illegal.






Download attachment "signature.asc" of type "application/pgp-signature" (474 bytes)