Re: [Qemu-devel] [PATCH 1/5] nbd: Improve per-export flag handling in server

From: Eric Blake <eblake@redhat.com>
To: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>,
	"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>
Cc: Kevin Wolf <kwolf@redhat.com>,
	"open list:Network Block Dev..." <qemu-block@nongnu.org>,
	Max Reitz <mreitz@redhat.com>
Subject: Re: [Qemu-devel] [PATCH 1/5] nbd: Improve per-export flag handling in server
Date: Tue, 3 Sep 2019 11:39:46 -0500	[thread overview]
Message-ID: <143fea30-cc9a-246a-4aff-5bb27a082a0e@redhat.com> (raw)
In-Reply-To: <55ebe9db-5a28-d844-89f7-2dc9a7d977d7@redhat.com>


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On 8/30/19 6:32 PM, Eric Blake wrote:

>>>> @@ -458,10 +458,13 @@ static int nbd_negotiate_handle_export_name(NBDClient *client,
>>>>           return -EINVAL;
>>>>       }
>>>>
>>>> -    trace_nbd_negotiate_new_style_size_flags(client->exp->size,
>>>> -                                             client->exp->nbdflags | myflags);
>>>> +    myflags = client->exp->nbdflags;
>>>> +    if (client->structured_reply) {
>>>> +        myflags |= NBD_FLAG_SEND_DF;
>>>> +    }
>>>
>>>
>>> why we cant do just
>>> client->exp->nbdflags |= NBD_FLAG_SEND_DF ?
>>
>> Because myflags is the runtime flags for _this_ client, while
>> client->exp->nbdflags are the base flags shared by _all_ clients.  If
>> client A requests structured reply, but client B does not, then we don't
>> want to advertise DF to client B; but amending client->exp->nbdflags
>> would have that effect.
> 
> I stand corrected - it looks like a fresh client->exp is created per
> client, as evidenced by:

I need to quit replying to myself, but my test was flawed.  Modern
clients don't go through NBD_OPT_EXPORT_NAME, so my added line...


> +++ w/nbd/server.c
> @@ -457,6 +457,7 @@ static int
> nbd_negotiate_handle_export_name(NBDClient *client, bool no_zeroes,
>      myflags = client->exp->nbdflags;
>      if (client->structured_reply) {
>          myflags |= NBD_FLAG_SEND_DF;
> +        client->exp->nbdflags |= NBD_FLAG_SEND_DF;
>      }

...was not getting reached.  If I instead tweak NBD_OPT_GO:

diff --git i/nbd/server.c w/nbd/server.c
index 6f3a83704fb3..da1ef793f6df 100644
--- i/nbd/server.c
+++ w/nbd/server.c
@@ -640,6 +640,7 @@ static int nbd_negotiate_handle_info(NBDClient
*client, Error **errp)
     myflags = exp->nbdflags;
     if (client->structured_reply) {
         myflags |= NBD_FLAG_SEND_DF;
+        exp->nbdflags |= NBD_FLAG_SEND_DF;
     }
     trace_nbd_negotiate_new_style_size_flags(exp->size, myflags);
     stq_be_p(buf, exp->size);


> $ ./qemu-nbd -r -f raw file -t &
> 
> $  ~/qemu/qemu-io -r -f raw --trace=nbd_\*size_flags
> nbd://localhost:10809 -c quit
> 32145@1567207628.519883:nbd_receive_negotiate_size_flags Size is
> 1049088, export flags 0x48f
> 
> $ MY_HACK=1 ~/qemu/qemu-io -r -f raw --trace=nbd_\*size_flags
> nbd://localhost:10809 -c quit
> 32156@1567207630.417815:nbd_receive_negotiate_size_flags Size is
> 1049088, export flags 0x40f
> 

Then this reports 0x48f, proving that my initial reaction was correct:
client->exp is a shared resource across multiple connections, but
advertising DF must be a per-connection decision.

> $  ~/qemu/qemu-io -r -f raw --trace=nbd_\*size_flags
> nbd://localhost:10809 -c quit
> 32167@1567207635.202940:nbd_receive_negotiate_size_flags Size is
> 1049088, export flags 0x48f
> 
> The export flags change per client, so I _can_ store into
> client->exp->nbdflags.  Will do that for v2.

I see nothing to change for v2, so I'm inclined to take this patch as is.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org


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