Re: [ENBD] [Fwd: Re: [PATCH] 2.5.8 IDE 36]

[Pavel Machek <pavel@suse.cz>]

Hi!

> With it, the protocol in the client daemon will ack the kernel _before_
> it gets an ack from the remote server. That should help relieve the
> deadlock. Things then go like this:
> 
>      kernel runs low on memory
>      kernel flushes buffers to device drivers under pressure
>      nbd client daemon sends to the net
>         * client acks kernel and releases buffers in kernel
>      server on the _same machine_ receives request over the net
>      server tries to write request to disk
>      server process needs buffers to write to
>         * server gets buffers released by client in kernel
> 
> At least, potentially. If I recall right, there's still a deadlock
> window in-kernel, but it's small. I don't recall the details. Oh -
> well, the request still hangs around in the driver until the client
> daemon acks .. maybe the client daemon can't ack without being swapped
> in first, and that'd be deadlock.
> 
> Well, there's a "-s" flag (for swap devices) that does an mlockall()
> that might take care of that. So "-a -s" might do it. Really needs
> a "-aa" option (release write request in kernel asap).

Well, with mlockall(), I'd believe it could be made to work. Okay.
									Pavel
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