To: akpm@osdl.org
CC: pmarques@grupopie.com, jgarzik@pobox.com, axboe@suse.de, neilb@suse.de,
       linux-kernel@vger.kernel.org
In-reply-to: <20051110161259.1189a040.akpm@osdl.org> (message from Andrew
	Morton on Thu, 10 Nov 2005 16:12:59 -0800)
Subject: Re: userspace block driver?
References: <4371A4ED.9020800@pobox.com>
	<17265.42782.188870.907784@cse.unsw.edu.au>
	<4371A944.6070302@pobox.com>
	<20051109075455.GN3699@suse.de>
	<4371ACE6.7010503@pobox.com>
	<4371EEBA.2080706@grupopie.com>
	<E1EZpTU-0001KK-00@dorka.pomaz.szeredi.hu>
	<E1EZqZK-0001RV-00@dorka.pomaz.szeredi.hu> <20051110161259.1189a040.akpm@osdl.org>
Message-Id: <E1EaRwq-00065e-00@dorka.pomaz.szeredi.hu>
From: Miklos Szeredi <miklos@szeredi.hu>
Date: Fri, 11 Nov 2005 06:59:40 +0100
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 959
Lines: 24

> >
> > N.B. though FUSE itself is free of deadlocks, as soon as you put
> > something on top of it which has asyncronous page writeback it will
> > not be safe anymore.
> 
> Why?   What goes wrong?

Filesystem daemon can't use GFP_NOIO, and can't set PF_MEMALLOC.  Even
if it could, there's the problem with reply packets from network,
which are not even handled in kernel yet (?).

FUSE sidesteps the issue, by doing writes synchronously and not
allowing shared-writable mappings, hence never dirtying any pages.

The sync write is actually not so bad, the filesystem daemon can do
it's own buffering safely (it's swapabble memory), or do async writes
over the network (letting TCP handle the buffering).

Miklos
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/