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From: David Howells <dhowells@redhat.com>
In-Reply-To: <200712141521.24227.nickpiggin@yahoo.com.au>
References: <200712141521.24227.nickpiggin@yahoo.com.au> <20071205193818.24617.79771.stgit@warthog.procyon.org.uk> <20071205194020.24617.28880.stgit@warthog.procyon.org.uk>
To: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: dhowells@redhat.com, viro@ftp.linux.org.uk, hch@infradead.org,
       Trond.Myklebust@netapp.com, sds@tycho.nsa.gov, casey@schaufler-ca.com,
       linux-kernel@vger.kernel.org, selinux@tycho.nsa.gov,
       linux-security-module@vger.kernel.org
Subject: Re: [PATCH 24/28] AFS: Add a function to excise a rejected write from the pagecache [try #2]
Date: Mon, 17 Dec 2007 22:54:31 +0000
Message-ID: <731.1197932071@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1461
Lines: 32

Nick Piggin <nickpiggin@yahoo.com.au> wrote:

> This reintroduces the fault vs truncate race window, which must be fixed.

Hmmm...  perhaps.  I remember that cropped up in NFS, but I'm doing things a
bit differently to NFS.  Remind me again how that worked please.

> Also, it is adding a fair bit of complexity in an area where we should
> instead be reducing it. I think your filesystem should not be doing
> writeback caching of dirty data in the cases where it is so problematic
> (or at least, disallow mmap and read on the dirty data until it has been
> written back or failed).

Eh?  It's a stateless network filesystem.  There's a gap between writing to a
file (perhaps though an mmap) and the pagecache pages being written back in
which someone may change the security on a file and block the writeback.
There's nothing I can do to prevent it, so I have to instead deal with the
consequences should they arise.  See the description of patch 25 for examples.

So you say I shouldn't do any writeback caching at all?

> But otherwise I guess if you really want to discard the dirty data after
> a failed writeback attempt, what's wrong with just invalidate_inode_pages2?

Erm...  Because it deadlocks?

David
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