linux-kernel - Re: [PATCH 24/28] AFS: Add a function to excise a rejected write from the pagecache [try #2]

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Date:	Mon, 17 Dec 2007 22:54:31 +0000
From:	David Howells <dhowells@...hat.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	dhowells@...hat.com, viro@....linux.org.uk, hch@...radead.org,
	Trond.Myklebust@...app.com, sds@...ho.nsa.gov,
	casey@...aufler-ca.com, linux-kernel@...r.kernel.org,
	selinux@...ho.nsa.gov, linux-security-module@...r.kernel.org
Subject: Re: [PATCH 24/28] AFS: Add a function to excise a rejected write from the pagecache [try #2]

Nick Piggin <nickpiggin@...oo.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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