From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Howells Subject: Re: [PATCH] AFS: Implement shared-writable mmap [try #2] Date: Wed, 16 May 2007 19:45:23 +0100 Message-ID: <31162.1179341123@redhat.com> References: <464B3F2D.9030603@yahoo.com.au> <464B3209.4010003@yahoo.com.au> <464B07EC.4050308@yahoo.com.au> <464AF3F3.30204@yahoo.com.au> <20070516100225.18685.51699.stgit@warthog.cambridge.redhat.com> <17173.1179321391@redhat.com> <19714.1179331928@redhat.com> <23262.1179334587@redhat.com> Cc: akpm@osdl.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org To: Nick Piggin Return-path: Received: from mx1.redhat.com ([66.187.233.31]:56700 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756223AbXEPSqH (ORCPT ); Wed, 16 May 2007 14:46:07 -0400 In-Reply-To: <464B3F2D.9030603@yahoo.com.au> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org Nick Piggin wrote: > You can drop the lock, do the invalidation, Hmmm... There's a danger of incurring a race by doing that. Consider two processes both trying to write to a dirty page for which writeback will be rejected: (1) The first process gets EKEYREJECTED from the server, drops its lock and is then preempted. (2) The second process gets EKEYREJECTED from the server, drops its lock, truncates the page, reloads the page and modifies it. (3) The first process resumes and truncates the page, thereby splatting the second process's write. Or: (1) The first process gets EKEYREJECTED from the server, clears the writeback information from the page, drops its lock and is then preempted. (2) The second process attaches its own writeback information to the page and modifies it. (3) The first process resumes and truncates the page, thereby splatting the second process's write. Really, what I want to do is pass the page lock to truncate to deal with. Better still, I want truncate to be selective, based on whether or not a page is still associated with the rejected writeback. I wonder if I should call truncate_complete_page() or invalidate_complete_page() directly. What might help is that PG_writeback is set on all pages in the rejected run, even those that are unlocked. Network filesystems are such fun:-) David