From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nick Piggin Subject: Re: lockless pagecache Cassini regression Date: Fri, 4 Jan 2008 15:27:33 +0100 Message-ID: <20080104142733.GA31052@wotan.suse.de> References: <20080103.193245.15134133.davem@davemloft.net> <20080104113351.GA12706@wotan.suse.de> <20080104.035831.52071724.davem@davemloft.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: netdev@vger.kernel.org To: David Miller Return-path: Received: from mx1.suse.de ([195.135.220.2]:47374 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752208AbYADO1f (ORCPT ); Fri, 4 Jan 2008 09:27:35 -0500 Content-Disposition: inline In-Reply-To: <20080104.035831.52071724.davem@davemloft.net> Sender: netdev-owner@vger.kernel.org List-ID: On Fri, Jan 04, 2008 at 03:58:31AM -0800, David Miller wrote: > From: Nick Piggin > Date: Fri, 4 Jan 2008 12:33:52 +0100 > > > Just for interest, the lockless pagecache actually makes > > page->_count unstable for all pages that _have ever_ been pagecache > > pages (since the last quiescent rcu state, anyway). Basically, it > > looks up and takes a ref on the struct page without ever having a > > prior pin or reference on that page. It can do this because it knows > > the struct page won't actually get freed. After taking the ref, it > > rechecks that it has got the right page... > > Ok, I understand the needs now. > > I think the way the drivers/net/niu.c driver handles things > would work better. It only performs get_page(), atomic > increments on compound_head(page)->_count, and __free_page(). > Is that all legal with the lockless pagecache? Yes, you can use the regular refcounting and freeing operations. If the lockless pagecache has got a speculative reference on the page after the driver drops all "real" references, it will take care of freeing the page. > If so I can likely rework the Cassini page management to > behave similarly. If you got the chance, that would be very nice. Thanks, Nick