From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S261431AbVAMAyR (ORCPT ); Wed, 12 Jan 2005 19:54:17 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S261497AbVAMAm5 (ORCPT ); Wed, 12 Jan 2005 19:42:57 -0500 Received: from smtp207.mail.sc5.yahoo.com ([216.136.129.97]:15266 "HELO smtp207.mail.sc5.yahoo.com") by vger.kernel.org with SMTP id S261450AbVAMAm0 (ORCPT ); Wed, 12 Jan 2005 19:42:26 -0500 Message-ID: <41E5C3E6.90906@yahoo.com.au> Date: Thu, 13 Jan 2005 11:42:14 +1100 From: Nick Piggin User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7.3) Gecko/20041007 Debian/1.7.3-5 X-Accept-Language: en MIME-Version: 1.0 To: Christoph Lameter CC: Andrew Morton , torvalds@osdl.org, ak@muc.de, hugh@veritas.com, linux-mm@kvack.org, linux-ia64@vger.kernel.org, linux-kernel@vger.kernel.org, benh@kernel.crashing.org Subject: Re: page table lock patch V15 [0/7]: overview References: <41E4BCBE.2010001@yahoo.com.au> <20050112014235.7095dcf4.akpm@osdl.org> <20050112104326.69b99298.akpm@osdl.org> <41E5AFE6.6000509@yahoo.com.au> <20050112153033.6e2e4c6e.akpm@osdl.org> <41E5B7AD.40304@yahoo.com.au> <41E5BC60.3090309@yahoo.com.au> In-Reply-To: Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Christoph Lameter wrote: > On Thu, 13 Jan 2005, Nick Piggin wrote: > > >>>Pointer operations and word size operations are atomic. So this is mostly >>>okay. >>> >>>The issue arises on architectures that have a large pte size than the >>>wordsize. This is only on i386 PAE mode and S/390. S/390 falls back to >>>the page table lock for these operations. PAE mode should do the same and >>>not use atomic ops if they cannot be made to work in a reasonable manner. >>> >> >>Yep well you should be OK then. Your implementation has the advantage >>that it only instantiates previously clear ptes... hmm, no I'm wrong, >>your ptep_set_access_flags path modifies an existing pte. I think this >>can cause subtle races in copy_page_range, and maybe other places, >>can't it? > > > ptep_set_access_flags is only used after acquiring the page_table_lock and > does not clear a pte. That is safe. The only critical thing is if a pte > would be cleared while holding the page_table_lock. That used to occur in > the swapper code but we modified that. > I mean what used to be the ptep_set_access_flags path. Where you are now modifying a pte without the ptl. However after a second look, it seems like that won't be a problem. > There is still an issue as Hugh rightly observed. One cannot rely on a > read of a pte/pud/pmd being atomic if the pte is > word size. This occurs > for all higher levels in handle_mm_fault. Thus we would need to either > acuire the page_table_lock for some architectures or provide primitives > get_pgd, get_pud etc that take the page_table_lock on PAE mode. ARGH. > Yes I know. I would say that having arch-definable accessors for the page tables wouldn't be a bad idea anyway, and the flexibility may come in handy for other things. It would be a big, annoying patch though :(