From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932613Ab2DZAHj (ORCPT ); Wed, 25 Apr 2012 20:07:39 -0400 Received: from fgwmail6.fujitsu.co.jp ([192.51.44.36]:37495 "EHLO fgwmail6.fujitsu.co.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932453Ab2DZAHi (ORCPT ); Wed, 25 Apr 2012 20:07:38 -0400 X-SecurityPolicyCheck: OK by SHieldMailChecker v1.7.4 Message-ID: <4F98914C.2060505@jp.fujitsu.com> Date: Thu, 26 Apr 2012 09:05:32 +0900 From: KAMEZAWA Hiroyuki User-Agent: Mozilla/5.0 (Windows NT 6.0; rv:11.0) Gecko/20120327 Thunderbird/11.0.1 MIME-Version: 1.0 To: Andrew Morton CC: KOSAKI Motohiro , Nick Piggin , Minchan Kim , Ingo Molnar , x86@kernel.org, Hugh Dickins , Johannes Weiner , Rik van Riel , Mel Gorman , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [RFC] propagate gfp_t to page table alloc functions References: <1335171318-4838-1-git-send-email-minchan@kernel.org> <4F963742.2030607@jp.fujitsu.com> <4F963B8E.9030105@kernel.org> <4F965413.9010305@kernel.org> <20120424143015.99fd8d4a.akpm@linux-foundation.org> <4F973BF2.4080406@jp.fujitsu.com> <4F973FB8.6050103@jp.fujitsu.com> <20120424172554.c9c330dd.akpm@linux-foundation.org> In-Reply-To: <20120424172554.c9c330dd.akpm@linux-foundation.org> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org (2012/04/25 9:25), Andrew Morton wrote: > On Wed, 25 Apr 2012 09:05:12 +0900 > KAMEZAWA Hiroyuki wrote: > >> (2012/04/25 8:55), KOSAKI Motohiro wrote: >> >>> On Tue, Apr 24, 2012 at 7:49 PM, KAMEZAWA Hiroyuki >>> wrote: >>>> (2012/04/25 6:30), Andrew Morton wrote: >>>> >>>>> On Tue, 24 Apr 2012 17:48:29 +1000 >>>>> Nick Piggin wrote: >>>>> >>>>>>> Hmm, there are several places to use GFP_NOIO and GFP_NOFS even, GFP_ATOMIC. >>>>>>> I believe it's not trivial now. >>>>>> >>>>>> They're all buggy then. Unfortunately not through any real fault of their own. >>>>> >>>>> There are gruesome problems in block/blk-throttle.c (thread "mempool, >>>>> percpu, blkcg: fix percpu stat allocation and remove stats_lock"). It >>>>> wants to do an alloc_percpu()->vmalloc() from the IO submission path, >>>>> under GFP_NOIO. >>>>> >>>>> Changing vmalloc() to take a gfp_t does make lots of sense, although I >>>>> worry a bit about making vmalloc() easier to use! >>>>> >>>>> I do wonder whether the whole scheme of explicitly passing a gfp_t was >>>>> a mistake and that the allocation context should be part of the task >>>>> context. ie: pass the allocation mode via *current. >>>> >>>> yes...that's very interesting. >>> >>> I think GFP_ATOMIC is used non task context too. ;-) >> >> Hmm, in interrupt context or some ? Can't we detect it ? > > There are lots of practical details and I haven't begun to think it > through, mainly because it Isn't Going To Happen! > > For example how do we handle spin_lock()? Does spin_lock() now do > > gfp_t spin_lock_2(spinlock_t *lock) > { > gfp_t old_gfp = set_current_gfp(GFP_ATOMIC); > spin_lock(lock); > return old_gfp; > } > > void spin_unlock_2(spinlock_t *lock, gfp_t old_gfp) > { > spin_unlock(lock); > set_current_gfp(old_gfp); > } > > Well that's bad. Currently we require programmers to keep track of > what context they're running in. So they think about what they're > doing. If we made it this easy, we'd see a big proliferation of > GFP_ATOMIC allocations, which is bad. > > Requiring the spin_lock() caller to run set_current_gfp() would have > the same effect. > > > > Or do we instead do this: > > - some_function(foo, bar, GFP_NOIO); > + old_gfp = set_current_gfp(GFP_NOIO); > + some_function(foo, bar); > + set_current_gfp(old_gfp); > > So the rule is "if the code was using an explicit GFP_foo then convert > it to use set_current_gfp(). If the code was receiving a gfp_t > variable from the caller then delete that arg". > > Or something like that. It's all too hopelessly impractical to bother > discussing - 20 years too late! > > > otoh, maybe a constrained version of this could be used to address the > vmalloc() problem alone. > Yes, I think it will be good start. > > otoh2, I didn't *want* blk-throttle.c to use GFP_NOIO for vmalloc(). > GFP_NOIO is weak, unreliable and lame. blk-throttle should find a way > of using GFP_KERNEL! I agree. Thanks, -Kame