From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, UNPARSEABLE_RELAY,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5B0A6C43331 for ; Thu, 7 Nov 2019 20:19:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3867B2166E for ; Thu, 7 Nov 2019 20:19:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726830AbfKGUTn (ORCPT ); Thu, 7 Nov 2019 15:19:43 -0500 Received: from out30-133.freemail.mail.aliyun.com ([115.124.30.133]:37203 "EHLO out30-133.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725792AbfKGUTm (ORCPT ); Thu, 7 Nov 2019 15:19:42 -0500 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R121e4;CH=green;DM=||false|;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e01419;MF=yang.shi@linux.alibaba.com;NM=1;PH=DS;RN=5;SR=0;TI=SMTPD_---0ThRudlZ_1573157976; Received: from US-143344MP.local(mailfrom:yang.shi@linux.alibaba.com fp:SMTPD_---0ThRudlZ_1573157976) by smtp.aliyun-inc.com(127.0.0.1); Fri, 08 Nov 2019 04:19:38 +0800 Subject: Re: [PATCH] mm: shmem: use proper gfp flags for shmem_writepage() To: Hugh Dickins Cc: Michal Hocko , akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <1572991351-86061-1-git-send-email-yang.shi@linux.alibaba.com> <20191106151820.GB8138@dhcp22.suse.cz> <733100ea-97aa-db27-4b43-cf42317afaf8@linux.alibaba.com> From: Yang Shi Message-ID: Date: Thu, 7 Nov 2019 12:19:35 -0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/6/19 10:59 AM, Hugh Dickins wrote: > On Wed, 6 Nov 2019, Yang Shi wrote: >> On 11/6/19 7:18 AM, Michal Hocko wrote: >>> On Wed 06-11-19 06:02:31, Yang Shi wrote: >>>> The shmem_writepage() uses GFP_ATOMIC to allocate swap cache. >>>> GFP_ATOMIC used to mean __GFP_HIGH, but now it means __GFP_HIGH | >>>> __GFP_ATOMIC | __GFP_KSWAPD_RECLAIM. However, shmem_writepage() should >>>> write out to swap only in response to memory pressure, so >>>> __GFP_KSWAPD_RECLAIM looks useless since the caller may be kswapd itself >>>> or in direct reclaim already. >>> What kind of problem are you trying to fix here? >> I didn't run into any visible problem. I just happened to find this >> inconsistency when I was looking into the other problem. > Yes, I don't think it fixes any actual problem: just a cleanup to > make the two calls look the same when they don't need to be different > (whereas the call from __read_swap_cache_async() rightly uses a > lower priority gfp). I'm supposed it is because __read_swap_cache_async()is typically called from page fault context which is less crucial than reclaim. Shall I consider this as an ack but with commit log rephrased to reflect the cleanup? > > If it does fix a problem, then you need to worry also about the > * TODO: this could cause a theoretical memory reclaim > * deadlock in the swap out path. > comment still against the call in add_to_swap(): but I think that > is equally theoretical, demanding no attention since 2.6.12. > >> The add_to_swap() does: >> >> int add_to_swap(struct page *page) >> { >> ... >> err = add_to_swap_cache(page, entry, >>                         __GFP_HIGH|__GFP_NOMEMALLOC|__GFP_NOWARN); >> ... >> } >> >> Actually, shmem_writepage() does almost the same thing and both of them are >> called in reclaim context, so I didn't see why they should use different gfp >> flag. And, GFP_ATOMIC is also different from the old definition as I >> mentioned in the commit log. >> >>>> In addition, XArray node allocations from PF_MEMALLOC contexts could >>>> completely exhaust the page allocator, __GFP_NOMEMALLOC stops emergency >>>> reserves from being allocated. >>> I am not really familiar with XArray much, could you be more specific >>> please? >> It comes from the comments of add_to_swap(), says: >> >> /* >>          * XArray node allocations from PF_MEMALLOC contexts could >>          * completely exhaust the page allocator. __GFP_NOMEMALLOC >>          * stops emergency reserves from being allocated. >> >> And, it looks the original comment came from pre-git time, TBH I'm not quite >> sure about the specific problem which incurred this. I suspect it may be >> because PF_MEMALLOC context allows ALLOC_NO_WATERMARK. >> >>>> Here just copy the gfp flags used by add_to_swap(). >>>> >>>> Cc: Hugh Dickins >>>> Signed-off-by: Yang Shi >>>> --- >>>> mm/shmem.c | 3 ++- >>>> 1 file changed, 2 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/mm/shmem.c b/mm/shmem.c >>>> index 220be9f..9691dec 100644 >>>> --- a/mm/shmem.c >>>> +++ b/mm/shmem.c >>>> @@ -1369,7 +1369,8 @@ static int shmem_writepage(struct page *page, >>>> struct writeback_control *wbc) >>>> if (list_empty(&info->swaplist)) >>>> list_add(&info->swaplist, &shmem_swaplist); >>>> - if (add_to_swap_cache(page, swap, GFP_ATOMIC) == 0) { >>>> + if (add_to_swap_cache(page, swap, >>>> + __GFP_HIGH | __GFP_NOMEMALLOC | __GFP_NOWARN) == 0) { >>>> spin_lock_irq(&info->lock); >>>> shmem_recalc_inode(inode); >>>> info->swapped++; >>>> -- >>>> 1.8.3.1