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 53E47C5DF63 for ; Wed, 6 Nov 2019 18:23:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2F3CD20869 for ; Wed, 6 Nov 2019 18:23:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732528AbfKFSXd (ORCPT ); Wed, 6 Nov 2019 13:23:33 -0500 Received: from out30-42.freemail.mail.aliyun.com ([115.124.30.42]:41353 "EHLO out30-42.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727397AbfKFSXd (ORCPT ); Wed, 6 Nov 2019 13:23:33 -0500 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R631e4;CH=green;DM=||false|;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01f04391;MF=yang.shi@linux.alibaba.com;NM=1;PH=DS;RN=5;SR=0;TI=SMTPD_---0ThMUXs8_1573064607; Received: from US-143344MP.local(mailfrom:yang.shi@linux.alibaba.com fp:SMTPD_---0ThMUXs8_1573064607) by smtp.aliyun-inc.com(127.0.0.1); Thu, 07 Nov 2019 02:23:29 +0800 Subject: Re: [PATCH] mm: shmem: use proper gfp flags for shmem_writepage() To: Michal Hocko Cc: hughd@google.com, 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> From: Yang Shi Message-ID: <733100ea-97aa-db27-4b43-cf42317afaf8@linux.alibaba.com> Date: Wed, 6 Nov 2019 10:23:24 -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: <20191106151820.GB8138@dhcp22.suse.cz> 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 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. 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