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=-0.7 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,UNPARSEABLE_RELAY 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 00DF6C31E46 for ; Wed, 12 Jun 2019 19:59:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CC264215EA for ; Wed, 12 Jun 2019 19:59:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389098AbfFLT7e (ORCPT ); Wed, 12 Jun 2019 15:59:34 -0400 Received: from out30-133.freemail.mail.aliyun.com ([115.124.30.133]:36121 "EHLO out30-133.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2387605AbfFLT7c (ORCPT ); Wed, 12 Jun 2019 15:59:32 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R191e4;CH=green;DM=||false|;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04420;MF=yang.shi@linux.alibaba.com;NM=1;PH=DS;RN=9;SR=0;TI=SMTPD_---0TU03sVe_1560369565; Received: from US-143344MP.local(mailfrom:yang.shi@linux.alibaba.com fp:SMTPD_---0TU03sVe_1560369565) by smtp.aliyun-inc.com(127.0.0.1); Thu, 13 Jun 2019 03:59:28 +0800 Subject: Re: [v2 PATCH] mm: thp: fix false negative of shmem vma's THP eligibility To: Hugh Dickins Cc: mhocko@suse.com, vbabka@suse.cz, rientjes@google.com, kirill@shutemov.name, kirill.shutemov@linux.intel.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <1556037781-57869-1-git-send-email-yang.shi@linux.alibaba.com> <578b7903-40ef-e616-d700-473713f438c0@linux.alibaba.com> From: Yang Shi Message-ID: <185ccaa5-c380-f84a-ddbb-b89c8f49445a@linux.alibaba.com> Date: Wed, 12 Jun 2019 12:59:24 -0700 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: 7bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6/12/19 11:44 AM, Hugh Dickins wrote: > On Mon, 10 Jun 2019, Yang Shi wrote: >> On 6/7/19 8:58 PM, Hugh Dickins wrote: >>> Yes, that is correct; and correctly placed. But a little more is needed: >>> see how mm/memory.c's transhuge_vma_suitable() will only allow a pmd to >>> be used instead of a pte if the vma offset and size permit. smaps should >>> not report a shmem vma as THPeligible if its offset or size prevent it. >>> >>> And I see that should also be fixed on anon vmas: at present smaps >>> reports even a 4kB anon vma as THPeligible, which is not right. >>> Maybe a test like transhuge_vma_suitable() can be added into >>> transparent_hugepage_enabled(), to handle anon and shmem together. >>> I say "like transhuge_vma_suitable()", because that function needs >>> an address, which here you don't have. >> Thanks for the remind. Since we don't have an address I'm supposed we just >> need check if the vma's size is big enough or not other than other alignment >> check. >> >> And, I'm wondering whether we could reuse transhuge_vma_suitable() by passing >> in an impossible address, i.e. -1 since it is not a valid userspace address. >> It can be used as and indicator that this call is from THPeligible context. > Perhaps, but sounds like it will abuse and uglify transhuge_vma_suitable() > just for smaps. Would passing transhuge_vma_suitable() the address > ((vma->vm_end & HPAGE_PMD_MASK) - HPAGE_PMD_SIZE) > give the the correct answer in all cases? Yes, it looks better. > >>> The anon offset situation is interesting: usually anon vm_pgoff is >>> initialized to fit with its vm_start, so the anon offset check passes; >>> but I wonder what happens after mremap to a different address - does >>> transhuge_vma_suitable() then prevent the use of pmds where they could >>> actually be used? Not a Number#1 priority to investigate or fix here! >>> but a curiosity someone might want to look into. >> Will mark on my TODO list. >> >>> Even with your changes >>> ShmemPmdMapped: 4096 kB >>> THPeligible: 0 >>> will easily be seen: THPeligible reflects whether a huge page can be >>> allocated and mapped by pmd in that vma; but if something else already >>> allocated the huge page earlier, it will be mapped by pmd in this vma >>> if offset and size allow, whatever THPeligible says. We could change >>> transhuge_vma_suitable() to force ptes in that case, but it would be >>> a silly change, just to make what smaps shows easier to explain. >> Where did this come from? From the commit log? If so it is the example for >> the wrong smap output. If that case really happens, I think we could document >> it since THPeligible should just show the current status. > Please read again what I explained there: it's not necessarily an example > of wrong smaps output, it's reasonable smaps output for a reasonable case. > > Yes, maybe Documentation/filesystems/proc.txt should explain "THPeligble" > a little better - "eligible for allocating THP pages" rather than just > "eligible for THP pages" would be good enough? we don't want to write > a book about the various cases. Yes, I agree. > > Oh, and the "THPeligible" output lines up very nicely there in proc.txt: > could the actual alignment of that 0 or 1 be fixed in smaps itself too? Sure. Thanks, Yang > > Thanks, > Hugh