From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f200.google.com (mail-pf0-f200.google.com [209.85.192.200]) by kanga.kvack.org (Postfix) with ESMTP id 2D0926B0008 for ; Tue, 17 Apr 2018 17:51:28 -0400 (EDT) Received: by mail-pf0-f200.google.com with SMTP id f19so11780688pfn.6 for ; Tue, 17 Apr 2018 14:51:28 -0700 (PDT) Received: from out30-132.freemail.mail.aliyun.com (out30-132.freemail.mail.aliyun.com. [115.124.30.132]) by mx.google.com with ESMTPS id bh10-v6si12097829plb.322.2018.04.17.14.51.26 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 17 Apr 2018 14:51:26 -0700 (PDT) Subject: Re: [RFC PATCH] fs: introduce ST_HUGE flag and set it to tmpfs and hugetlbfs References: <1523999293-94152-1-git-send-email-yang.shi@linux.alibaba.com> <20180417143144.b7ffb07fad28875bad546247@linux-foundation.org> From: Yang Shi Message-ID: <8f01845c-51dd-20a6-1d75-64f9de0ccb0b@linux.alibaba.com> Date: Tue, 17 Apr 2018 14:51:17 -0700 MIME-Version: 1.0 In-Reply-To: <20180417143144.b7ffb07fad28875bad546247@linux-foundation.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: viro@zeniv.linux.org.uk, nyc@holomorphy.com, mike.kravetz@oracle.com, kirill.shutemov@linux.intel.com, hughd@google.com, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-man@vger.kernel.org, mtk.manpages@gmail.com, linux-api@vger.kernel.org On 4/17/18 2:31 PM, Andrew Morton wrote: > On Wed, 18 Apr 2018 05:08:13 +0800 Yang Shi wrote: > >> Since tmpfs THP was supported in 4.8, hugetlbfs is not the only >> filesystem with huge page support anymore. tmpfs can use huge page via >> THP when mounting by "huge=" mount option. >> >> When applications use huge page on hugetlbfs, it just need check the >> filesystem magic number, but it is not enough for tmpfs. So, introduce >> ST_HUGE flag to statfs if super block has SB_HUGE set which indicates >> huge page is supported on the specific filesystem. >> >> Some applications could benefit from this change, for example QEMU. >> When use mmap file as guest VM backend memory, QEMU typically mmap the >> file size plus one extra page. If the file is on hugetlbfs the extra >> page is huge page size (i.e. 2MB), but it is still 4KB on tmpfs even >> though THP is enabled. tmpfs THP requires VMA is huge page aligned, so >> if 4KB page is used THP will not be used at all. The below /proc/meminfo >> fragment shows the THP use of QEMU with 4K page: >> >> ShmemHugePages: 679936 kB >> ShmemPmdMapped: 0 kB >> >> With ST_HUGE flag, QEMU can get huge page, then /proc/meminfo looks >> like: >> >> ShmemHugePages: 77824 kB >> ShmemPmdMapped: 6144 kB >> >> With this flag, the applications can know if huge page is supported on >> the filesystem then optimize the behavior of the applications >> accordingly. Although the similar function can be implemented in >> applications by traversing the mount options, it looks more convenient >> if kernel can provide such flag. >> >> Even though ST_HUGE is set, f_bsize still returns 4KB for tmpfs since >> THP could be split, and it also my fallback to 4KB page silently if >> there is not enough huge page. >> >> And, set the flag for hugetlbfs as well to keep the consistency, and the >> applications don't have to know what filesystem is used to use huge >> page, just need to check ST_HUGE flag. >> > Patch is simple enough, although I'm having trouble forming an opinion > about it ;) > > It will call for an update to the statfs(2) manpage. I'm not sure > which of linux-man@vger.kernel.org, mtk.manpages@gmail.com and > linux-api@vger.kernel.org is best for that, so I'd cc all three... Thanks, Andrew. Added cc to those 3 lists.