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Wed, 20 Jul 2022 18:00:22 -0700 (PDT) Message-ID: <62d8a526.1c69fb81.e02e3.04ac@mx.google.com> X-Google-Original-Message-ID: <20220721010020.GA1498096@cgel.zte@gmail.com> Date: Thu, 21 Jul 2022 01:00:20 +0000 From: CGEL To: Matthew Wilcox Cc: viro@zeniv.linux.org.uk, hughd@google.com, akpm@linux-foundation.org, hch@infradead.org, hsiangkao@linux.alibaba.com, yang.yang29@zte.com.cn, axboe@kernel.dk, yangerkun@huawei.com, johannes.thumshirn@wdc.com, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Ran Xiaokai Subject: Re: [PATCH] fs: drop_caches: skip dropping pagecache which is always dirty References: <20220720022118.1495752-1-yang.yang29@zte.com.cn> <62d79a79.1c69fb81.e4cba.37f5@mx.google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: ARC-Authentication-Results: i=1; imf04.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=PcvjKGlO; dmarc=pass (policy=none) header.from=gmail.com; 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dkim=pass header.d=gmail.com header.s=20210112 header.b=PcvjKGlO; dmarc=pass (policy=none) header.from=gmail.com; spf=pass (imf04.hostedemail.com: domain of cgel.zte@gmail.com designates 209.85.216.46 as permitted sender) smtp.mailfrom=cgel.zte@gmail.com X-Stat-Signature: 8ke9cns54ammtmss65ishn8bfebhigrx X-HE-Tag: 1658365223-352060 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Jul 20, 2022 at 04:02:04PM +0100, Matthew Wilcox wrote: > On Wed, Jul 20, 2022 at 06:02:32AM +0000, CGEL wrote: > > On Wed, Jul 20, 2022 at 04:02:40AM +0100, Matthew Wilcox wrote: > > > On Wed, Jul 20, 2022 at 02:21:19AM +0000, cgel.zte@gmail.com wrote: > > > > From: Yang Yang > > > > > > > > Pagecache of some kind of fs has PG_dirty bit set once it was > > > > allocated, so it can't be dropped. These fs include ramfs and > > > > tmpfs. This can make drop_pagecache_sb() more efficient. > > > > > > Why do we want to make drop_pagecache_sb() more efficient? > > > > Some users may use drop_caches besides testing or debugging. > > This is a terrible reason. > Another case that may use drop_caches: "Migration of virtual machines will go faster if there are fewer pages to copy, so administrators would like to be able to force a virtual machine to reclaim as much memory as possible before the migration begins. " See https://lwn.net/Articles/894849/ > > For example, some systems will create a lot of pagecache when boot up > > while reading bzImage, ramdisk, docker images etc. Most of this pagecache > > is useless after boot up. It may has a longterm negative effects for the > > workload when trigger page reclaim. It is especially harmful when trigger > > direct_reclaim or we need allocate pages in atomic context. So users may > > chose to drop_caches after boot up. > > If that's actually a problem, work on fixing that.