linux-kernel - Re: [PATCH] fs: drop_caches: skip dropping pagecache which is always dirty

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Date:   Wed, 20 Jul 2022 07:01:56 +0000
From:   CGEL <cgel.zte@...il.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Matthew Wilcox <willy@...radead.org>, viro@...iv.linux.org.uk,
        hughd@...gle.com, akpm@...ux-foundation.org,
        hsiangkao@...ux.alibaba.com, yang.yang29@....com.cn,
        axboe@...nel.dk, yangerkun@...wei.com, johannes.thumshirn@....com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Ran Xiaokai <ran.xiaokai@....com.cn>
Subject: Re: [PATCH] fs: drop_caches: skip dropping pagecache which is always
 dirty

On Tue, Jul 19, 2022 at 11:04:36PM -0700, Christoph Hellwig wrote:
> On Wed, Jul 20, 2022 at 06:02:32AM +0000, CGEL wrote:
> > 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.
> 
> It is purely a debug interface.  If you want to drop specific page cache
> that needs to be done through madvise.

It's not easy for users to use madvise in complex system, it takes cost.
Since drop_caches is not forbidden, and users may want simply use it in
the scene above mail described.