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Date:   Fri, 14 Aug 2020 13:10:34 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Zhaoyang Huang <huangzhaoyang@gmail.com>
Cc:     Roman Gushchin <klamm@yandex-team.ru>,
        Zhaoyang Huang <zhaoyang.huang@unisoc.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm : sync ra->ra_pages with bdi->ra_pages
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On Fri, 14 Aug 2020 17:03:44 +0800 Zhaoyang Huang <huangzhaoyang@gmail.com> wrote:

> Some system(like android) will turbo read during startup via expanding the
> readahead window and then set it back to normal(128kb as usual). However, some
> files in the system process context will keep to be opened since it is opened
> up and has no chance to sync with the updated value as it is almost impossible
> to change the files attached to the inode(processes are unaware of these things)

How about making VM_READAHEAD_PAGES a variable?  Provide a kernel boot
parameter to alter the default setting.  Userspace can then restore the
usual value later in boot?