linux-kernel - Re: [PATCH] mm : update ra->ra_pages if it's NOT equal to bdi->ra

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Date:   Fri, 14 Aug 2020 18:46:29 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Zhaoyang Huang <huangzhaoyang@...il.com>,
        Roman Gushchin <klamm@...dex-team.ru>,
        Zhaoyang Huang <zhaoyang.huang@...soc.com>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm : update ra->ra_pages if it's NOT equal to
 bdi->ra_pages

On Thu, Aug 13, 2020 at 07:33:07PM -0700, Andrew Morton wrote:
> On Fri, 14 Aug 2020 10:20:11 +0800 Zhaoyang Huang <huangzhaoyang@...il.com> wrote:
> > No, What I want to fix is the file within one process's context  keeps
> > using the initialized value when it is opened and not sync with new
> > value when bdi->ra_pages changes.
> 
> So you're saying that
> 
> 	echo xxx > /sys/block/dm/queue/read_ahead_kb
> 
> does not affect presently-open files, and you believe that it should do
> so?
> 
> I guess that could be a reasonable thing to want - it's reasonable for
> a user to expect that writing to a global tunable will take immediate
> global effect.  I guess.

But it's also reasonable for someone to have written an application
assuming that the current behaviour won't change.

As I understand it, if we change net.ipv4.tcp_window_scaling, that will
take effect only for new connections, and not for existing ones.

I think the _real_ problem is that readahead never scales down, except
for EIO.

I don't have time to take on another project right now, but I think this
patch is too simplistic and has too many downsides.  Someone needs to
really think the readahead situation through properly.