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Date:   Fri, 14 Aug 2020 03:31:55 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Zhaoyang Huang <huangzhaoyang@gmail.com>
Cc:     Roman Gushchin <klamm@yandex-team.ru>,
        Andrew Morton <akpm@linux-foundation.org>,
        Zhaoyang Huang <zhaoyang.huang@unisoc.com>,
        "open list:MEMORY MANAGEMENT" <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm : update ra->ra_pages if it's NOT equal to
 bdi->ra_pages
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On Fri, Aug 14, 2020 at 10:26:23AM +0800, Zhaoyang Huang wrote:
> On Fri, Aug 14, 2020 at 10:20 AM Zhaoyang Huang <huangzhaoyang@gmail.com> wrote:
> >
> > On Fri, Aug 14, 2020 at 10:07 AM Matthew Wilcox <willy@infradead.org> wrote:
> > >
> > > On Fri, Aug 14, 2020 at 02:43:55AM +0100, Matthew Wilcox wrote:
> > > > On Fri, Aug 14, 2020 at 09:30:11AM +0800, Zhaoyang Huang wrote:
> > > > > file->f_ra->ra_pages will remain the initialized value since it opend, which may
> > > > > be NOT equal to bdi->ra_pages as the latter one is updated somehow(etc,
> > > > > echo xxx > /sys/block/dm/queue/read_ahead_kb).So sync ra->ra_pages to the
> > > > > updated value when sync read.
> > > >
> > > > It still ignores the work done by shrink_readahead_size_eio()
> > > > and fadvise(POSIX_FADV_SEQUENTIAL).
> > >
> > > ... by the way, if you're trying to update one particular file's readahead
> > > state, you can just call fadvise(POSIX_FADV_NORMAL) on it.
> > >
> > > If you want to update every open file's ra_pages by writing to sysfs,
> > > then just no.  We don't do that.
> > 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 mean it is just the desired behavior as having the opened file
> use the initialized value even if bdi->ra_pages changed via sysfs?

That's right.  If that's not the behaviour you want, call
fadvise(POSIX_FADV_NORMAL).

> > >
> > > You haven't said what problem you're facing, so I really can't be more
> > > helpful.