linux-kernel - Re: interaction of MADV_PAGEOUT with CoW anonymous mappings?

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Message-ID: <20200317172022.GD73302@google.com>
Date:   Tue, 17 Mar 2020 10:20:22 -0700
From:   Minchan Kim <minchan@...nel.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Jann Horn <jannh@...gle.com>, Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Daniel Colascione <dancol@...gle.com>,
        Dave Hansen <dave.hansen@...el.com>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: interaction of MADV_PAGEOUT with CoW anonymous mappings?

On Tue, Mar 17, 2020 at 04:58:55PM +0100, Michal Hocko wrote:
> On Tue 17-03-20 08:00:55, Minchan Kim wrote:
> > On Tue, Mar 17, 2020 at 08:12:39AM +0100, Michal Hocko wrote:
> [...]
> > > Just to make it clear, are you really suggesting to special case
> > > page_check_references for madvise path?
> > > 
> > 
> > No, (page_mapcount() >  1) checks *effectively* fixes the performance
> > bug as well as vulnerability issue.
> 
> Ahh, ok then we are on the same page. You were replying to the part
> where I have pointed out that you can control aging by these calls
> and your response suggested that this is somehow undesirable behavior or
> even a bug.

Sorry about the confusing.

I want to clarify my speaking.

If we don't have vulnerability issue Jann raised, the performance issue
Daniel pointed should be fixed by introducing a special flag in
page_check_references from madvise path to avoid cleaning of access bit
from other processes's pte. With it, we don't need to limit semantic of
MADV_PAGEOUT as "exclusive page only" so that MADV_PAGEOUT will work
*cold* shared pages as well as exclusive one.

However, since we have the vulnerability issue, *unfortunately*, we need
to make MADV_PAGEOUT's semantic working with only exclusive page. 
Thus, page_mapcount check in madvise patch will fix both issues
*effectively*.

Thanks.