linux-kernel - Re: [PATCH 3/3] mm/mincore: provide mapped status when cached status is not allowed

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Message-ID: <nycvar.YFH.7.76.1902062111120.11598@cbobk.fhfr.pm>
Date:   Wed, 6 Feb 2019 21:14:26 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>,
        Linus Torvalds <torvalds@...ux-foundation.org>
cc:     Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-api@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Jann Horn <jannh@...gle.com>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Chinner <david@...morbit.com>,
        Kevin Easton <kevin@...rana.org>,
        Matthew Wilcox <willy@...radead.org>,
        Cyril Hrubis <chrubis@...e.cz>, Tejun Heo <tj@...nel.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Daniel Gruss <daniel@...ss.cc>, Josh Snyder <joshs@...flix.com>
Subject: Re: [PATCH 3/3] mm/mincore: provide mapped status when cached status
 is not allowed

On Fri, 1 Feb 2019, Vlastimil Babka wrote:

> > Well, but rss update will not tell you that the page has been faulted in
> > which is the most interesting part.
> 
> Sure, but the patch doesn't add back that capability neither. It allows
> to recognize page being reclaimed, and I argue you can infer that from
> rss change as well. That change is mentioned in the last paragraph in
> changelog, and I thought "add a hard to evaluate side channel" in your
> reply referred to that. It doesn't add back the "original" side channel
> to detect somebody else accessed a page.

On Fri, 1 Feb 2019, Vlastimil Babka wrote:

> > Is this really worth it? Do we know about any specific usecase that
> > would benefit from this change? TBH I would rather wait for the report
> > than add a hard to evaluate side channel.
> 
> Well it's not that complicated IMHO. Linus said it's worth trying, so
> let's see how he likes the result. The side channel exists anyway as
> long as process can e.g. check if its rss shrinked, and I doubt we are
> going to remove that possibility.

Linus, do you have any opinion here?

I have a hunch that mm maintainers are keeping this on a backburner 
because there might still open question(s) in the air.

Thanks,

-- 
Jiri Kosina
SUSE Labs