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Subject: Re: [PATCH 3/3] mm/mincore: provide mapped status when cached status
 is not allowed
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linux-api@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        Jann Horn <jannh@google.com>, Jiri Kosina <jikos@kernel.org>,
        Dominique Martinet <asmadeus@codewreck.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Dave Chinner <david@fromorbit.com>,
        Kevin Easton <kevin@guarana.org>,
        Matthew Wilcox <willy@infradead.org>,
        Cyril Hrubis <chrubis@suse.cz>, Tejun Heo <tj@kernel.org>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Daniel Gruss <daniel@gruss.cc>, Jiri Kosina <jkosina@suse.cz>,
        Josh Snyder <joshs@netflix.com>
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 <99ee4d3e-aeb2-0104-22be-b028938e7f88@suse.cz>
 <20190201091152.GG11599@dhcp22.suse.cz>
From:   Vlastimil Babka <vbabka@suse.cz>
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Date:   Fri, 1 Feb 2019 10:27:38 +0100
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On 2/1/19 10:11 AM, Michal Hocko wrote:
> On Fri 01-02-19 10:04:23, Vlastimil Babka wrote:
>> 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.
> 
> 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.

> You shouldn't be able to sniff on
> /proc/$vicimt/smaps as an attacker.