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Subject: Re: [PATCH] mm/mincore: allow for making sys_mincore() privileged
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Josh Snyder <joshs@netflix.com>,
        Dominique Martinet <asmadeus@codewreck.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Dave Chinner <david@fromorbit.com>,
        Jiri Kosina <jikos@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Jann Horn <jannh@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Michal Hocko <mhocko@suse.com>, Linux-MM <linux-mm@kvack.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>
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From:   Vlastimil Babka <vbabka@suse.cz>
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Date:   Fri, 18 Jan 2019 19:58:13 +0100
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On 1/18/19 5:49 AM, Linus Torvalds wrote:
> On Fri, Jan 18, 2019 at 9:45 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>>
>> Or maybe we could resort to the 5.0-rc1 page table check (that is now being
>> reverted) but only in cases when we are not allowed the page cache residency
>> check? Or would that be needlessly complicated?
> 
> I think it would  be good fallback semantics, but I'm not sure it's
> worth it. Have you tried writing a patch for it? I don't think you'd
> want to do the check *when* you find a hole, so you'd have to do it
> upfront and then pass the cached data down with the private pointer
> (or have a separate "struct mm_walk" structure, perhaps?
> 
> So I suspect we're better off with the patch we have. But if somebody
> *wants* to try to do that fancier patch, and it doesn't look
> horrendous, I think it might be the "quality" solution.

I thought to drop the idea because of leaking that page has been
evicted, but then I realized there are other ways to check for that
anyway in /proc. So I'll try, but probably not until after next week. If
somebody else wants to, they are welcome. As you say, the current
solution should be ok, so that would be a patch on top anyway, for
bisectability etc.

Vlastimil

>               Linus
>