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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 16 Jan 2019 17:00:25 +1200
Message-ID: <CAHk-=wgF9p9xNzZei_-ejGLy1bJf4VS1C5E9_V0kCTEpCkpCTQ@mail.gmail.com>
Subject: Re: [PATCH] mm/mincore: allow for making sys_mincore() privileged
To:     Josh Snyder <joshs@netflix.com>
Cc:     Dominique Martinet <asmadeus@codewreck.org>,
        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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On Wed, Jan 16, 2019 at 12:42 PM Josh Snyder <joshs@netflix.com> wrote:
>
> For Netflix, losing accurate information from the mincore syscall would
> lengthen database cluster maintenance operations from days to months.  We
> rely on cross-process mincore to migrate the contents of a page cache from
> machine to machine, and across reboots.

Ok, this is the kind of feedback we need, and means I guess we can't
just use the mapping existence for mincore.

The two other ways that we considered were:

 (a) owner of the file gets to know cache information for that file.

 (b) having the fd opened *writably* gets you cache residency information.

Sadly, taking a look at happycache, you open the file read-only, so
(b) doesn't work.

Judging just from the source code, I can't tell how the user ownership
works. Any input on that?

And if you're not the owner of the file, do you have another
suggestion for that "Yes, I have the right to see what's in-core for
this file". Because the problem is literally that if it's some random
read-only system file, the kernel shouldn't leak access patterns to
it..

                     Linus