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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 18 Jan 2019 16:49:30 +1200
Message-ID: <CAHk-=wi0MXm4zTC6jjS1TBfbHW_sQq_OcyfeLBNGJ29m88pt+g@mail.gmail.com>
Subject: Re: [PATCH] mm/mincore: allow for making sys_mincore() privileged
To:     Vlastimil Babka <vbabka@suse.cz>
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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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.

              Linus