linux-kernel - Re: [PATCH] mm/madvise: allow MADV_DONTNEED to free memory that is MLOCK

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Date:   Wed, 20 Jun 2018 13:00:22 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Jason Baron <jbaron@...mai.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Mel Gorman <mgorman@...e.de>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-api@...r.kernel.org, emunson@...bm.net
Subject: Re: [PATCH] mm/madvise: allow MADV_DONTNEED to free memory that is
 MLOCK_ONFAULT

On Fri 15-06-18 15:36:07, Jason Baron wrote:
> 
> 
> On 06/13/2018 03:15 AM, Michal Hocko wrote:
> > On Wed 13-06-18 08:32:19, Vlastimil Babka wrote:
[...]
> >> BTW I didn't get why we should allow this for MADV_DONTNEED but not
> >> MADV_FREE. Can you expand on that?
> > 
> > Well, I wanted to bring this up as well. I guess this would require some
> > more hacks to handle the reclaim path correctly because we do rely on
> > VM_LOCK at many places for the lazy mlock pages culling.
> > 
> 
> The point of not allowing MADV_FREE on mlock'd pages for me was that
> with mlock and even MLOCK_ON_FAULT, one can always can always determine
> if a page is present or not (and thus avoid the major fault). Allowing
> MADV_FREE on lock'd pages breaks that assumption.

But once you have called MADV_FREE you cannot assume anything about the
content until you touch the memory again. So you can safely assume a
major fault for the worst case. Btw. why knowing whether you major fault
is important in the first place? What is an application going to do
about that information?
-- 
Michal Hocko
SUSE Labs