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

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Date:   Tue, 12 Jun 2018 09:46:46 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Jason Baron <jbaron@...mai.com>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Vlastimil Babka <vbabka@...e.cz>,
        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 Mon 11-06-18 12:23:58, Jason Baron wrote:
> On 06/11/2018 11:03 AM, Michal Hocko wrote:
> > So can we start discussing whether we want to allow MADV_DONTNEED on
> > mlocked areas and what downsides it might have? Sure it would turn the
> > strong mlock guarantee to have the whole vma resident but is this
> > acceptable for something that is an explicit request from the owner of
> > the memory?
> > 
> 
> If its being explicity requested by the owner it makes sense to me. I
> guess there could be a concern about this breaking some userspace that
> relied on MADV_DONTNEED not freeing locked memory?

Yes, this is always the fear when changing user visible behavior.  I can
imagine that a userspace allocator calling MADV_DONTNEED on free could
break. The same would apply to MLOCK_ONFAULT/MCL_ONFAULT though. We
have the new flag much shorter so the probability is smaller but the
problem is very same. So I _think_ we should treat both the same because
semantically they are indistinguishable from the MADV_DONTNEED POV. Both
remove faulted and mlocked pages. Mlock, once applied, should guarantee
no later major fault and MADV_DONTNEED breaks that obviously.

So the more I think about it the more I am worried about this but I am
more and more convinced that making ONFAULT special is just a wrong way
around this.
-- 
Michal Hocko
SUSE Labs