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

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Message-ID: <20180613083707.GE13364@dhcp22.suse.cz>
Date:   Wed, 13 Jun 2018 10:37:07 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Jason Baron <jbaron@...mai.com>, 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 Wed 13-06-18 09:51:23, Vlastimil Babka wrote:
> On 06/13/2018 09:15 AM, Michal Hocko wrote:
> > On Wed 13-06-18 08:32:19, Vlastimil Babka wrote:
[...]
> >> I think more concerning than guaranteeing no later major fault is
> >> possible data loss, e.g. replacing data with zero-filled pages.
> > 
> > But MADV_DONTNEED is an explicit call for data loss. Or do I miss your
> > point?
> 
> My point is that if somebody is relying on MADV_DONTNEED not affecting
> mlocked pages, the consequences will be unexpected data loss, not just
> extra page faults.

OK, I see your point now. I would consider this an application bug
though. Calling MADV_DONTNEED and wondering that the content is gone is,
ehm, questionable at best. Why would anybody do that in the first place?

Anyway, I think that we cannot change the behavior because of mlockall
semantic as mentioned earlier.
-- 
Michal Hocko
SUSE Labs