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

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Date:   Fri, 8 Jun 2018 12:57:17 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Jason Baron <jbaron@...mai.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Mel Gorman <mgorman@...e.de>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH] mm/madvise: allow MADV_DONTNEED to free memory that is
 MLOCK_ONFAULT

On Fri,  8 Jun 2018 14:56:52 -0400 Jason Baron <jbaron@...mai.com> wrote:

> In order to free memory that is marked MLOCK_ONFAULT, the memory region
> needs to be first unlocked, before calling MADV_DONTNEED. And if the region
> is to be reused as MLOCK_ONFAULT, we require another call to mlock2() with
> the MLOCK_ONFAULT flag.
> 
> Let's simplify freeing memory that is set MLOCK_ONFAULT, by allowing
> MADV_DONTNEED to work directly for memory that is set MLOCK_ONFAULT. The
> locked memory limits, tracked by mm->locked_vm do not need to be adjusted
> in this case, since they were charged to the entire region when
> MLOCK_ONFAULT was initially set.

Seems useful.

Is a manpage update planned?

Various updates to tools/testing/selftests/vm/* seem appropriate.

> Further, I don't think allowing MADV_FREE for MLOCK_ONFAULT regions makes
> sense, since the point of MLOCK_ONFAULT is for userspace to know when pages
> are locked in memory and thus to know when page faults will occur.

This sounds non-backward-compatible?