linux-kernel - Re: [PATCH v2] mm: Add MREMAP

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Message-ID: <20200210103556.wz72op4g554qn5jm@box>
Date:   Mon, 10 Feb 2020 13:35:56 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Brian Geffon <bgeffon@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, linux-api@...r.kernel.org,
        Andy Lutomirski <luto@...capital.net>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Sonny Rao <sonnyrao@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Yu Zhao <yuzhao@...gle.com>, Jesse Barnes <jsbarnes@...gle.com>
Subject: Re: [PATCH v2] mm: Add MREMAP_DONTUNMAP to mremap().

On Fri, Feb 07, 2020 at 12:42:15PM -0800, Brian Geffon wrote:
> Hi Kirill,
> I started a new thread https://lkml.org/lkml/2020/2/7/640 for my v4
> patch. But I wanted to quickly address your comments. Regarding the
> concern around the rmap, no changes actually need to be made. If we
> were to unlink_anon_vma(vma) and then set vma->anon_vma = NULL, that
> would be fine but then as soon as there was a fault the same anon_vma
> would be attached since it's a private anonymous mapping. So there is
> really nothing to do regarding the rmap.

Okay.

My worry was that we create a new VMA with the same anon_vma *and*
vm_pgoff, but I just realized we can do the same with the current
mremap(2) plus following mmap(2) in the old place. So it's not regression.

I guess we are fine here.

> I considered the two flag approach but since I could not come up with
> a concrete use case of MREMAP_MUSTMOVE I decided to just leave the
> single MREMAP_DONTUNMAP flag, the two flag approach would be only for
> clarifying the operations so I'm not sure it's worth it. (Still trying
> to come up with a better name). But I've attached a man page diff to
> the latest patch.

At least it doesn't have 'FIXED' semantics forced on user. It's fine with
me.

-- 
 Kirill A. Shutemov