linux-kernel - Re: [PATCH] mm: Allow shmem mappings with MREMAP

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Message-ID: <alpine.LSU.2.11.2103181828300.3440@eggly.anvils>
Date:   Thu, 18 Mar 2021 18:58:23 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Peter Xu <peterx@...hat.com>
cc:     Hugh Dickins <hughd@...gle.com>, Brian Geffon <bgeffon@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andy Lutomirski <luto@...capital.net>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Sonny Rao <sonnyrao@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Dmitry Safonov <dima@...sta.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Alejandro Colomar <alx.manpages@...il.com>
Subject: Re: [PATCH] mm: Allow shmem mappings with MREMAP_DONTUNMAP

On Tue, 16 Mar 2021, Peter Xu wrote:
> 
> I'm curious whether it's okay to expand MREMAP_DONTUNMAP to PFNMAP too..
> E.g. vfio maps device MMIO regions with both VM_DONTEXPAND|VM_PFNMAP, to me it
> makes sense to allow the userspace to get such MMIO region remapped/duplicated
> somewhere else as long as the size won't change.  With the strict check as
> above we kill all those possibilities.
> 
> Though in that case we'll still need commits like cd544fd1dc92 to protect any
> customized ->mremap() when they're not supported.

It would take me many hours to arrive at a conclusion on that:
I'm going to spend the time differently, and let whoever ends up
wanting MREMAP_DONTUNMAP on a VM_PFNMAP area research the safety
of that for existing users.

I did look to see what added VM_PFNMAP to the original VM_DONTEXPAND:

v2.6.15
commit 4d7672b46244abffea1953e55688c0ea143dd617
Author: Linus Torvalds <torvalds@...osdl.org>
Date:   Fri Dec 16 10:21:23 2005 -0800

Make sure we copy pages inserted with "vm_insert_page()" on fork

The logic that decides that a fork() might be able to avoid copying a VM
area when it can be re-created by page faults didn't know about the new
vm_insert_page() case.

Also make some things a bit more anal wrt VM_PFNMAP.

Pointed out by Hugh Dickins <hugh@...itas.com>

Signed-off-by: Linus Torvalds <torvalds@...l.org>

So apparently I do bear some anal responsibility.  My concern seems
to have been that in those days an unexpected page fault in a special
driver area would end up allocating an anonymous page, which would
never get freed later.  Nowadays it looks like there's a SIGBUS for
the equivalent situation.

So probably VM_DONTEXPAND is less important than it was, and the
additional VM_PFNMAP safety net no longer necessary, and you could
strip it out of the old size check and Brian's new dontunmap check.

But I give no guarantee: I don't know VM_PFNMAP users at all well.

Hugh