Re: [PATCH] mm: Allow shmem mappings with MREMAP_DONTUNMAP

From: Hugh Dickins
Date: Thu Mar 18 2021 - 21:59:27 EST

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:

commit 4d7672b46244abffea1953e55688c0ea143dd617
Author: Linus Torvalds <torvalds@xxxxxxxxxxx>
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@xxxxxxxxxxx>

Signed-off-by: Linus Torvalds <torvalds@xxxxxxxx>

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.