Re: Sealed memfd & no-fault mmap

[Ming Lin]

On 5/31/2021 11:24 PM, Linus Torvalds wrote:
> On Mon, May 31, 2021 at 11:13 AM Ming Lin <minggr@xxxxxxxxx> wrote:
> > OK, I borrowed code from filemap_xip.c and implemented this behavior.
>
> I think that "unmap page" is too complicated and fragile.
>
> The only page that could possibly validly be unmapped is a stale zero
> page, but that code in shmem_unmap_nofault_page() seems to try to
> handle other cases too (ie that whole page_remove_rmap() - afaik a
> zero page has no rmap).
>
> I get the feeling that the simpler thing to do is to just say "if you
> use MAP_NOSIGBUS, and you access pages that don't have a backing
> store, you will get zero pages, and they will NOT BE SYNCHRONIZED with
> the backing store possibly later being updated".
>
> IOW, just document the fact that a MAP_NOSIGBUS mapping isn't coherent
> wrt shmem contents that are expanded and filled in later.
>
> Don't try to "fix" it - because any user that uses MAP_NOSIGBUS had
> better just accept that it's not compatible with expanding the shmem
> backing store later.
>
> Keep it simple and stupid. Hmm?

Simon,

Is this "simple" solution good enough for Wayland compositor use case?