linux-kernel - Re: Sealed memfd & no-fault mmap

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Date:   Tue, 1 Jun 2021 00:08:48 -0700
From:   Ming Lin <minggr@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Hugh Dickins <hughd@...gle.com>, Simon Ser <contact@...rsion.fr>,
        Peter Xu <peterx@...hat.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Will Deacon <will@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Herrmann <dh.herrmann@...il.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Greg Kroah-Hartman <greg@...ah.com>,
        "tytso@....edu" <tytso@....edu>
Subject: Re: Sealed memfd & no-fault mmap

On 5/31/2021 11:24 PM, Linus Torvalds wrote:
> On Mon, May 31, 2021 at 11:13 AM Ming Lin <minggr@...il.com> 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?