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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 3 Jun 2021 11:25:15 -0700
Message-ID: <CAHk-=wiHJ2GF503wnhCC4jsaSWNyq5=NqOy7jpF_v_t82AY0UA@mail.gmail.com>
Subject: Re: [PATCH 2/2] mm: adds NOSIGBUS extension for out-of-band shmem read
To:     Hugh Dickins <hughd@google.com>
Cc:     Ming Lin <mlin@kernel.org>, Simon Ser <contact@emersion.fr>,
        Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>
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On Wed, Jun 2, 2021 at 5:46 PM Hugh Dickins <hughd@google.com> wrote:
>
> Ideally you can simply call do_anonymous_page() from __do_fault()
> in the VM_FAULT_SIGBUS on VM_NOSIGBUS case.

Heh.

We're actually then back to my original patch.

That one doesn't handle shared mappings (even read-only ones), for the
simple reason that do_anonymous_page() refuses to insert anonymous
pages into a shared mapping, and has

        /* File mapping without ->vm_ops ? */
        if (vma->vm_flags & VM_SHARED)
                return VM_FAULT_SIGBUS;

at the very top.

But yes, if we just remove that check, I think my original patch
should actually "JustWork(tm)".

I'm attaching it again, with old name and old commentary (ie that

    /* FIXME! We don't have a VM_NOFAULT bit */

should just be replaced with that VM_NOSIGBUS bit instead, and the
#if'ed out region should be enabled.

Oh, and we need to think hard about one more case: mprotect().

In particular, I think the attached patch fails horribly for the case
of a shared mapping that starts out read-only, then inserts a zero
page, then somebody does mprotect(MAP_WRITE), and then writes to the
page. I haven't checked what the write protect fault handler does, but
I think that for a shared mapping it will just make the page dirty and
writable.

Which would be horribly wrong for VM_NOSIGBUS.

So that support infrastructure that adds MAP_NOSIGBUS, and checks that
it is only done on a read-only mapping, also has to make sure that it
clears the VM_MAYWRITE bit when it sets VM_NOSIGBUS.

That way mprotect can't then later make it writable.

Hugh, comments on this approach?

Again: this patch is my *OLD* one, I didn't try to update it to the
new world order. It requires

 - Ming's MAP_NOSIGBUS ccode

 - removal of that "File mapping without ->vm_ops" case

 - that FIXME fixed and name updated

 - and that VM_MAYWRITE clearing if VM_NOSIGBUS is set, to avoid the
mprotect issue.

Hmm?

                  Linus

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