linux-kernel - Re: [PATCH 0/5] 4.14 backports of fixes for "CoW after fork() issue"

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Date:   Wed, 21 Apr 2021 13:01:34 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Mikulas Patocka <mpatocka@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>, Peter Xu <peterx@...hat.com>,
        stable <stable@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jann Horn <jannh@...gle.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>, Shaohua Li <shli@...com>,
        Nadav Amit <namit@...are.com>, Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Android Kernel Team <kernel-team@...roid.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>
Subject: Re: [PATCH 0/5] 4.14 backports of fixes for "CoW after fork() issue"

On Wed, Apr 7, 2021 at 2:53 PM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> On Wed, Apr 7, 2021 at 12:23 PM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > On Wed, Apr 7, 2021 at 11:47 AM Mikulas Patocka <mpatocka@...hat.com> wrote:
> > >
> > > So, we fixed it, but we don't know why.
> > >
> > > Peter Xu's patchset that fixed it is here:
> > > https://lore.kernel.org/lkml/20200821234958.7896-1-peterx@redhat.com/
> >
> > Yeah, that's the part that ends up being really painful to backport
> > (with all the subsequent fixes too), so the 4.14 people would prefer
> > to avoid it.
> >
> > But I think that if it's a "requires dax pmem and ptrace on top", it
> > may simply be a non-issue for those users. Although who knows - maybe
> > that ends up being a real issue on Android..
>
> A lot to digest, so I need to do some reading now. Thanks everyone!

After a delay due to vacation I prepared backports of 17839856fd58
("gup: document and work around "COW can break either way" issue") for
4.14 and 4.19 kernels. As Linus pointed out, uffd-wp was introduced
later in 5.7, so is not an issue for 4.x kernels. The issue with THPs
is still unresolved, so with or without this patch it's still there
(Android is not affected by this since we do not use THPs with older
kernels).
Andrea pointed out that there are other issues and to properly fix
them his COR approach is needed. However it has not been accepted yet,
so I can't really backport it. I'll be happy to do that though if it
is accepted in the future.

Peter, you mentioned https://lkml.org/lkml/2020/8/10/439 patch to
distinguish real writes vs enforced COW read requests, however I also
see that you had a later version of this patch here:
https://lore.kernel.org/patchwork/patch/1286506/. Which one should I
backport? Or is it not needed in the absence of uffd-wp support in the
earlier kernels?
Thanks,
Suren.

>
> >
> >             Linus