linux-kernel - Re: [PATCH 1/5] mm: Introduce mm_struct.has

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Message-ID: <CAHk-=wj61s30pt8POVtKYVamYTh6h=7-_ser2Hx9sEjqeACkDA@mail.gmail.com>
Date:   Sun, 27 Sep 2020 11:45:30 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Leon Romanovsky <leonro@...dia.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, Peter Xu <peterx@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Michal Hocko <mhocko@...e.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Kirill Shutemov <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>,
        Christoph Hellwig <hch@....de>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned

On Sun, Sep 27, 2020 at 11:16 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> Btw, I'm not convinced about the whole "turn the pte read-only and
> then back". If the fork races with another thread doing a pinning
> fast-GUP on another CPU, there are memory ordering issues etc too.
> That's not necessarily visible on x86 (the "turn read-only being a
> locked op will force serialization), but it all looks dodgy as heck.

.. looking at it more, I also think it could possibly lose the dirty
bit for the case where another CPU did a HW dirty/accessed bit update
in between the original read of the pte, and then us writing back the
writable pte again.

Us holding the page table lock means that no _software_ accesses will
happen to the PTE, but dirty/accessed bits can be modified by hardware
despite the lock.

That is, of course, a completely crazy case, and I think that since we
only do this for a COW mapping, and only do the PTE changes if the pte
was writable, the pte will always have been dirty already.

So I don't think it's an _actual_ bug, but it's another "this looks
dodgy as heck" marker. It may _work_, but it sure ain't pretty.

But despite having looked at this quite a bit, I don't see anything
that looks actively wrong, so I think the series is fine. This is more
of a note for people to perhaps think about.

                Linus