linux-kernel - Re: [PATCH 1/4] mm/mprotect: Retry on pmd_trans

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Date:   Sun, 4 Jun 2023 19:58:18 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Yang Shi <shy828301@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        David Hildenbrand <david@...hat.com>,
        Alistair Popple <apopple@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Johannes Weiner <hannes@...xchg.org>,
        John Hubbard <jhubbard@...dia.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Muhammad Usama Anjum <usama.anjum@...labora.com>,
        Hugh Dickins <hughd@...gle.com>,
        Mike Rapoport <rppt@...nel.org>
Subject: Re: [PATCH 1/4] mm/mprotect: Retry on pmd_trans_unstable()

On Fri, Jun 02, 2023 at 07:04:48PM -0700, Yang Shi wrote:
> On Fri, Jun 2, 2023 at 4:06 PM Peter Xu <peterx@...hat.com> wrote:
> >
> > When hit unstable pmd, we should retry the pmd once more because it means
> > we probably raced with a thp insertion.
> >
> > Skipping it might be a problem as no error will be reported to the caller.
> > I assume it means the user will expect prot changed (e.g. mprotect or
> > userfaultfd wr-protections) applied but it's actually not.
> 
> IIRC, mprotect() holds write mmap_lock, so it should not matter. PROT
> NUMA holds read mmap_lock, but returning 0 also doesn't matter (of
> course retry is fine too). just skip that 2M area.

True.

> The userfaultfd-wp is your call :-)

Yeah I think uffd should still be a problem.  I'll reword the commit
message (by dropping mprotect example) in the new version.

If you have time feel free to have a look at patch 4, where I think it's a
bug for pagemap too (I didn't check as close as all the rest; the memcg one
might be suspecious, that's also in patch 4).

Thanks!

-- 
Peter Xu