linux-kernel - Re: [PATCH v1] mm: fix PageAnonExclusive clearing racing with concurrent RCU GUP-fast

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Message-Id: <20220901153512.a59e9e584fb00a350788f56e@linux-foundation.org>
Date:   Thu, 1 Sep 2022 15:35:12 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        stable@...r.kernel.org, Jason Gunthorpe <jgg@...dia.com>,
        John Hubbard <jhubbard@...dia.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Hugh Dickins <hughd@...gle.com>, Peter Xu <peterx@...hat.com>,
        Alistair Popple <apopple@...dia.com>,
        Nadav Amit <namit@...are.com>, Yang Shi <shy828301@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Christoph von Recklinghausen <crecklin@...hat.com>,
        Don Dutile <ddutile@...hat.com>
Subject: Re: [PATCH v1] mm: fix PageAnonExclusive clearing racing with
 concurrent RCU GUP-fast

On Thu,  1 Sep 2022 10:35:59 +0200 David Hildenbrand <david@...hat.com> wrote:

> The possible issues due to reordering are of theoretical nature so far
> and attempts to reproduce the race failed.
> 
> Especially the "no PTE change" case isn't the common case, because we'd
> need an exclusive anonymous page that's mapped R/O and the PTE is clean
> in KSM code -- and using KSM with page pinning isn't extremely common.
> Further, the clear+TLB flush we used for now implies a memory barrier.
> So the problematic missing part should be the missing memory barrier
> after pinning but before checking if the PTE changed.

Obscure bug, large and tricky patch.  Is a -stable backport really
justifiable?