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From:   Yu Zhao <yuzhao@google.com>
Date:   Wed, 24 May 2023 16:54:59 -0600
Message-ID: <CAOUHufbwyCRHx07e5VrDuG60X5wRUJ795qcBOz3u+ZEmx0Bgog@mail.gmail.com>
Subject: Re: [PATCH 01/31] mm: use pmdp_get_lockless() without surplus barrier()
To:     Hugh Dickins <hughd@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        David Hildenbrand <david@redhat.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Qi Zheng <zhengqi.arch@bytedance.com>,
        Yang Shi <shy828301@gmail.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Peter Xu <peterx@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>,
        Alistair Popple <apopple@nvidia.com>,
        Ralph Campbell <rcampbell@nvidia.com>,
        Ira Weiny <ira.weiny@intel.com>,
        Steven Price <steven.price@arm.com>,
        SeongJae Park <sj@kernel.org>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Zack Rusin <zackr@vmware.com>, Jason Gunthorpe <jgg@ziepe.ca>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Pasha Tatashin <pasha.tatashin@soleen.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Minchan Kim <minchan@kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        Song Liu <song@kernel.org>,
        Thomas Hellstrom <thomas.hellstrom@linux.intel.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
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On Sun, May 21, 2023 at 10:49=E2=80=AFPM Hugh Dickins <hughd@google.com> wr=
ote:
>
> Use pmdp_get_lockless() in preference to READ_ONCE(*pmdp), to get a more
> reliable result with PAE (or READ_ONCE as before without PAE); and remove
> the unnecessary extra barrier()s which got left behind in its callers.
>
> HOWEVER: Note the small print in linux/pgtable.h, where it was designed
> specifically for fast GUP, and depends on interrupts being disabled for
> its full guarantee: most callers which have been added (here and before)
> do NOT have interrupts disabled, so there is still some need for caution.
>
> Signed-off-by: Hugh Dickins <hughd@google.com>

Acked-by: Yu Zhao <yuzhao@google.com>

The previous ask here:
https://lore.kernel.org/r/CAOUHufZo=3DfB2HcaCrj2aidLJ2zEhOpi7ou5M_7qOQiuQq8=
+wTQ@mail.gmail.com/