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From:   Jann Horn <jannh@google.com>
Date:   Wed, 31 May 2023 19:06:23 +0200
Message-ID: <CAG48ez0tnYTVjr7zw3Vp4GTcQ=960EodatjqE5bM9a3EVYM16Q@mail.gmail.com>
Subject: Re: [PATCH 01/12] mm/pgtable: add rcu_read_lock() and rcu_read_unlock()s
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>, Yu Zhao <yuzhao@google.com>,
        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>,
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        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
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        Christian Borntraeger <borntraeger@linux.ibm.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        linux-arm-kernel@lists.infradead.org, sparclinux@vger.kernel.org,
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On Mon, May 29, 2023 at 8:15=E2=80=AFAM Hugh Dickins <hughd@google.com> wro=
te:
> Before putting them to use (several commits later), add rcu_read_lock()
> to pte_offset_map(), and rcu_read_unlock() to pte_unmap().  Make this a
> separate commit, since it risks exposing imbalances: prior commits have
> fixed all the known imbalances, but we may find some have been missed.
[...]
> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
> index c7ab18a5fb77..674671835631 100644
> --- a/mm/pgtable-generic.c
> +++ b/mm/pgtable-generic.c
> @@ -236,7 +236,7 @@ pte_t *__pte_offset_map(pmd_t *pmd, unsigned long add=
r, pmd_t *pmdvalp)
>  {
>         pmd_t pmdval;
>
> -       /* rcu_read_lock() to be added later */
> +       rcu_read_lock();
>         pmdval =3D pmdp_get_lockless(pmd);
>         if (pmdvalp)
>                 *pmdvalp =3D pmdval;

It might be a good idea to document that this series assumes that the
first argument to __pte_offset_map() is a pointer into a second-level
page table (and not a local copy of the entry) unless the containing
VMA is known to not be THP-eligible or the page table is detached from
the page table hierarchy or something like that. Currently a bunch of
places pass references to local copies of the entry, and while I think
all of these are fine, it would probably be good to at least document
why these are allowed to do it while other places aren't.

$ vgrep 'pte_offset_map(&'
Index File                  Line Content
    0 arch/sparc/mm/tlb.c    151 pte =3D pte_offset_map(&pmd, vaddr);
    1 kernel/events/core.c  7501 ptep =3D pte_offset_map(&pmd, addr);
    2 mm/gup.c              2460 ptem =3D ptep =3D pte_offset_map(&pmd, add=
r);
    3 mm/huge_memory.c      2057 pte =3D pte_offset_map(&_pmd, haddr);
    4 mm/huge_memory.c      2214 pte =3D pte_offset_map(&_pmd, haddr);
    5 mm/page_table_check.c  240 pte_t *ptep =3D pte_offset_map(&pmd, addr)=
;