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Wed, 31 May 2023 10:06:59 -0700 (PDT) MIME-Version: 1.0 References: <35e983f5-7ed3-b310-d949-9ae8b130cdab@google.com> <88c445ae-552-5243-31a4-2674bac62d4d@google.com> In-Reply-To: <88c445ae-552-5243-31a4-2674bac62d4d@google.com> From: Jann Horn Date: Wed, 31 May 2023 19:06:23 +0200 Message-ID: Subject: Re: [PATCH 01/12] mm/pgtable: add rcu_read_lock() and rcu_read_unlock()s To: Hugh Dickins Cc: Andrew Morton , Mike Kravetz , Mike Rapoport , "Kirill A. Shutemov" , Matthew Wilcox , David Hildenbrand , Suren Baghdasaryan , Qi Zheng , Yang Shi , Mel Gorman , Peter Xu , Peter Zijlstra , Will Deacon , Yu Zhao , Alistair Popple , Ralph Campbell , Ira Weiny , Steven Price , SeongJae Park , Naoya Horiguchi , Christophe Leroy , Zack Rusin , Jason Gunthorpe , Axel Rasmussen , Anshuman Khandual , Pasha Tatashin , Miaohe Lin , Minchan Kim , Christoph Hellwig , Song Liu , Thomas Hellstrom , Russell King , "David S. Miller" , Michael Ellerman , "Aneesh Kumar K.V" , Heiko Carstens , Christian Borntraeger , Claudio Imbrenda , Alexander Gordeev , linux-arm-kernel@lists.infradead.org, sparclinux@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 9A77A80024 X-Rspam-User: X-Stat-Signature: 3uapnie3dqbruc41n3wpadc37pbr6jcc X-Rspamd-Server: rspam03 X-HE-Tag: 1685552820-48504 X-HE-Meta: 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 dW9IvVyu EhYydrIaLV5Dsc/K60Y2iav2T1a79kihVMMx/O2EklzxVHsLbhqvQ9rix+Faqm+38ZGuaJse9CkO3EuI7rqYB7FpkYhMLHTrZLPAr2ekliyZ1xXQAF0s39AdBT09DofOKsxwtEXJnBpkdY6cXtZIOMz/raNdgBrplr+tTQL09U6H3V3w1X7V20f5Lj1qv0C5/6ZGmf6eL/55KTGGCWrayEmHLq029ycZwbYpG9v9EC30n3owY8JNoV4iOKOVRHZ4TToCdSkM+kki78urwalfCUSvI6T/OMC67stStJDiA/Hei2fE= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, May 29, 2023 at 8:15=E2=80=AFAM Hugh Dickins 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)= ;