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Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Guo Ren , Brian Cain , Geert Uytterhoeven , Dinh Nguyen , Simon Schuster , Jonas Bonn , Stefan Kristiansson , Stafford Horne , Yoshinori Sato , Rich Felker , John Paul Adrian Glaubitz , Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti , Russell King , Vineet Gupta , Michal Simek , Chris Zankel , Max Filippov , Will Deacon , "Aneesh Kumar K.V" , Nick Piggin , Peter Zijlstra , "David S. Miller" , Andreas Larsson , Richard Henderson , Matt Turner , Magnus Lindholm , Catalin Marinas , Mark Rutland , Huacai Chen , WANG Xuerui , Thomas Bogendoerfer , "James E.J. Bottomley" , Helge Deller , Madhavan Srinivasan , Michael Ellerman , "Christophe Leroy (CS GROUP)" , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Christian Borntraeger , Sven Schnelle , Richard Weinberger , Anton Ivanov , Johannes Berg , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Arnd Bergmann , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jason Gunthorpe , John Hubbard , Peter Xu , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-csky@vger.kernel.org, linux-hexagon@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-openrisc@vger.kernel.org, linux-sh@vger.kernel.org, linux-riscv@lists.infradead.org, linux-arm-kernel@lists.infradead.org, linux-snps-arc@lists.infradead.org, linux-arch@vger.kernel.org, sparclinux@vger.kernel.org, linux-alpha@vger.kernel.org, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-um@lists.infradead.org, Hugh Dickins , Qi Zheng Subject: Re: [PATCH 12/12] mm: change the contract for free_pgtables(), update docs Message-ID: References: <20260901-rcu-pagetable-freeing-v1-0-5456a81c8212@kernel.org> <20260901-rcu-pagetable-freeing-v1-12-5456a81c8212@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260901-rcu-pagetable-freeing-v1-12-5456a81c8212@kernel.org> X-Stat-Signature: k35np49n1yro4q7yogq19js9aknu6gur X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 508541A0008 X-Rspam-User: X-HE-Tag: 1788271059-592185 X-HE-Meta: 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 p4QEfhIv BP/CDa5YyhYxEwzgsq2B6oKI22XSYenqpvKTx7VlG4/rSBP6N6RrEe1zo4ythgmHlLn8A2WS9LjBFutRa3WKUMtLkKTZA9qlu88mJ36RBuiy2Az3SMlsLUm3Ft713ifT7HPlauK6M8m3x9XVjVe94gjctkCpllqX92gWgpniJW8WE2pwZrdAxRkXosm1u31Xsl4i2Wg4ZrBNgKcVJX3M8x3G2BTzFQTM3zq6NWjj6itvtvdFkXNv0vwW51xdZZlLUV+aGC1n6gCuVnDt5eOvI/Qh8DUh1owjIED1Oty8w/dd3mf6XCx0cQw6MandHcOoYg0aXwmWCFCdRjR7usGe2IRXkG/Bwp3XafUYODfEzPLkUXUKXZOQEAmwqgI8oH4wUdxT1NXz8eX8SjgX5pfj2zQuOYQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Sep 01, 2026 at 12:01:32PM +0100, Lorenzo Stoakes (ARM) wrote: > diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c > index b91b1a98029c..ff8ff3706485 100644 > --- a/mm/pgtable-generic.c > +++ b/mm/pgtable-generic.c > @@ -386,9 +386,21 @@ pte_t *pte_offset_map_rw_nolock(struct mm_struct *mm, pmd_t *pmd, > * be read-only/read-write protected. > * > * Note that free_pgtables(), used after unmapping detached vmas, or when > - * exiting the whole mm, does not take page table lock before freeing a page > - * table, and may not use RCU at all: "outsiders" like khugepaged should avoid > - * pte_offset_map() and co once the vma is detached from mm or mm_users is zero. > + * exiting the whole mm, does not take the page table lock before freeing a > + * table. > + * > + * However, the PMD entry is cleared first, and the table freed only after > + * an RCU grace period, so a walker that mapped the table under > + * rcu_read_lock() stays safe, and the pmd_same() recheck in > + * pte_offset_map_lock() detects the teardown. > + * > + * Therefore it is safe for "outsiders" like khugepaged to use > + * pte_offset_map() and co. for VMAs that might be undergoing page table > + * teardown. Is it strictly true? free_pte_range() clears the pmd without taking the PTL, so the pmd_same() recheck in pte_offset_map_lock() can pass and the pmd gets cleared right after. Readers are fine -- __pte_offset_map() holds rcu_read_lock() until pte_unmap(), so the table cannot go away. khugepaged is an odd example here. collapse_pte_mapped_thp() and retract_page_tables() don't rely on this recheck -- they take pmd_lock() plus the ptl and do their own pmd_same() under both. Can we say walks are safe and leave the write rule where it is? -- Kiryl Shutsemau / Kirill A. Shutemov