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b=lvFYpHpVMwPBfEiIuvmYeJUUzULg6cdEWSx4iWYVWk727A5rZV5bpt/5Sjkl2pSTJ +koGcSegVzDsG20kO8t41OXmh6KWyi8RNfZl2ErJwNIuTh23tjzoeT8Dv37tsGeg79 61gp8tpMx2xIK3zP8kYJIEm2UGPz0MsuiqmNKxY9v5C+ubE70585sEQgHP0rrKt73G 695MVjPFlFxGNvVaOlYp2A83auWFA74egHtR5nruPWwDU0nzqWrYwTTl+K4hgYPEPV DQ3pwEy3zgSLOIKKJ8W+2SiDFy6P5qvxi1PY+qb9cXOQdaC3iHQ+VtthIr/FlkyQ/I POZQV/h73LJFA== Date: Thu, 24 Sep 2026 08:54:49 +0100 From: "Lorenzo Stoakes (ARM)" To: Lance Yang Cc: akpm@linux-foundation.org, david@kernel.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, liam@infradead.org, nico.pache@linux.dev, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, usama.arif@linux.dev, kas@kernel.org, guoren@kernel.org, bcain@kernel.org, geert@linux-m68k.org, dinguyen@kernel.org, schuster.simon@siemens-energy.com, jonas@southpole.se, stefan.kristiansson@saunalahti.fi, shorne@gmail.com, dalias@libc.org, glaubitz@physik.fu-berlin.de, pjw@kernel.org, palmer@dabbelt.com, aou@eecs.berkeley.edu, alex@ghiti.fr, linux@armlinux.org.uk, vgupta@kernel.org, monstr@monstr.eu, chris@zankel.net, jcmvbkbc@gmail.com, will@kernel.org, aneesh.kumar@kernel.org, npiggin@gmail.com, peterz@infradead.org, davem@davemloft.net, andreas@gaisler.com, richard.henderson@linaro.org, mattst88@gmail.com, linmag7@gmail.com, catalin.marinas@arm.com, mark.rutland@arm.com, chenhuacai@kernel.org, kernel@xen0n.name, tsbogend@alpha.franken.de, James.Bottomley@hansenpartnership.com, deller@gmx.de, maddy@linux.ibm.com, mpe@ellerman.id.au, chleroy@kernel.org, hca@linux.ibm.com, gor@linux.ibm.com, agordeev@linux.ibm.com, borntraeger@linux.ibm.com, svens@linux.ibm.com, richard@nod.at, anton.ivanov@cambridgegreys.com, johannes@sipsolutions.net, tglx@kernel.org, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, arnd@arndb.de, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, jgg@ziepe.ca, jhubbard@nvidia.com, peterx@redhat.com, ysato@users.sourceforge.jp, shakeel.butt@linux.dev, corbet@lwn.net, rdunlap@infradead.org, 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, hughd@google.com, qi.zheng@linux.dev, linux-doc@vger.kernel.org Subject: Re: [PATCH v4 12/12] mm: change the contract for free_pgtables(), update docs Message-ID: References: <20260922-rcu-pagetable-freeing-v4-12-fe1ad1f1e303@kernel.org> <20260924032625.28555-1-lance.yang@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260924032625.28555-1-lance.yang@linux.dev> X-Rspamd-Queue-Id: C13CF180005 X-Rspam-User: X-Rspamd-Server: rspam07 X-Stat-Signature: enb8dfaafpyzzxam869qmgoose8eaitq X-HE-Tag: 1790236513-558170 X-HE-Meta: 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 n5toSHq/ MkI3VI7q6vtZ4bddP4Dsg4wOr4wVAR1QSPfPmT402rz7xbR/fRL62BWbMcvq132Ebp5hCOGJI6PsxQIiBOUdxScwQDPTcPjOfdt/3EiN4UUREniOQRTyYzEfih+GSJBgdQLWmtQYl+sUA4j5mR0dgoxp0IF08yMMnE2N3gASolVyK6uR2WuC4jIVHF7lFiYKDbEi70ytECLFhG1e9OXz1RqVnU3BOZJq5K9f9H22LaSk5QZ3GqXR/zza4q/gZnnh6XQWcL6JGUaizIVWBZR8qhfqLC8Otak77vY7h34PwnyWmEHI5MACjbxcYykyk1FPI5USrC5fFi+Amj2dAK+vGZRM9gNzvpDYHqC9vqX4ENc3KheQBgBuVxFYa03Dpr2pqlR6B3r7Y1tdHAxyEmbTS3xOGSQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Sep 24, 2026 at 11:26:25AM +0800, Lance Yang wrote: > > On Tue, Sep 22, 2026 at 04:35:43PM +0100, Lorenzo Stoakes (ARM) wrote: > >Now that page tables are freed after an RCU grace period, it is safe for > >read-only page table walkers to walk page table ranges that are being > >concurrently torn down, provided the mm is kept alive via mmgrab(). > > > >It is however unsafe for writers to do so, as they must obtain an > >appropriate lock to do so safely. > > > >Update the pte_offset_map_lock()'s comment block to reflect this. > > > >Similarly update the process addresses documentation. > > > >Acked-by: Kiryl Shutsemau (Meta) > >Signed-off-by: Lorenzo Stoakes (ARM) > >--- > > Documentation/mm/process_addrs.rst | 6 ++++++ > > mm/pgtable-generic.c | 15 +++++++++++---- > > 2 files changed, 17 insertions(+), 4 deletions(-) > > > >diff --git a/Documentation/mm/process_addrs.rst b/Documentation/mm/process_addrs.rst > >index a7296f251799..b1f4f44d75eb 100644 > >--- a/Documentation/mm/process_addrs.rst > >+++ b/Documentation/mm/process_addrs.rst > >@@ -537,6 +537,12 @@ We establish basic locking rules when interacting with page tables: > > * When changing a page table entry the page table lock for that page table > > **must** be held, except if you can safely assume nobody can access the page > > tables concurrently (such as on invocation of :c:func:`!free_pgtables`). > >+* Page tables may be *walked* under RCU alone, as page tables are freed only > >+ after an RCU grace period has elapsed. However, any entry found must be > >+ revalidated after the page table lock is taken (such as the > >+ :c:func:`!pmd_same` recheck performed by :c:func:`!pte_offset_map_lock`) > >+ before it is acted upon. Changing an entry requires the page table > >+ lock and one of the locks that excludes teardown (mmap or VMA lock). > > What about rmap walkers? try_to_unmap() clears PTEs under the rmap lock > and PTL, without an mmap or VMA lock. That's an abomination but yep will update. > > Cheers, Lance > > > * Reads from and writes to page table entries must be *appropriately* > > atomic. See the section on atomicity below for details. > > * Populating previously empty entries requires that the mmap or VMA locks are > >diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c > >index b91b1a98029c..a127e3e8f9b9 100644 > >--- a/mm/pgtable-generic.c > >+++ b/mm/pgtable-generic.c > >@@ -385,10 +385,17 @@ pte_t *pte_offset_map_rw_nolock(struct mm_struct *mm, pmd_t *pmd, > > * Note: "RO" / "RW" expresses the intended semantics, not that the *kmap* will > > * 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. > >+ * Note that free_pgtables(), used after unmapping detached vmas or when exiting > >+ * the whole mm, does not take a page table lock before freeing a page table. > >+ * > >+ * As page table freeing itself is RCU-safe, page table readers can safely run > >+ * concurrently with page table teardown. > >+ * > >+ * However, writers CANNOT as, without a lock being held, nothing prevents > >+ * concurrent teardown. > >+ * > >+ * Also note that the PGD itself is freed at mmdrop() time, not under RCU - so > >+ * the walker must keep the mm alive either by pinning the mm or the VMA. > > */ > > pte_t *pte_offset_map_lock(struct mm_struct *mm, pmd_t *pmd, > > unsigned long addr, spinlock_t **ptlp) > > > >-- > >2.55.0 > > > > -- Cheers, Lorenzo