From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 8283BC61DD3 for ; Tue, 1 Sep 2026 14:31:52 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 8551A6B01FB; Tue, 1 Sep 2026 10:31:51 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 8089A6B0252; Tue, 1 Sep 2026 10:31:51 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 6F5786B0253; Tue, 1 Sep 2026 10:31:51 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id 413B56B01FB for ; Tue, 1 Sep 2026 10:31:51 -0400 (EDT) Received: from smtpin27.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay07.hostedemail.com (Postfix) with ESMTP id BF54216041D for ; Tue, 1 Sep 2026 14:31:50 +0000 (UTC) X-FDA: 85165432380.27.C5DB09B Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by imf04.hostedemail.com (Postfix) with ESMTP id 2CD4F4000A for ; Tue, 1 Sep 2026 14:31:49 +0000 (UTC) Authentication-Results: imf04.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b="My/Zkw7E"; spf=pass (imf04.hostedemail.com: domain of ljs@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=ljs@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1788273109; b=etsE8aITDypWF20K989K7SwztMD4KT2NYAhDua5TklpM8sSSzGtnAv+CyQ7sKKHQjYuxJz qj8HkDD0sHEhn13epXFEek1nvV2ilK50qq9Lf+8EbSWvnZ4biESVl7THjoHviokS6Wlfk/ +ZswyMg2Dm9srQUROOCaQIkQMbREC7U= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1788273109; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=+SM87dKxgHKoTgR6BSYO1fL6Rd9rD874uyPlzcJDnuw=; b=S9EEYuCn93owV2Uc5AZodfiM6M1NACe8uvy6r/evuRBeFxYANCHsj4XGMtqTe1e4YKKuF6 C5Vifsf23YG4Z1PNtordWv87mL1NHcpj5to24tqAlGjwT8z4iHlX7Kz4aOQE0yGG3DbGE+ Bcrs0BQEOrDzoEoqCwlmcp4fkD0+5CM= ARC-Authentication-Results: i=1; imf04.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b="My/Zkw7E"; spf=pass (imf04.hostedemail.com: domain of ljs@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=ljs@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id B07AB60267; Tue, 1 Sep 2026 14:31:48 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5767E1F000E9; Tue, 1 Sep 2026 14:31:29 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788273108; bh=+SM87dKxgHKoTgR6BSYO1fL6Rd9rD874uyPlzcJDnuw=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=My/Zkw7EkDQLUSB5jkacAppt65djuYrOjFPJWtAE+uuNKaseNbscCeIJiLFL5tjOF zK/l3KkQ2N9S29hI3UzfgzFa0i3rNCpVRYFstyNcwPJV5mzts5D3rGRQPGBcL2r5sl M4LoV6VJ/3D2KAGExFMGP7y9KIyumhdCGZjgQtts1w0HwS2NaKaLtzngF9+7NuNxTq 97Ypv59rrpwA8Xfdjav2hWACORC+/7eWDiEOl7fC+2HguCIpqRQRocLFjIYq8tNPwN 1Rl+PvXhytTcmeBeZ03XozFMVE7XZhTphKZmP8mF89keRLxkpfMQ1iGRJldtR0luyc IzD2SuW+PoJHA== Date: Tue, 1 Sep 2026 15:31:26 +0100 From: "Lorenzo Stoakes (ARM)" To: Kiryl Shutsemau Cc: Andrew Morton , David Hildenbrand , Zi Yan , Baolin Wang , "Liam R. 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: X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 2CD4F4000A X-Stat-Signature: pcbj36tpgn4u4q3mrzu7hwrcj8xbyrrt X-HE-Tag: 1788273109-426860 X-HE-Meta: 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 TzsVtMwm szKT2WasxzWlelDPOTPmRlP8PZUyVouCSMn4UvaubDuKAe4o7df5HZo+hriW1BuqFJgfUUzk/cvTW33jVRcYQhAaOJlCedJWvWPqf2WlhdD5umvgVZZVXkOMeVso58pE6+DdW1nJUliR6lVQhR3llGFGVhcrUxYhCByRrATcAuoD9/9vA05HrZFGtL/G6yKWxIpik1aJhYjNrK0+sn3zeZVUY2XIxwNxAMLHEW6VTBxLbm8QE39lLjz5OK8S1UJiGyd2lT8v2Uo1i0DWz9LINaNX0EhDUCXg5vDtH1rrDOCd6+a4bbjwBUqZP4Z+ltltO1ajZe/37CN9YtHm0t+x0W5vgUwD5HquR/OA1XX7wp4xAmcCtBDIUT2g+6A== 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 02:57:13PM +0100, Kiryl Shutsemau wrote: > 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? Hmm yeah, ah page tables, what a rabbit hole of horror :) I guess this is effectively a reflection of the 'write lock on write, RCU load on read' pattern in RCU itself generally. So, maybe something like: * Note that free_pgtables(), used after unampping detached vmas, or when * exiting the whoel 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 nothing prevents concurrent teardown. */ Instead? What do you think? > > -- > Kiryl Shutsemau / Kirill A. Shutemov -- Cheers, Lorenzo