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 34C03C61DD3 for ; Tue, 1 Sep 2026 17:25:48 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id D8EC16B0095; Tue, 1 Sep 2026 13:25:46 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id D67726B0099; Tue, 1 Sep 2026 13:25:46 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id C7DF06B009B; Tue, 1 Sep 2026 13:25:46 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0010.hostedemail.com [216.40.44.10]) by kanga.kvack.org (Postfix) with ESMTP id 968106B0095 for ; Tue, 1 Sep 2026 13:25:46 -0400 (EDT) Received: from smtpin28.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay10.hostedemail.com (Postfix) with ESMTP id 2F932C04B8 for ; Tue, 1 Sep 2026 17:25:46 +0000 (UTC) X-FDA: 85165870692.28.BC6A9F5 Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by imf26.hostedemail.com (Postfix) with ESMTP id 95D2314000A for ; Tue, 1 Sep 2026 17:25:44 +0000 (UTC) Authentication-Results: imf26.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=Yi2t1tkD; spf=pass (imf26.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-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1788283544; 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=RU/XHZsQrPB/KjzCrGwIunh3KbYqDQIpxjG/zB2SrYU=; b=ljDdA13gtquzoML39F1tGHHnQNextDf4wq+UWAl+N7oYGzZIY6ZpfQDq0eYOWYWLtceUGd 3qGPytTPasO8cy3A0JvgwruaDDVKyD7pkKli40JSZ3sRxc7HNB09lt/dzKszSj0qp4qvH+ KB82bWXfmnO23j4iYm5rmDraehUxiRY= ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1788283544; b=yd2NBpgfRAAES3zR8hNfrQljgUnw2cOQP4pU9pDYReL1mZwjUkmlZS8+VviMFpDtqy8xOg dGcRj8PoIG4mj2i63yrbbWFrZAulwKBWj0SgYKfGAOvHB08gQBPbtWUZ1IgsiZPcQ5Mqpx n4ETRD+ZSygekH49kSjMk0Y7xmK/hX8= ARC-Authentication-Results: i=1; imf26.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=Yi2t1tkD; spf=pass (imf26.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 9F5C86020D; Tue, 1 Sep 2026 17:25:43 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2F38E1F000E9; Tue, 1 Sep 2026 17:25:29 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788283543; bh=RU/XHZsQrPB/KjzCrGwIunh3KbYqDQIpxjG/zB2SrYU=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=Yi2t1tkD7CbVI2GlNVLMPxmx2y6uQ0N17KQVN6tfdVVmsc8e3U70HKz60pHNwlQs3 1uaubu11GEhlMNqEwwCAnmGwy18sQmPBDQlkl4HFerFnkf9qqSXhXZRQB4AbUCIy65 +F5eg8cH3mV8+ZyrzWtQ7QlVH0tfsCC88xh+SXAYBAqn1Leh14VC/K7s9GATAG+x4E KI4pAxyEdR503wQD7kpPC4DxKOGSs/zzQQ249JbOJTrJb8Tg6ICIFvJvGHbD91pyny gkZbMWCaIAhY75HIF/43FdM8kRnN6lg6QLV7LnJhpcpnHrbOS/0Iht1oAeMIckn9b0 Sr0NCrNgxEH6Q== Date: Tue, 1 Sep 2026 18:25:26 +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: rspam01 X-Rspamd-Queue-Id: 95D2314000A X-Stat-Signature: bs1bqs7qmafnnzxafkj5s6zkrad69htg X-HE-Tag: 1788283544-303694 X-HE-Meta: U2FsdGVkX189wio4ZB3g8+eMlm5hJWgI7h4ma/mv8pP9hpKfWFg01jYmdiFedYLn07RG1IIE9hJE0Pqt7tKy8wMkMe5v+k/Po8hYZ17BTKm60ae34L2oOglETjwiuHljdqpq353tQ+rTGibDKv259KpgniTSolD9Sr7rQDm01aOCQKJic2iQeexXXKKai1Z4siJeS7x6bczqAvlpn7oZVsggGFbef8EGb6eGC6Nb/cjKETzO8vH9/Y+RdH28VO3iZtpaqLdtiM9q5F6Nhzsk8Bz9sfVPW/92qeRo3PqK7rq8q43xe6TtXf3tnlUqn6C0qwBiY+eqQVgbU+lGr/ux6FcPM0GeakY9By3GrsW99X5T69MyS2ma8XBMk5fXZqzADRATkZuNFi7Y1Svm+dhWMT85XxP0Jvj7ypj4dUc6XSOcX1yl3fIHJqti7adsbDS+b+2zp4hO9ODxlU9WCvsEU0VAC8QhMniwsZ1pD2w0VtUbSUHcKO4pbjenEVTEV7fYIQCjUkjXFrT5OSReU0R47Thwsvy2kEGdvCdF9H1ddInr640ajYBpAwKBZ3RMXIQvF6OSGSFmxO4bMHuweVsfJYaVzYfgaPtQIyoCrBrIqITPZ4nutTHRFattnwxg8oAou1IphBVABkI8XzlHx1uC9AWf81ER/H8bg6w2ujGFqm4Lx0YSekKxytdvZk0FrsKjBTaUGJe/bckxACVromHG1pKKSnoMm3qc4S89BXs1mrw0TAgDboDa09Ais4k6wub4nU0a83ScogrhtQ5EQqJqLALrtGbciouSTFMwvFZbG2+83FJLOGlUIX7p41vAKYZmt6RbzvjEGiIJfcs8SNoWv1xor5/N6DGBgyw4gsWxJQuI3arWX7gIy7kDO5M882z7cSfLCeDPDqvuquibJiEFsx19y30dn5BkzSgVM2caTu5lGEg2EkziMHNOAeUMpI+tVqjvQ4HjSBOQHtQMKmQ FDpuWnxM nSSm7EcrjoOOIPZKLyFpV6qnjIdrHGefebBxjF+8+VVBywbWPeMCs2JH6GTF7TYrVKoekUBI2kNOAkk4U6K26zXobQsBx1hNZvlnI3/g6sN4azSERM3s11OdHRZUT5ACEbSjR+by9yXt0D/DDvHmzNCcpzzWwEmOrt83+n1kxfGBXN8P6So4rIZyYwWeChgbyNba3+MeAzMtsfRh41fRnTe8q00wemK7olPCStIX69zCdojokqN8h2wwsuXWefYbZv2ue0i45gfmuG0000iXrpqPLpkbMZmreJn7zkpL/qqE2RERmghBqZ03u9VOiDuhv2ORdc8ZU+kUTb4LAce+2MhKXCJNPJ7IBI/ZPeQPOnB5f+bcn52DWZ9YEoQ== 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 06:15:17PM +0100, Kiryl Shutsemau wrote: > On Tue, Sep 01, 2026 at 03:31:26PM +0100, Lorenzo Stoakes (ARM) wrote: > > 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? > > s/whoel/whole/ Haha yup the inevitable typo... > > Otherwise, LGTM. Thanks! :) > > -- > Kiryl Shutsemau / Kirill A. Shutemov -- Cheers, Lorenzo