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 lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 71E4EC98314 for ; Wed, 23 Sep 2026 19:59:36 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [127.0.0.1]) by lists.ozlabs.org (Postfix) with ESMTP id 4hqnqV4qXdz3048; Thu, 24 Sep 2026 05:59:34 +1000 (AEST) Authentication-Results: lists.ozlabs.org; arc=none smtp.remote-ip=172.105.4.254 ARC-Seal: i=1; a=rsa-sha256; d=lists.ozlabs.org; s=201707; t=1790193574; cv=none; b=diku4ZrFzRsHlis8dq+fds3lOyt/o5n1qm0i9Bwooe0b+Qa0TzbLBZNM5TQBRvJm5us4tAMtTbazXA8SaVxvIUPve9OTFpaeH6OIC2BNAbNZhbbNqedexirXjp06OWZYs3+Gm6G84bFg3Ck29gqx75UBh/u7SswgRr0IRWKZjB+AuWvXw46BoyRc4o9IIjQuVZ0WkdEpeTewqPgMsgN01VHBO59L815bPTioRXLBKz2bpTVgtHBWD+z+19o765R9pUjiDc1Zm6bmoOZkL+ZdEyV2zA/57Ugzm3cD0Xg5itPqKSocuzQ9wUaJ/tSONhCrzutl8Sb1qQ1ozvkxp+Zp+g== ARC-Message-Signature: i=1; a=rsa-sha256; d=lists.ozlabs.org; s=201707; t=1790193574; c=relaxed/relaxed; bh=6Vw00INHTiJj7/4QiQUDB/NHC+AlawfaZqFc89mI3Uo=; h=Date:From:To:Cc:Subject:Message-Id:In-Reply-To:References: Mime-Version:Content-Type; b=F111XYdemiRa4p7C8l6MvkACe/kPPkfNT+9LscI5qC2XMDw+bTkOGPDwabKSqEBEGR4aD21l99A87CGDya+Zy7bqBtusn57b3FByRosGVov5o6sGdc/6WvKrkqnU/LmNUJichnOgFZRiJBbDJVwsTgXaJLuSeIZaowmWvOtu+h/uYTr6UFHK4TK+7u+Hq3VqE37NUMR9AlE6m3KJ1QON25/eoL2YgT9tuXO++x04Adsc3yt7gTrZ6E8fs3AMAAttWKcrAOaoXG8ZAJ84EQhGwsBAT7uDDKNPCN13ia9BvtbaMsyTipMc/LlORU0I7wUBFB90b9s0WZ381myJ0tXl3w== ARC-Authentication-Results: i=1; lists.ozlabs.org; dmarc=none (p=none dis=none) header.from=linux-foundation.org; dkim=pass (1024-bit key; unprotected) header.d=linux-foundation.org header.i=@linux-foundation.org header.a=rsa-sha256 header.s=korg header.b=kno41qWF; dkim-atps=neutral; spf=pass (client-ip=172.105.4.254; helo=tor.source.kernel.org; envelope-from=akpm@linux-foundation.org; receiver=lists.ozlabs.org) smtp.mailfrom=linux-foundation.org Authentication-Results: lists.ozlabs.org; dmarc=none (p=none dis=none) header.from=linux-foundation.org Authentication-Results: lists.ozlabs.org; dkim=pass (1024-bit key; unprotected) header.d=linux-foundation.org header.i=@linux-foundation.org header.a=rsa-sha256 header.s=korg header.b=kno41qWF; dkim-atps=neutral Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=linux-foundation.org (client-ip=172.105.4.254; helo=tor.source.kernel.org; envelope-from=akpm@linux-foundation.org; receiver=lists.ozlabs.org) Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange x25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4hqnqT28LSz2xlw for ; Thu, 24 Sep 2026 05:59:32 +1000 (AEST) Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id 69C12600AA; Wed, 23 Sep 2026 19:59:28 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6B16C1F000FF; Wed, 23 Sep 2026 19:59:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux-foundation.org; s=korg; t=1790193568; bh=6Vw00INHTiJj7/4QiQUDB/NHC+AlawfaZqFc89mI3Uo=; h=Date:From:To:Cc:Subject:In-Reply-To:References; b=kno41qWFahoXgz8d+ShejJCZHoWEcJVL1leHlRD1kffko8g8JqMF0swHQKAX1HqpM oSFWcsNfwRPPZPiT8oiRGQWKzrdI0TLKeP9oQtYbei92ILkPB3/p1L+hKq6td77JFF LtG5ONUzFTYy73cWN7de/JfRwIjTq8OlV8hOUlho= Date: Wed, 23 Sep 2026 12:59:25 -0700 From: Andrew Morton To: "Lorenzo Stoakes (ARM)" Cc: David Hildenbrand , Zi Yan , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Guo Ren , Brian Cain , Geert Uytterhoeven , Dinh Nguyen , Simon Schuster , Jonas Bonn , Stefan Kristiansson , Stafford Horne , 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 , Yoshinori Sato , Shakeel Butt , Jonathan Corbet , Randy Dunlap , 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 , linux-doc@vger.kernel.org Subject: Re: [PATCH v4 00/12] mm: make userland page table freeing RCU-safe Message-Id: <20260923125925.105d7575838c3a154a258a4c@linux-foundation.org> In-Reply-To: <20260922-rcu-pagetable-freeing-v4-0-fe1ad1f1e303@kernel.org> References: <20260922-rcu-pagetable-freeing-v4-0-fe1ad1f1e303@kernel.org> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Tue, 22 Sep 2026 16:35:31 +0100 "Lorenzo Stoakes (ARM)" wrote: > The majority of architectures in the kernel defer page table freeing until > an RCU grace period has elapsed, this series converts all remaining > architectures to do so too and eliminates CONFIG_MMU_GATHER_RCU_TABLE_FREE > altogether. > > This is important because it enables safe lockless page table walking under > RCU alone. > > Doing so allows for reduced lock contention, avoids lock ordering concerns > and enables fast, efficient and correct page table walking as a result. > > Additionally it removes a bunch of code and architecture-specific behaviour > which is always a beneficial thing to do. > > There has been much recent work on this: Thanks, I've updated mm.git's mm-unstable branch to this version. Plus one -fix for [01/12]. > v4: > * Updated the 1st patch to allocate a new page table on PTE deposit rather > than deposit a page table that might currently be being walked by an > RCU-only page table walker, as per David. Here's how v4 altered mm.git: mm/huge_memory.c | 2 +- mm/khugepaged.c | 34 ++++++++++++++++++++++++++++++++-- 2 files changed, 33 insertions(+), 3 deletions(-) --- a/mm/huge_memory.c~b +++ a/mm/huge_memory.c @@ -2478,7 +2478,7 @@ static inline void zap_deposited_table(s pgtable_t pgtable; pgtable = pgtable_trans_huge_withdraw(mm, pmd); - pte_free_defer(mm, pgtable); + pte_free(mm, pgtable); mm_dec_nr_ptes(mm); } --- a/mm/khugepaged.c~b +++ a/mm/khugepaged.c @@ -1278,6 +1278,23 @@ static enum scan_result alloc_charge_fol return SCAN_SUCCEED; } +static pgtable_t alloc_deposit_pte(struct mm_struct *mm) +{ + /* + * khugepaged is run from a kernel thread, so need to manually set the + * correct memcg so the allocation gets charged correctly. + */ + struct mem_cgroup *memcg = get_mem_cgroup_from_mm(mm); + struct mem_cgroup *old_memcg = set_active_memcg(memcg); + pgtable_t pgtable; + + pgtable = pte_alloc_one(mm); + + set_active_memcg(old_memcg); + mem_cgroup_put(memcg); + return pgtable; +} + /* * collapse_huge_page() expects the mmap_lock to be unlocked before entering and * will always return with the lock unlocked, to avoid holding the mmap_lock @@ -1293,7 +1310,7 @@ static enum scan_result collapse_huge_pa LIST_HEAD(compound_pagelist); pmd_t *pmd, _pmd; pte_t *pte = NULL; - pgtable_t pgtable; + pgtable_t pgtable = NULL; struct folio *folio; spinlock_t *pmd_ptl, *pte_ptl; enum scan_result result = SCAN_FAIL; @@ -1310,6 +1327,14 @@ static enum scan_result collapse_huge_pa goto out_nolock; } + if (is_pmd_order(order)) { + pgtable = alloc_deposit_pte(mm); + if (!pgtable) { + result = SCAN_ALLOC_HUGE_PAGE_FAIL; + goto out_nolock; + } + } + mmap_read_lock(mm); result = hugepage_vma_revalidate(mm, pmd_addr, /*expect_anon=*/ true, &vma, cc, order); @@ -1433,8 +1458,8 @@ static enum scan_result collapse_huge_pa spin_lock(pmd_ptl); VM_WARN_ON_ONCE(!pmd_none(*pmd)); if (is_pmd_order(order)) { - pgtable = pmd_pgtable(_pmd); pgtable_trans_huge_deposit(mm, pmd, pgtable); + pgtable = NULL; map_anon_folio_pmd_nopf(folio, pmd, vma, pmd_addr); } else { /* @@ -1453,6 +1478,9 @@ static enum scan_result collapse_huge_pa } spin_unlock(pmd_ptl); + if (is_pmd_order(order)) + pte_free_defer(mm, pmd_pgtable(_pmd)); + folio = NULL; result = SCAN_SUCCEED; @@ -1463,6 +1491,8 @@ out_up_write: anon_vma_unlock_write(vma->anon_vma); mmap_write_unlock(mm); out_nolock: + if (pgtable) + pte_free(mm, pgtable); if (folio) folio_put(folio); trace_mm_collapse_huge_page(mm, result == SCAN_SUCCEED, result, order); _