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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (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 01429C982FA for ; Wed, 23 Sep 2026 07:14:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=UqS6xPqxydZSejBMbvPyFqCPMp578ySnRbPTmya72dM=; b=jLLOPkXG8dkAyK E/5anntdZVn6LKI/jNb+2MpLfckS7Lv9sjL1rJS5RNnJR6N1LVNp/pTtqp08YAfyaU3D0w7380xcJ RSBELEG1q4ajFTSlNG+haSc8pzTu5W+YEut+u6idY/ImBR0mLx7V7eH5M99EnztUB0zyCVDv6oRRM o2eGh+i+rKGke+IYxwYFssF2QgrIa0fNgcM/mP3p8cSEwrxyXNt5MUAfFVs9YGyC3/bJxgiO40MJI JWetach/m5irTQmzLbNcIZYvfQSwXIi/oHSZMOmW5SJTR/h9dfawguyvnnGtlNSkK9cCNRSv3IBD8 MkvPI43OMhnnFiKXhIfA==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1x9HBU-00000007NBS-0rZ1; Wed, 23 Sep 2026 07:14:28 +0000 Received: from sea.source.kernel.org ([172.234.252.31]) by bombadil.infradead.org with esmtps (Exim 4.99.1 #2 (Red Hat Linux)) id 1x9HBS-00000007NBC-0N0o; Wed, 23 Sep 2026 07:14:26 +0000 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 2C12B4478B; Wed, 23 Sep 2026 07:14:25 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5F5D21F000FF; Wed, 23 Sep 2026 07:14:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790147665; bh=eUbT4b8cpuhzpeO+5wvLG35IoWThWdBXchOpscPEUbo=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=LGAn7A/ufhqitMqo1pvJejNQDWdQFSBFm9O5wWGXBsgPApH8MsQd8GanjDi0vOklx JPgYDPGv92XgtwBRRfIxcfedlZc7mxw6avAAAZiYZAWCHR0kQGLCUYDlXWnt5v+Nih LrsMIAfrHao5oCPNM8rjGlCjqcZeY2jbCE7wVS3dd2Ur8Oq8OWK2URPvFmzGjgOGlU OE2f62zMcsXZqAtjnJkDBOw0xEMR4mVQwmtvwLDo9CW309HQIhS1xawhMcaQlXkyHd FYSeV6pMmios7JXgGp6FMQd67u3MhQvT3u9olACCz7hTHq7EBKXHgWba+J4ldFKNuc H9C4as6GwzkRg== Date: Wed, 23 Sep 2026 08:14:01 +0100 From: "Lorenzo Stoakes (ARM)" To: Andrew Morton , 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 Cc: 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 01/12] mm/khugepaged: deposit a newly allocated page table on collapse Message-ID: References: <20260922-rcu-pagetable-freeing-v4-0-fe1ad1f1e303@kernel.org> <20260922-rcu-pagetable-freeing-v4-1-fe1ad1f1e303@kernel.org> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20260922-rcu-pagetable-freeing-v4-1-fe1ad1f1e303@kernel.org> X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org Andrew - would it be possible to make one quick fix up below? On Tue, Sep 22, 2026 at 04:35:32PM +0100, Lorenzo Stoakes (ARM) wrote: > collapse_huge_page() deposits a PTE page table on PMD collapse in order > that it can be utilised for subsequent split operations, meaning that those > operations do not need to perform an allocation (as they are in a context > where it might be unwise). > > However the PTE page table which is deposited is the one which is currently > mapped by the PMD entry that is in the process of being collapsed. > > Once deposited, the PTE page table may be used in a split of any other > unrelated PMD entry. > > This is currently not an issue as this operation is performed with VMA/mmap > write lock + anon rmap locks held, so ordinary page table walkers will > never accidentally end up walking the wrong thing, and GUP-fast is > protected by an IPI via tlb_remove_table_sync_one(). > > However, the series to which this commit belongs implements RCU-safe page > table traversal, at which point this becomes problematic. > > This can be resolved by using pte_offset_map_lock() which gates on a PTE > PTL and a pmd_same() check, but lockless walks are unsafe as things stand. > > Resolve this by simply allocating a new, zeroed, PTE page table to deposit > at the point of collapse. > > This path is already costly and an allocation has already been performed > for the huge folio, so this allocation is statistical noise in terms of > performance and memory usage at this point. > > With this PTE page table deposited, RCU-free the existing PTE page table > so it is safe for page table walkers to traverse within a grace period. > > This also brings this deposit case in line with all other page table > deposit logic which deposit a fresh page table. > > Additionally, this was the only place in the kernel that displaced a page > table like this, so eliminating it also helps consistency. > > An edge case for deposit exists for powerpc and its hash-based MMU - it > stores hash slot data in deposited page tables and zeroes them on withdraw, > so a zeroed deposited page table works correctly for it. > > Since khugepaged runs as a kernel thread, do a little dance in > alloc_deposit_pte() to correctly charge the allocation. > > This is already done for the folio allocation via alloc_charge_folio() but > no such wrapper exists for a page table allocation. > > Signed-off-by: Lorenzo Stoakes (ARM) > --- > mm/khugepaged.c | 32 ++++++++++++++++++++++++++++++-- > 1 file changed, 30 insertions(+), 2 deletions(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index f49a6710933b..dab421f8233e 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -1278,6 +1278,23 @@ static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_stru > 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_page(struct mm_struct *mm, unsigned long s > 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 +1327,12 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s > goto out_nolock; > } > > + if (is_pmd_order(order)) { > + pgtable = alloc_deposit_pte(mm); > + if (!pgtable) > + goto out_nolock; Fixup is here: - if (!pgtable) - goto out_nolock; + if (!pgtable) { + result = SCAN_ALLOC_HUGE_PAGE_FAIL; + goto out_nolock; + } Thanks! > + } > + > mmap_read_lock(mm); > result = hugepage_vma_revalidate(mm, pmd_addr, /*expect_anon=*/ true, > &vma, cc, order); > @@ -1433,8 +1456,8 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s > 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 +1476,9 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s > } > spin_unlock(pmd_ptl); > > + if (is_pmd_order(order)) > + pte_free_defer(mm, pmd_pgtable(_pmd)); > + > folio = NULL; > > result = SCAN_SUCCEED; > @@ -1463,6 +1489,8 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s > 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); > > -- > 2.55.0 > -- Cheers, Lorenzo _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv