From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id CE29847988A; Wed, 23 Sep 2026 08:54:42 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790153692; cv=none; b=J3JP7tKG4MBIQUJZqQIpQr/fQDwOIRaeMKSHgIkzRJTjNo8y+FOC946FA9+w3ZyGT2aREnw3Mm1XvIGYBHVU3F9lMkRsEJd1l6LuywzGmk+fIOshgNsNnFvrWNiRZZ55sAD6RbkNMJuxGbI6EmWdozxVLzT7LVGWsFTVyq2ya7Q= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790153692; c=relaxed/simple; bh=V48D3YJJycpWHMpgJ18QmYaRDBS9H66RjCggZi/PcmE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Cy4v082hNPZHHVkzIFaItfCb8hL0Lt1Pun80XdYS4+XQHS+/Kp+e7cYRpw/p0k4rYv6JiLYPEr5G3VtFpM3nR0wNV2a1NDwR6MRjDC3lWxaCabSk9X2EhrmVNEzZOF6fj06OxAQSQGv1T1O6PEFUp/FlPis0HMqHUx32/3OF9GM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=e6JGCywJ; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="e6JGCywJ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id DE4BE1F000FF; Wed, 23 Sep 2026 08:54:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790153679; bh=mfm+dgT5lNMdwvcoEsUCo1RatnytaF+7FQ0IE0uMup8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=e6JGCywJTcC9yMcSrzIixlrJgX8t5PhsTA6gwRUYjo2Rg9Npys6OSwJnM5LDnfDEy u20Tu9Pa0QyHyAfTx2ghB9J6QP6gBr7EWbZSjEiIrJOl2CCCILU7j46EcI407+b3G4 JbplyhQQmC8k8Zcc7ceIRcIj+e47k7MI6571ZOr6OU5vJGqRgnheYl4mX81DH9D74x K5+g7m/nU0Qqxmm6i11YWDVVuq8qJUznAnAl7zgN5WMG00TrgLu8BDRDA/ps23/+iE rfbVBTyqQij0mFvDHjN9Z4Vy8VJvV/Rfswxrcl25y5XhPcf3HoVl/l6+cj4FDRLumn kUpp72cLS9TEg== Message-ID: <76b2f611-b74c-4d6d-b166-491e1cfcb9a5@kernel.org> Date: Wed, 23 Sep 2026 10:54:07 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 01/12] mm/khugepaged: deposit a newly allocated page table on collapse To: "Lorenzo Stoakes (ARM)" , Andrew Morton , 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 References: <20260922-rcu-pagetable-freeing-v4-0-fe1ad1f1e303@kernel.org> <20260922-rcu-pagetable-freeing-v4-1-fe1ad1f1e303@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/22/26 17:35, 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. ack > > 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. I think we would have zeroed out all page table entries either way already right now before depositing. See clear_ptes() in __collapse_huge_page_copy_succeeded(). So this paragraph might just be dropped. > > 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) alloc_deposit_pte_table or alloc_deposit_pgtable should be clearer. > +{ > + /* > + * 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); I'm no memcg expert but that looks good to me. > + mem_cgroup_put(memcg); > + return pgtable; > +} > + With the fixup Acked-by: David Hildenbrand (Arm) -- Cheers, David