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 0F69D12C534; Fri, 5 Jun 2026 07:18:44 +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=1780643926; cv=none; b=RRf0ftqn9Kn4g640o8iOCA9DTA+5kmhhf7EI+0gfsqfmvbZn/t28ft828RXV+kHtOTXmHw9TXi2+TBPBovvrJj3qXr7l208AFTm6x9qBNpi0o3O4pS/fneXnJn71jGVTDYfgAjvVe9bznBXTY2cOoKAD7TRPcQ2J7Pu8VyzLjJk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780643926; c=relaxed/simple; bh=DM9UlyIAHn15CmL8vPXapVhy+5dazkz7jhUmqLbUntY=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=VaVZap9wKLTc1fOf9hj95+0lipMHIAQi7MmTKqSCq+vrUMS8cBjAlGglmaDSgAXPy12cmD++pqaVe3wdxiqd+RydBk9APGPGnMCP75OJa+rc/ZoDJLUukkg0hs6901gXq7ge8o0X6vj1sUlahqo34BXpIRSUfTlqxMdqJul5dgM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=UFKSoHsq; 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="UFKSoHsq" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 689791F00893; Fri, 5 Jun 2026 07:18:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780643924; bh=3+V341z4AFJZZMWBuVoX5xq/8STGfWgb2w/e64lYkj8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=UFKSoHsqpiw5gvp4gxvIEeJV+ca7a8BTyA6Fv3MGXG6GJ0w6FjqLoAmGNkrI40bzi LPiHrU5+KmRBiz308PEbAzP4ENiIfnH+V734xr/2xp4sCaeu+zcMgecxFNyfqSjb+v WgHMFtYwkcSrA1pSkQ4v7Vz+wEkoqzQ2+YcHMqXkD4dWkMnKIGsUE6Gms3U33ndlPx YEzC14yJ1npKFNlXHQvj4x2sIQCn12AgtAexYzvK7XvBVdBfb0Yr4JhkqRHnpuQseK 3jQOBtfcUkG3FG8kYz13aDf522Wg8k+yRie67+W03EZFQpEHhxZxIU0G0rzOPDryev ng00bDx57SfaA== Message-ID: <0ef96c28-9e6c-4d04-90ae-ac43c81d465d@kernel.org> Date: Fri, 5 Jun 2026 09:18:27 +0200 Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-unstable v18 06/14] mm/khugepaged: generalize collapse_huge_page for mTHP collapse To: Nico Pache Cc: Lance Yang , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, aarcange@redhat.com, akpm@linux-foundation.org, anshuman.khandual@arm.com, apopple@nvidia.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, byungchul@sk.com, catalin.marinas@arm.com, cl@gentwo.org, corbet@lwn.net, dave.hansen@linux.intel.com, dev.jain@arm.com, gourry@gourry.net, hannes@cmpxchg.org, hughd@google.com, jack@suse.cz, jackmanb@google.com, jannh@google.com, jglisse@google.com, joshua.hahnjy@gmail.com, kas@kernel.org, liam@infradead.org, ljs@kernel.org, mathieu.desnoyers@efficios.com, matthew.brost@intel.com, mhiramat@kernel.org, mhocko@suse.com, peterx@redhat.com, pfalcato@suse.de, rakie.kim@sk.com, raquini@redhat.com, rdunlap@infradead.org, richard.weiyang@gmail.com, rientjes@google.com, rostedt@goodmis.org, rppt@kernel.org, ryan.roberts@arm.com, shivankg@amd.com, sunnanyong@huawei.com, surenb@google.com, thomas.hellstrom@linux.intel.com, tiwai@suse.de, usamaarif642@gmail.com, vbabka@suse.cz, vishal.moola@gmail.com, wangkefeng.wang@huawei.com, will@kernel.org, willy@infradead.org, yang@os.amperecomputing.com, ying.huang@linux.alibaba.com, ziy@nvidia.com, zokeefe@google.com, usama.arif@linux.dev References: <2024af56-5e99-4799-a586-e9ba756cecb9@kernel.org> <20260601032804.96122-1-lance.yang@linux.dev> <616de1a8-1cfd-40b8-b04f-7b324be40bfd@linux.dev> <6b11bf0a-769c-4ef2-ac6f-2af38200a6bc@kernel.org> <06d9b665-945f-4967-9ed9-b06514478996@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: 8bit On 6/4/26 19:04, Nico Pache wrote: > On Mon, Jun 1, 2026 at 9:00 AM Nico Pache wrote: >> >> On Mon, Jun 1, 2026 at 5:14 AM David Hildenbrand (Arm) wrote: >>> >>> >>> Yeah. BTW, I think we'd need a spin_lock_nested(), so @Nico, treat my code as a >>> draft. >> >> Okay, I read the above and did some investigating. >> >> I will try to implement and verify the changes you suggested :) > > I've implemented something slightly different actually and I *think* its better! > > } else { > /* this is map_anon_folio_pte_nopf with no mmu update */ > __map_anon_folio_pte_nopf(folio, pte, vma, start_addr, > /*uffd_wp=*/ false); > smp_wmb(); > pmd_populate(mm, pmd, pmd_pgtable(_pmd)); > /* > * Some architectures (e.g. MIPS) walk the live page table in > * their implementation. update_mmu_cache_range() must be called > * with a valid page table hierarchy and the PTE lock held. > * Acquire it nested inside pmd_ptl when they are distinct locks. > */ > if (pte_ptl != pmd_ptl) > spin_lock_nested(pte_ptl, SINGLE_DEPTH_NESTING); > update_mmu_cache_range(NULL, vma, start_addr, pte, nr_pages); > if (pte_ptl != pmd_ptl) > spin_unlock(pte_ptl); > } > spin_unlock(pmd_ptl); > > The logic here is that when the PMD becomes visible, PTEs are already > populated (no possibility of spurious faults on local CPU) > > the SMP_WMB makes sure of the above > > And the pmd is installed with the pte and pmd lock both held through > the mmu_cache update. > > This follows the conventions used in pmd_install() and clears the > potential for local CPU faults hitting cleared PTE entries. After the pmdp_collapse_flush() we'd be getting CPU faults due to the cleared PMD already? So the case here is rather different. -- Cheers, David