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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 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 X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: D8FBC120005 X-Stat-Signature: 9gbpq6bh977n9kf1y48rnefozbw4371w X-Rspam-User: X-HE-Tag: 1780643925-256920 X-HE-Meta: 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 iS5zJORh 8WUMpOp11RaYuFQdlIEpBUQuHgE3vmlKjWRvsCUggtAQLqUyELOmVtRQfGLXJ9CwaJSXIIDI3zvmix5VrNbsUW2V80x5yYOf95mWHygz/MbQ3Fq0tNtuS/7b6p5mRQa1610timyDzwF22LUazsKMRpcZ1gQokrNmdzmqZ5Shcwi6R7Yqk0G62qD/TzWV9c+7k3DIi7Mzgfnq/0QFwmkmB9HoCllh71Ol7c7TWx4B2aXsgPx4CzzmsJERWUU5XJsg41LYIIMY09XKRLdHc4iHDMFG4YpVn/Rv7qXcjuDaojgqTpJjbHWYWYLH+s5vMI5nI9v5utKoGlMwg2EwuOYhKTpH2KH5YiEUSsuUBem2lXWvWE4zF03TYoVlRgYePvetY20tToDgBFR1rgqNXSw9J06+TjgOmWNWZwqxQXZz8xdM/L+WPYPt4Z2ny56UEpMIq0uengs9AcbCDGn8= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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