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(p200300d82f239c00d1f6f7fe8f147e34.dip0.t-ipconnect.de. [2003:d8:2f23:9c00:d1f6:f7fe:8f14:7e34]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3e7521be57esm2887963f8f.2.2025.09.09.07.28.43 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 09 Sep 2025 07:28:45 -0700 (PDT) Message-ID: <29383ee2-d6d6-4435-9052-d75a263a5c45@redhat.com> Date: Tue, 9 Sep 2025 16:28:43 +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 v2 2/7] mm: introduce local state for lazy_mmu sections To: Kevin Brodsky , Alexander Gordeev Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andreas Larsson , Andrew Morton , Boris Ostrovsky , Borislav Petkov , Catalin Marinas , Christophe Leroy , Dave Hansen , "David S. Miller" , "H. Peter Anvin" , Ingo Molnar , Jann Horn , Juergen Gross , "Liam R. Howlett" , Lorenzo Stoakes , Madhavan Srinivasan , Michael Ellerman , Michal Hocko , Mike Rapoport , Nicholas Piggin , Peter Zijlstra , Ryan Roberts , Suren Baghdasaryan , Thomas Gleixner , Vlastimil Babka , Will Deacon , Yeoreum Yun , linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, sparclinux@vger.kernel.org, xen-devel@lists.xenproject.org References: <20250908073931.4159362-1-kevin.brodsky@arm.com> <20250908073931.4159362-3-kevin.brodsky@arm.com> <2fecfae7-1140-4a23-a352-9fd339fcbae5-agordeev@linux.ibm.com> <47ee1df7-1602-4200-af94-475f84ca8d80@arm.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZoEEwEIAEQCGwMCF4ACGQEFCwkIBwICIgIG FQoJCAsCBBYCAwECHgcWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaJzangUJJlgIpAAKCRBN 3hD3AP+DWhAxD/9wcL0A+2rtaAmutaKTfxhTP0b4AAp1r/eLxjrbfbCCmh4pqzBhmSX/4z11 opn2KqcOsueRF1t2ENLOWzQu3Roiny2HOU7DajqB4dm1BVMaXQya5ae2ghzlJN9SIoopTWlR 0Af3hPj5E2PYvQhlcqeoehKlBo9rROJv/rjmr2x0yOM8qeTroH/ZzNlCtJ56AsE6Tvl+r7cW 3x7/Jq5WvWeudKrhFh7/yQ7eRvHCjd9bBrZTlgAfiHmX9AnCCPRPpNGNedV9Yty2Jnxhfmbv Pw37LA/jef8zlCDyUh2KCU1xVEOWqg15o1RtTyGV1nXV2O/mfuQJud5vIgzBvHhypc3p6VZJ lEf8YmT+Ol5P7SfCs5/uGdWUYQEMqOlg6w9R4Pe8d+mk8KGvfE9/zTwGg0nRgKqlQXrWRERv cuEwQbridlPAoQHrFWtwpgYMXx2TaZ3sihcIPo9uU5eBs0rf4mOERY75SK+Ekayv2ucTfjxr Kf014py2aoRJHuvy85ee/zIyLmve5hngZTTe3Wg3TInT9UTFzTPhItam6dZ1xqdTGHZYGU0O otRHcwLGt470grdiob6PfVTXoHlBvkWRadMhSuG4RORCDpq89vu5QralFNIf3EysNohoFy2A LYg2/D53xbU/aa4DDzBb5b1Rkg/udO1gZocVQWrDh6I2K3+cCs7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <47ee1df7-1602-4200-af94-475f84ca8d80@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit >>>>>> Can't we just use an "enum lazy_mmu_state" and call it a day? >>>>> >>>>> I could envision something completely different for this type on s390, >>>>> e.g. a pointer to a per-cpu structure. So I would really ask to stick >>>>> with the current approach. > > This is indeed the motivation - let every arch do whatever it sees fit. > lazy_mmu_state_t is basically an opaque type as far as generic code is > concerned, which also means that this API change is the first and last > one we need (famous last words, I know). It makes the API more complicated, though. :) > > I mentioned in the cover letter that the pkeys-based page table > protection series [1] would have an immediate use for lazy_mmu_state_t. > In that proposal, any helper writing to pgtables needs to modify the > pkey register and then restore it. To reduce the overhead, lazy_mmu is > used to set the pkey register only once in enter(), and then restore it > in leave() [2]. This currently relies on storing the original pkey > register value in thread_struct, which is suboptimal and most Can you elaborate why this is suboptimal? See below regarding the size of task_struct. > importantly doesn't work if lazy_mmu sections nest. Can you elaborate why it would be problematic with nesting (if we would have a count and can handle the transition from 0->1 and 1->0)? > With this series, we > could instead store the pkey register value in lazy_mmu_state_t > (enlarging it to 64 bits or more). Yes. > > I also considered going further and making lazy_mmu_state_t a pointer as > Alexander suggested - more complex to manage, but also a lot more flexible. > >>>> Would that integrate well with LAZY_MMU_DEFAULT etc? >>> >>> Hmm... I though the idea is to use LAZY_MMU_* by architectures that >>> want to use it - at least that is how I read the description above. >>> >>> It is only kasan_populate|depopulate_vmalloc_pte() in generic code >>> that do not follow this pattern, and it looks as a problem to me. > > This discussion also made me realise that this is problematic, as the > LAZY_MMU_{DEFAULT,NESTED} macros were meant only for architectures' > convenience, not for generic code (where lazy_mmu_state_t should ideally > be an opaque type as mentioned above). It almost feels like the kasan > case deserves a different API, because this is not how enter() and > leave() are meant to be used. This would mean quite a bit of churn > though, so maybe just introduce another arch-defined value to pass to > leave() for such a situation - for instance, > arch_leave_lazy_mmu_mode(LAZY_MMU_FLUSH)? The discussion made me realize that it's a bit hack right now :) If LAZY_MMU_DEFAULT etc. are not for common code, then please maintain them for the individual archs as well, just like you do with the opaque type. > >> >> Yes, that's why I am asking. >> >> What kind of information (pointer to a per-cpu structure) would you >> want to return, and would handling it similar to how >> pagefault_disable()/pagefault_enable() e.g., using a variable in >> "current" to track the nesting level avoid having s390x to do that? > > The pagefault_disabled approach works fine for simple use-cases, but it > doesn't scale well. The space allocated in task_struct/thread_struct to > track that state is wasted (unused) most of the time. I'm not sure that's a concern. Fitting an int into existing holes should work and even another 64bit (8byte )... I just checked with pahole using the Fedora config on current mm-unstable. /* size: 9792, cachelines: 153, members: 276 */ /* sum members: 9619, holes: 20, sum holes: 125 */ /* sum bitfield members: 85 bits, bit holes: 2, sum bit holes: 43 bits */ /* padding: 32 */ /* member types with holes: 4, total: 6, bit holes: 2, total: 2 */ /* paddings: 6, sum paddings: 49 */ /* forced alignments: 12, forced holes: 2, sum forced holes: 60 */ Due to some "arch_task_struct_size" we might actually allocate more space. Staring at my live system: $ sudo slabinfo Name Objects Objsize Space Slabs/Part/Cpu O/S O %Fr %Ef Flg ... task_struct 1491 12376 24.8M 721/25/37 2 3 3 74 I am not sure if even an additional 8byte would move the needle here. Worse, it does not > truly enable states to be nested: it allows the outermost section to > store some state, but nested sections cannot allocate extra space. This > is really what the stack is for. If it's really just 8 bytes I don't really see the problem. So likely there is more to it? -- Cheers David / dhildenb