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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-3e75223898csm2497249f8f.39.2025.09.09.04.55.00 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 09 Sep 2025 04:55:02 -0700 (PDT) Message-ID: Date: Tue, 9 Sep 2025 13:54:59 +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: Alexander Gordeev Cc: Kevin Brodsky , 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> 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: <2fecfae7-1140-4a23-a352-9fd339fcbae5-agordeev@linux.ibm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 09.09.25 13:45, Alexander Gordeev wrote: > On Tue, Sep 09, 2025 at 12:09:48PM +0200, David Hildenbrand wrote: >> On 09.09.25 11:40, Alexander Gordeev wrote: >>> On Tue, Sep 09, 2025 at 11:07:36AM +0200, David Hildenbrand wrote: >>>> On 08.09.25 09:39, Kevin Brodsky wrote: >>>>> arch_{enter,leave}_lazy_mmu_mode() currently have a stateless API >>>>> (taking and returning no value). This is proving problematic in >>>>> situations where leave() needs to restore some context back to its >>>>> original state (before enter() was called). In particular, this >>>>> makes it difficult to support the nesting of lazy_mmu sections - >>>>> leave() does not know whether the matching enter() call occurred >>>>> while lazy_mmu was already enabled, and whether to disable it or >>>>> not. >>>>> >>>>> This patch gives all architectures the chance to store local state >>>>> while inside a lazy_mmu section by making enter() return some value, >>>>> storing it in a local variable, and having leave() take that value. >>>>> That value is typed lazy_mmu_state_t - each architecture defining >>>>> __HAVE_ARCH_ENTER_LAZY_MMU_MODE is free to define it as it sees fit. >>>>> For now we define it as int everywhere, which is sufficient to >>>>> support nesting. >>> ... >>>>> { >>>>> + lazy_mmu_state_t lazy_mmu_state; >>>>> ... >>>>> - arch_enter_lazy_mmu_mode(); >>>>> + lazy_mmu_state = arch_enter_lazy_mmu_mode(); >>>>> ... >>>>> - arch_leave_lazy_mmu_mode(); >>>>> + arch_leave_lazy_mmu_mode(lazy_mmu_state); >>>>> ... >>>>> } >>>>> >>>>> * In a few cases (e.g. xen_flush_lazy_mmu()), a function knows that >>>>> lazy_mmu is already enabled, and it temporarily disables it by >>>>> calling leave() and then enter() again. Here we want to ensure >>>>> that any operation between the leave() and enter() calls is >>>>> completed immediately; for that reason we pass LAZY_MMU_DEFAULT to >>>>> leave() to fully disable lazy_mmu. enter() will then re-enable it >>>>> - this achieves the expected behaviour, whether nesting occurred >>>>> before that function was called or not. >>>>> >>>>> Note: it is difficult to provide a default definition of >>>>> lazy_mmu_state_t for architectures implementing lazy_mmu, because >>>>> that definition would need to be available in >>>>> arch/x86/include/asm/paravirt_types.h and adding a new generic >>>>> #include there is very tricky due to the existing header soup. >>>> >>>> Yeah, I was wondering about exactly that. >>>> >>>> In particular because LAZY_MMU_DEFAULT etc resides somewehere compeltely >>>> different. >>>> >>>> Which raises the question: is using a new type really of any benefit here? >>>> >>>> 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. >> >> 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. 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? -- Cheers David / dhildenb