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(p200300d82f4e3200c99da38b3f3ad4b3.dip0.t-ipconnect.de. [2003:d8:2f4e:3200:c99d:a38b:3f3a:d4b3]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-429898ee8a9sm5476369f8f.46.2025.10.23.13.00.24 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 23 Oct 2025 13:00:26 -0700 (PDT) Message-ID: <2073294c-8003-451a-93e0-9aab81de4d22@redhat.com> Date: Thu, 23 Oct 2025 22:00:23 +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 v3 07/13] mm: enable lazy_mmu sections to nest To: Kevin Brodsky , linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, Alexander Gordeev , 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, x86@kernel.org References: <20251015082727.2395128-1-kevin.brodsky@arm.com> <20251015082727.2395128-8-kevin.brodsky@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: <20251015082727.2395128-8-kevin.brodsky@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit [...] > > In summary (count/enabled represent the values *after* the call): > > lazy_mmu_mode_enable() -> arch_enter() count=1 enabled=1 > lazy_mmu_mode_enable() -> ΓΈ count=2 enabled=1 > lazy_mmu_mode_pause() -> arch_leave() count=2 enabled=0 The arch_leave..() is expected to do a flush itself, correct? > lazy_mmu_mode_resume() -> arch_enter() count=2 enabled=1 > lazy_mmu_mode_disable() -> arch_flush() count=1 enabled=1 > lazy_mmu_mode_disable() -> arch_leave() count=0 enabled=0 > > Note: in_lazy_mmu_mode() is added to to allow arch > headers included by to use it. > > Signed-off-by: Kevin Brodsky > --- > Alexander Gordeev suggested that a future optimisation may need > lazy_mmu_mode_{pause,resume}() to call distinct arch callbacks [1]. For > now arch_{leave,enter}() are called directly, but introducing new arch > callbacks should be straightforward. > > [1] https://lore.kernel.org/all/5a0818bb-75d4-47df-925c-0102f7d598f4-agordeev@linux.ibm.com/ > --- [...] > > +struct lazy_mmu_state { > + u8 count; I would have called this "enabled_count" or "nesting_level". > + bool enabled; "enabled" is a bit confusing when we have lazy_mmu_mode_enable(). I'd have called this "active". > +}; > + > #endif /* _LINUX_MM_TYPES_TASK_H */ > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h > index 194b2c3e7576..269225a733de 100644 > --- a/include/linux/pgtable.h > +++ b/include/linux/pgtable.h > @@ -228,28 +228,89 @@ static inline int pmd_dirty(pmd_t pmd) > * of the lazy mode. So the implementation must assume preemption may be enabled > * and cpu migration is possible; it must take steps to be robust against this. > * (In practice, for user PTE updates, the appropriate page table lock(s) are > - * held, but for kernel PTE updates, no lock is held). Nesting is not permitted > - * and the mode cannot be used in interrupt context. > + * held, but for kernel PTE updates, no lock is held). The mode cannot be used > + * in interrupt context. > + * > + * The lazy MMU mode is enabled for a given block of code using: > + * > + * lazy_mmu_mode_enable(); > + * > + * lazy_mmu_mode_disable(); > + * > + * Nesting is permitted: may itself use an enable()/disable() pair. > + * A nested call to enable() has no functional effect; however disable() causes > + * any batched architectural state to be flushed regardless of nesting. After a > + * call to disable(), the caller can therefore rely on all previous page table > + * modifications to have taken effect, but the lazy MMU mode may still be > + * enabled. > + * > + * In certain cases, it may be desirable to temporarily pause the lazy MMU mode. > + * This can be done using: > + * > + * lazy_mmu_mode_pause(); > + * > + * lazy_mmu_mode_resume(); > + * > + * This sequence must only be used if the lazy MMU mode is already enabled. > + * pause() ensures that the mode is exited regardless of the nesting level; > + * resume() re-enters the mode at the same nesting level. must not modify > + * the lazy MMU state (i.e. it must not call any of the lazy_mmu_mode_* > + * helpers). > + * > + * in_lazy_mmu_mode() can be used to check whether the lazy MMU mode is > + * currently enabled. > */ > #ifdef CONFIG_ARCH_LAZY_MMU > static inline void lazy_mmu_mode_enable(void) > { > - arch_enter_lazy_mmu_mode(); > + struct lazy_mmu_state *state = ¤t->lazy_mmu_state; > + > + VM_BUG_ON(state->count == U8_MAX); No VM_BUG_ON() please. > + /* enable() must not be called while paused */ > + VM_WARN_ON(state->count > 0 && !state->enabled); > + > + if (state->count == 0) { > + arch_enter_lazy_mmu_mode(); > + state->enabled = true; > + } > + ++state->count; Can do if (state->count++ == 0) { > } > > static inline void lazy_mmu_mode_disable(void) > { > - arch_leave_lazy_mmu_mode(); > + struct lazy_mmu_state *state = ¤t->lazy_mmu_state; > + > + VM_BUG_ON(state->count == 0); Dito. > + VM_WARN_ON(!state->enabled); > + > + --state->count; > + if (state->count == 0) { Can do if (--state->count == 0) { > + state->enabled = false; > + arch_leave_lazy_mmu_mode(); > + } else { > + /* Exiting a nested section */ > + arch_flush_lazy_mmu_mode(); > + } > } > > static inline void lazy_mmu_mode_pause(void) > { > + struct lazy_mmu_state *state = ¤t->lazy_mmu_state; > + > + VM_WARN_ON(state->count == 0 || !state->enabled); > + > + state->enabled = false; > arch_leave_lazy_mmu_mode(); > } > > static inline void lazy_mmu_mode_resume(void) > { > + struct lazy_mmu_state *state = ¤t->lazy_mmu_state; > + > + VM_WARN_ON(state->count == 0 || state->enabled); > + > arch_enter_lazy_mmu_mode(); > + state->enabled = true; > } > #else > static inline void lazy_mmu_mode_enable(void) {} > diff --git a/include/linux/sched.h b/include/linux/sched.h > index cbb7340c5866..2862d8bf2160 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -1441,6 +1441,10 @@ struct task_struct { > > struct page_frag task_frag; > > +#ifdef CONFIG_ARCH_LAZY_MMU > + struct lazy_mmu_state lazy_mmu_state; > +#endif > + > #ifdef CONFIG_TASK_DELAY_ACCT > struct task_delay_info *delays; > #endif > @@ -1724,6 +1728,18 @@ static inline char task_state_to_char(struct task_struct *tsk) > return task_index_to_char(task_state_index(tsk)); > } > > +#ifdef CONFIG_ARCH_LAZY_MMU > +static inline bool in_lazy_mmu_mode(void) So these functions will reveal the actual arch state, not whether _enabled() was called. As I can see in later patches, in interrupt context they are also return "not in lazy mmu mode". -- Cheers David / dhildenb