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[80.187.80.144]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-46e5c44bd75sm9846475e9.4.2025.09.30.02.43.20 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 30 Sep 2025 02:43:22 -0700 (PDT) Message-ID: <5d763fb0-b216-4b6c-ac93-510fa471575f@redhat.com> Date: Tue, 30 Sep 2025 11:43:19 +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 v7 10/16] mm: define clear_pages(), clear_user_pages() To: Ankur Arora Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org, akpm@linux-foundation.org, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, mingo@redhat.com, mjguzik@gmail.com, luto@kernel.org, peterz@infradead.org, acme@kernel.org, namhyung@kernel.org, tglx@linutronix.de, willy@infradead.org, raghavendra.kt@amd.com, boris.ostrovsky@oracle.com, konrad.wilk@oracle.com References: <20250917152418.4077386-1-ankur.a.arora@oracle.com> <20250917152418.4077386-11-ankur.a.arora@oracle.com> <5ec85b0b-9848-4cee-98f4-37953d504773@redhat.com> <87y0q4ewzf.fsf@oracle.com> <6743ead5-4d61-4274-a24f-13a8a8265aee@redhat.com> <87frcbayt2.fsf@oracle.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 25.09.25 07:25, Ankur Arora wrote: > > David Hildenbrand writes: > >> On 23.09.25 22:26, Ankur Arora wrote: >>> David Hildenbrand writes: >>> >>>> On 17.09.25 17:24, Ankur Arora wrote: >>>>> Define fallback versions of clear_pages(), clear_user_pages(). >>>>> In absence of architectural primitives, we just clear pages >>>>> sequentially. >>>>> Signed-off-by: Ankur Arora >>>>> --- >>>>> include/linux/mm.h | 38 ++++++++++++++++++++++++++++++++++++++ >>>>> 1 file changed, 38 insertions(+) >>>>> diff --git a/include/linux/mm.h b/include/linux/mm.h >>>>> index 1ae97a0b8ec7..0cde9b01da5e 100644 >>>>> --- a/include/linux/mm.h >>>>> +++ b/include/linux/mm.h >>>>> @@ -3768,6 +3768,44 @@ static inline void clear_page_guard(struct zone *zone, struct page *page, >>>>> unsigned int order) {} >>>>> #endif /* CONFIG_DEBUG_PAGEALLOC */ >>>>> +#ifndef clear_pages >>>>> +/** >>>>> + * clear_pages() - clear a page range using a kernel virtual address. >>>> >>>> I'd just call this "clear a page range for kernel-internal use" >>>> >>>>> + * @addr: start address >>>>> + * @npages: number of pages >>>>> + * >>>>> + * Assumes that (@addr, +@npages) references a kernel region. >>>> >>>> And say here simply that "Use clear_user_pages() instead for clearing a page >>>> range to be mapped to user space". >>> So, comments that actually speak to the use instead of technically >>> correct but unhelpful generalities :). Thanks, good lesson. >>> >>>>> + * Does absolutely no exception handling. >>>>> + */ >>>>> +static inline void clear_pages(void *addr, unsigned int npages) >>>>> +{ >>>>> + do { >>>>> + clear_page(addr); >>>>> + addr += PAGE_SIZE; >>>>> + } while (--npages); >>>>> +} >>>>> +#endif >>>>> + >>>>> +#ifndef clear_user_pages >>>>> +/** >>>>> + * clear_user_pages() - clear a page range mapped by the user. >>>> >>>> I'd call this then "clear a page range to be mapped to user space" >>>> >>>> Because it's usually called before we actually map it and it will properly flush >>>> the dcache if required. >>> Makes sense. >>> >>>>> + * @addr: kernel mapped address >>>> >>>> "start address" >>>> >>>>> + * @vaddr: user mapped address >>>> >>>> "start address of the user mapping" ? >>>> >>>>> + * @pg: start page >>>> >>>> Please just call it "page". I know, clear_user_page() has this weird page vs. pg >>>> thingy, but let's do it better here. >>>> >>>>> + * @npages: number of pages >>>>> + * >>>>> + * Assumes that the region (@addr, +@npages) has been validated >>>>> + * already so this does no exception handling. >>>>> + */ >>>>> +#define clear_user_pages(addr, vaddr, pg, npages) \ >>>>> +do { \ >>>>> + clear_user_page(addr, vaddr, pg); \ >>>>> + addr += PAGE_SIZE; \ >>>>> + vaddr += PAGE_SIZE; \ >>>>> + pg++; \ >>>>> +} while (--npages) >>>>> +#endif >>>> >>>> Should indent with one tab. >>> Will do. Also acking to the ones above. >>> >>>> Any reason this is not a static inline function? >>> Alas yes. Most architecture code defines clear_user_page() as a macro >>> where, if they need a to flush the dcache or otherwise do something >>> special, they need access to some external primitive. And this primitive >>> which might not be visible in contexts that we include this header. >>> For instance this one on sparc: >>> https://lore.kernel.org/lkml/202509030338.DlQJTxIk-lkp@intel.com/ >>> Defining as a macro to get around that. But maybe there's a better >>> way? >> >> Can we just move it to mm/utils.c and not have it be an inline function? > > Thanks. Yeah, that's a good place for it. If the function call overhead is ever a problem, we can look into reworking that. -- Cheers David / dhildenb