linux-kernel - Re: [PATCH v7 10/16] mm: define clear_pages(), clear_user

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Message-ID: <5d763fb0-b216-4b6c-ac93-510fa471575f@redhat.com>
Date: Tue, 30 Sep 2025 11:43:19 +0200
From: David Hildenbrand <david@...hat.com>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
 akpm@...ux-foundation.org, bp@...en8.de, dave.hansen@...ux.intel.com,
 hpa@...or.com, mingo@...hat.com, mjguzik@...il.com, luto@...nel.org,
 peterz@...radead.org, acme@...nel.org, namhyung@...nel.org,
 tglx@...utronix.de, willy@...radead.org, raghavendra.kt@....com,
 boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH v7 10/16] mm: define clear_pages(), clear_user_pages()

On 25.09.25 07:25, Ankur Arora wrote:
> 
> David Hildenbrand <david@...hat.com> writes:
> 
>> On 23.09.25 22:26, Ankur Arora wrote:
>>> David Hildenbrand <david@...hat.com> 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 <ankur.a.arora@...cle.com>
>>>>> ---
>>>>>     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, +@...ges) 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, +@...ges) 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