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

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Message-ID: <6743ead5-4d61-4274-a24f-13a8a8265aee@redhat.com>
Date: Wed, 24 Sep 2025 13:05:42 +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 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?


-- 
Cheers

David / dhildenb