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

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Message-ID: <87frcbayt2.fsf@oracle.com>
Date: Wed, 24 Sep 2025 22:25:29 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, 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()


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.

--
ankur