linux-kernel - Re: [PATCH v6 11/15] mm: define clear_pages(), clear_user

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Message-ID: <87frd4p49o.fsf@oracle.com>
Date: Tue, 02 Sep 2025 21:08:03 -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 v6 11/15] mm: define clear_pages(), clear_user_pages()


David Hildenbrand <david@...hat.com> writes:

> On 02.09.25 10:08, Ankur Arora wrote:
>> Define fallback versions of clear_pages(), clear_user_pages().
>> In absence of architectural primitives, these just do straight clearing
>> sequentially.
>> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
>> ---
>>   include/linux/mm.h | 32 ++++++++++++++++++++++++++++++++
>>   1 file changed, 32 insertions(+)
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 1ae97a0b8ec7..b8c3f265b497 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -3768,6 +3768,38 @@ static inline void clear_page_guard(struct zone *zone, struct page *page,
>>   				unsigned int order) {}
>>   #endif	/* CONFIG_DEBUG_PAGEALLOC */
>>   +#ifndef ARCH_PAGE_CONTIG_NR
>> +#define PAGE_CONTIG_NR	1
>> +#else
>> +#define PAGE_CONTIG_NR	ARCH_PAGE_CONTIG_NR
>> +#endif
>> +
>
> These likely don't belong into this aptch :)

Yeah :).

>> +#ifndef clear_pages
>> +/*
>
> /**
>
> for proper kernel doc
>
>> + * clear_pages() - clear kernel page range.
>> + * @addr: start address of page range
>> + * @npages: number of pages
>> + *
>> + * Assumes that (@addr, +@...ges) references a kernel region.
>> + * Like clear_page(), this does absolutely no exception handling.
>> + */
>> +static inline void clear_pages(void *addr, unsigned int npages)
>> +{
>> +	for (int i = 0; i < npages; i++)
>> +		clear_page(addr + i * PAGE_SIZE);
>
> If we know that we will clear at least one page (which we can document)
>
> do {
> 	clear_page(addr);
> 	addr += PAGE_SIZE;
> } while (--npages);
>
> Similarly for the case below.

Ack. Though how about the following instead? Slightly less clear but
probably better suited for caching the likely access pattern.

   addr += (npages - 1) * PAGE_SIZE;
   do {
           clear_page(addr);
           addr -= PAGE_SIZE;
   } while (--npages);

>> +}
>> +#endif
>> +
>> +#ifndef clear_user_pages
>
> Can we add kernel doc here as well?

Will do.

Thanks for the quick reviews.

--
ankur