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

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Message-ID: <5ec85b0b-9848-4cee-98f4-37953d504773@redhat.com>
Date: Tue, 23 Sep 2025 10:04:27 +0200
From: David Hildenbrand <david@...hat.com>
To: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, x86@...nel.org
Cc: 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 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".

> + * 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.

> + * @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. Any reason this is not a static inline 
function?

-- 
Cheers

David / dhildenb