linux-kernel - Re: [PATCH v9 2/7] mm: introduce clear_pages() and clear_user

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Message-ID: <10a90b15-3e26-451a-bebf-f7ad8ba67d1b@kernel.org>
Date: Sun, 23 Nov 2025 14:17:37 +0100
From: "Christophe Leroy (CS GROUP)" <chleroy@...nel.org>
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, david@...nel.org, bp@...en8.de,
 dave.hansen@...ux.intel.com, hpa@...or.com, mingo@...hat.com,
 mjguzik@...il.com, luto@...nel.org, peterz@...radead.org,
 tglx@...utronix.de, willy@...radead.org, raghavendra.kt@....com,
 boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH v9 2/7] mm: introduce clear_pages() and clear_user_pages()



Le 21/11/2025 à 21:23, Ankur Arora a écrit :
> Introduce clear_pages(), to be overridden by architectures that
> support more efficient clearing of consecutive pages.
> 
> Also introduce clear_user_pages(), however, we will not expect
> this function to be overridden anytime soon.
> 
> We have to place the clear_user_pages() variant that uses
> clear_user_page() into mm/util.c for now to work around
> macro magic on sparc and m68k.
> 
> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
> Acked-by: David Hildenbrand (Red Hat) <david@...nel.org>
> ---
> 
> Notes:
>     - Use macros clear_pages, clear_user_page, instead of __HAVE_ARCH_CLEAR_PAGES,
>       __HAVE_ARCH_CLEAR_USER_PAGE.
> 
>   include/linux/mm.h | 41 +++++++++++++++++++++++++++++++++++++++++
>   mm/util.c          | 13 +++++++++++++
>   2 files changed, 54 insertions(+)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 6fa6c188f99a..c397ee2f6dd5 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3879,6 +3879,26 @@ 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 for kernel-internal use.
> + * @addr: start address
> + * @npages: number of pages
> + *
> + * Use clear_user_pages() instead when 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);

Why a 'do while' instead of a 'while' ?

Are you certain that this function will never ever be called with a nul 
npages ?

> +}
> +#endif
> +
>   #ifndef clear_user_page
>   /**
>    * clear_user_page() - clear a page to be mapped to user space
> @@ -3901,6 +3921,27 @@ static inline void clear_user_page(void *addr, unsigned long vaddr, struct page
>   }
>   #endif
>   
> +/**
> + * clear_user_pages() - clear a page range to be mapped to user space
> + * @addr: start address
> + * @vaddr: start address of the user mapping
> + * @page: start page
> + * @npages: number of pages
> + *
> + * Assumes that the region (@addr, +@...ges) has been validated
> + * already so this does no exception handling.
> + */
> +#ifdef clear_user_pages
> +void clear_user_pages(void *addr, unsigned long vaddr,
> +		struct page *page, unsigned int npages);

By doing this you forbid architectures to define it as a static inline, 
is that wanted ?

> +#else
> +static inline void clear_user_pages(void *addr, unsigned long vaddr,
> +		struct page *page, unsigned int npages)
> +{
> +	clear_pages(addr, npages);
> +}
> +#endif
> +
>   #ifdef __HAVE_ARCH_GATE_AREA
>   extern struct vm_area_struct *get_gate_vma(struct mm_struct *mm);
>   extern int in_gate_area_no_mm(unsigned long addr);
> diff --git a/mm/util.c b/mm/util.c
> index 8989d5767528..3c6cd44db1bd 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -1344,3 +1344,16 @@ bool page_range_contiguous(const struct page *page, unsigned long nr_pages)
>   }
>   EXPORT_SYMBOL(page_range_contiguous);
>   #endif
> +
> +#ifdef clear_user_page
> +void clear_user_pages(void *addr, 

What happens if clear_user_page is defined but not clear_user_pages ? In 
that case it seems like the definition in linux/mm.h will conflict.

unsigned long vaddr,
> +		struct page *page, unsigned int npages)
> +{
> +	do {
> +		clear_user_page(addr, vaddr, page);
> +		addr += PAGE_SIZE;
> +		vaddr += PAGE_SIZE;
> +		page++;
> +	} while (--npages);

Same, are you sure npages will never be nul ?

> +}
> +#endif