linux-kernel - Re: [PATCH v9 1/7] treewide: provide a generic clear_user

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Message-ID: <189bc218-8f19-4686-b2ee-5ddc6a0b4684@kernel.org>
Date: Sun, 23 Nov 2025 12:53:07 +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 1/7] treewide: provide a generic clear_user_page()
 variant



Le 21/11/2025 à 21:23, Ankur Arora a écrit :
> From: David Hildenbrand <david@...hat.com>
> 
> Let's drop all variants that effectively map to clear_page() and
> provide it in a generic variant instead.
> 
> We'll use the macro clear_user_page to indicate whether an architecture
> provides it's own variant.
> 
> We have to be a bit careful if an architecture provides a custom
> clear_user_highpage(), because then it's very likely that some special
> flushing magic is happening behind the scenes.
> 
> Maybe at some point these should be CONFIG_ options.
> 
> Note that for parisc, clear_page() and clear_user_page() map to
> clear_page_asm(), so we can just get rid of the custom clear_user_page()
> implementation. There is a clear_user_page_asm() function on parisc,
> that seems to be unused. Not sure what's up with that.
> 
> Signed-off-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>

...

> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 7c79b3369b82..6fa6c188f99a 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3879,6 +3879,28 @@ static inline void clear_page_guard(struct zone *zone, struct page *page,
>   				unsigned int order) {}
>   #endif	/* CONFIG_DEBUG_PAGEALLOC */
>   
> +#ifndef clear_user_page
> +/**
> + * clear_user_page() - clear a page to be mapped to user space
> + * @addr: the address of the page
> + * @vaddr: the address of the user mapping
> + * @page: the page
> + */
> +static inline void clear_user_page(void *addr, unsigned long vaddr, struct page *page)
> +{
> +#ifdef clear_user_highpage
> +	/*
> +	 * If an architecture defines its own clear_user_highpage() variant,
> +	 * then we have to be a bit more careful here and cannot simply
> +	 * rely on clear_page().
> +	 */
> +	clear_user_highpage(page, vaddr);
> +#else
> +	clear_page(addr);
> +#endif
> +}
> +#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);


Isn't it chicken and egg with clear_user_highpage() in linux/highmem.h ? :

#ifndef clear_user_highpage
static inline void clear_user_highpage(struct page *page, unsigned long 
vaddr)
{
	void *addr = kmap_local_page(page);
	clear_user_page(addr, vaddr, page);
	kunmap_local(addr);
}
#endif

And at the end this function is the only caller of clear_user_page() so 
there is apparently no need for a generic clear_user_page(), at least 
not when clear_user_highpage() is defined.

I think is would be simpler and cleaner to instead add the following in 
linux/highmem.c:

#ifndef clear_user_page
/**
  * clear_user_page() - clear a page to be mapped to user space
  * @addr: the address of the page
  * @vaddr: the address of the user mapping
  * @page: the page
  */
static inline void clear_user_page(void *addr, unsigned long vaddr, 
struct page *page)
{
	clear_page(addr);
}
#endif