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

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Message-ID: <d3d68f78-bd76-4cf2-b20f-a0f7c838124a@kernel.org>
Date: Mon, 24 Nov 2025 11:17:31 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: "Christophe Leroy (CS GROUP)" <chleroy@...nel.org>,
 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, 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

On 11/23/25 12:53, Christophe Leroy (CS GROUP) wrote:
> 
> 
> 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 ? :

No really, because we make use of clear_user_highpage() only when the 
arch defines it, so the highmem.h variant is ignored?

Not that I particularly enjoy this way of handling it, so something 
cleaner would be nice :)

(in particular, relying on highmem.h defines in mm.h is a bit suboptimal)

> 
> #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:

I assume you mean highmem.h

It's not really highmem.h material, but if it makes things cleaner, sure.

Might be that the compiler will not be happy about that.

@Ankur can you play with that and see if we can make compilers happy one 
way or the other?

-- 
Cheers

David