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

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Message-ID: <ff7b2ca9-d4d7-41b3-acd8-2d90294e59cd@kernel.org>
Date: Mon, 24 Nov 2025 15:02:51 +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 11/24/25 11:17, David Hildenbrand (Red Hat) wrote:
> 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)

Correction: there is no highmem.h dependency, as clear_user_highpage() 
comes directly from the architecture helpers in page.h

-- 
Cheers

David