Re: [PATCH v9 1/7] treewide: provide a generic clear_user_page() variant

[David Hildenbrand (Red Hat)]

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@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
> > Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
>
> ...
>
> > 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