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

[David Hildenbrand (Red Hat)]

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@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)

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

--
Cheers

David