Re: [PATCH v5 10/14] x86/mm: Simplify clear_page_*

[David Hildenbrand]

On 11.07.25 21:24, Ankur Arora wrote:
> David Hildenbrand <david@xxxxxxxxxx> writes:
>
> > On 11.07.25 19:26, Ankur Arora wrote:
> > > David Hildenbrand <david@xxxxxxxxxx> writes:
> > >
> > > > On 10.07.25 02:59, Ankur Arora wrote:
> > > > > clear_page_rep() and clear_page_erms() are wrappers around "REP; STOS"
> > > > > variations. Inlining gets rid of an unnecessary CALL/RET (which isn't
> > > > > free when using RETHUNK speculative execution mitigations.)
> > > > > Fixup and rename clear_page_orig() to adapt to the changed calling
> > > > > convention.
> > > > > And, add a comment from Dave Hansen detailing various clearing mechanisms
> > > > > used in clear_page().
> > > > > Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
> > > > > ---
> > > > >     arch/x86/include/asm/page_32.h |  6 +++++
> > > > >     arch/x86/include/asm/page_64.h | 42 ++++++++++++++++++++++++++--------
> > > > >     arch/x86/lib/clear_page_64.S   | 39 +++++++------------------------
> > > > >     3 files changed, 46 insertions(+), 41 deletions(-)
> > > > > diff --git a/arch/x86/include/asm/page_32.h b/arch/x86/include/asm/page_32.h
> > > > > index 0c623706cb7e..a8ff43bb9652 100644
> > > > > --- a/arch/x86/include/asm/page_32.h
> > > > > +++ b/arch/x86/include/asm/page_32.h
> > > > > @@ -17,6 +17,12 @@ extern unsigned long __phys_addr(unsigned long);
> > > > >       #include <linux/string.h>
> > > > >     +/*
> > > >
> > > > /** if this was supposed to be kernel doc (which it looks like it is)
> > > >
> > > > > + * clear_page() - clear kernel page.
> > > >
> > > > "clear a kernel page"
> > > >
> > > > Although I am not sure what a "kernel page" is.
> > > >
> > > > Did you mean "clear a page using a kernel virtual address" ?
> > > Thanks. Yes, this makes way more sense.
> >
> > FWIW, most clear_user_page() will just call clear_page(). The ones that don't,
> > have to deal with the dcache flushing, as the page might have cache alias from
> > another (mm) address space.
> >
> > So clear_page() is just about clearing page content using a kernel virtual
> > address, and clear_user_page() is additionally taking care of any dcache
> > aliases.
>
> Yeah that's a good point.
>
> Right now I'm assuming that clear_user_page() and clear_page() are
> effectively identical. Which as you say, isn't true for all the
> architectures that can't disambiguate dcache aliases.
>
> So maybe I should be adjusting the naming of the helpers a bit?
>
> Specifically, by calling clear_user_pages() from clear_pages_resched().
> Which on x86 etc can just call clear_pages().

Right, and calling it from folio_zero_user() :)

--
Cheers,

David / dhildenb