Re: [PATCH 5/9] x86/clear_pages: add clear_pages()

Peter Zijlstra
2023-04-07T10:35:29Z

On Thu, Apr 06, 2023 at 05:50:18PM -0700, Ankur Arora wrote:
>
> Peter Zijlstra <peterz@infradead.org> writes:
>
> > On Sun, Apr 02, 2023 at 10:22:29PM -0700, Ankur Arora wrote:
> >> Add clear_pages() and define the ancillary clear_user_pages().
> >>
> >> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
> >> ---
> >> arch/x86/include/asm/page.h | 6 ++++++
> >> arch/x86/include/asm/page_32.h | 6 ++++++
> >> arch/x86/include/asm/page_64.h | 9 +++++++--
> >> 3 files changed, 19 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/arch/x86/include/asm/page.h b/arch/x86/include/asm/page.h
> >> index d18e5c332cb9..03e3c69fc427 100644
> >> --- a/arch/x86/include/asm/page.h
> >> +++ b/arch/x86/include/asm/page.h
> >> @@ -28,6 +28,12 @@ static inline void clear_user_page(void *page, unsigned long vaddr,
> >> clear_page(page);
> >> }
> >>
> >> +static inline void clear_user_pages(void *page, unsigned long vaddr,
> >> + struct page *pg, unsigned int nsubpages)
> >> +{
> >> + clear_pages(page, nsubpages);
> >> +}
> >
> > This seems dodgy, clear_user* has slightly different semantics. It needs
> > the access_ok() and stac/clac thing on at the very least.
>
> That can't be right. On x86, clear_user_page(), copy_user_page() (and
> now the multi-page versions) only write to kernel maps of user pages.
> That's why they can skip the access_ok(), stac/clac or uacess
> exception handling.

Bah, that namespace is a mess :/