linux-kernel - Re: [PATCH 5/9] x86/clear_pages: add clear

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Date:   Sun, 9 Apr 2023 14:26:26 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ankur Arora <ankur.a.arora@...cle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
        torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
        luto@...nel.org, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, mingo@...hat.com, juri.lelli@...hat.com,
        mgorman@...e.de, rostedt@...dmis.org, tglx@...utronix.de,
        vincent.guittot@...aro.org, jon.grimm@....com, bharata@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH 5/9] x86/clear_pages: add clear_pages()

On Fri, Apr 07, 2023 at 12:34:44PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 06, 2023 at 05:50:18PM -0700, Ankur Arora wrote:
> > 
> > Peter Zijlstra <peterz@...radead.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@...cle.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 :/

What (I think) it's suppsoed to be is that clear_page() works on kernel
pages that are never seen by userspace while clear_user_page() works
on kernel mappings of pages the user can definitely see.  This makes
no difference to x86, but some architectures can skip a lot of cache
flushing.