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

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Message-ID: <20230407103444.GB430894@hirez.programming.kicks-ass.net>
Date:   Fri, 7 Apr 2023 12:34:44 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ankur Arora <ankur.a.arora@...cle.com>
Cc:     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,
        willy@...radead.org, 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 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 :/