linux-kernel - Re: [PATCH] arm64: define __alloc_zeroed_user

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Date:   Thu, 12 Mar 2020 20:59:28 +0100
From:   Alexander Potapenko <glider@...gle.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        linux-arm-kernel@...ts.infradead.org,
        LKML <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] arm64: define __alloc_zeroed_user_highpage

On Thu, Mar 12, 2020 at 5:49 PM Mark Rutland <mark.rutland@....com> wrote:
>
> On Thu, Mar 12, 2020 at 04:59:20PM +0100, glider@...gle.com wrote:
> > When running the kernel with init_on_alloc=1, calling the default
> > implementation of __alloc_zeroed_user_highpage() from include/linux/highmem.h
> > leads to double-initialization of the allocated page (first by the page
> > allocator, then by clear_user_page().
> > Calling alloc_page_vma() with __GFP_ZERO, similarly to e.g. x86, seems
> > to be enough to ensure the user page is zeroed only once.
>
> Just to check, is there a functional ussue beyond the redundant zeroing,
> or is this jsut a performance issue?

This is just a performance issue that only manifests when running the
kernel with init_on_alloc=1.

> On architectures with real highmem, does GFP_HIGHUSER prevent the
> allocator from zeroing the page in this case, or is the architecture
> prevented from allocating from highmem?

I was hoping one of ARM maintainers can answer this question. My
understanding was that __GFP_ZERO should be sufficient, but there's
probably something I'm missing.



> Thanks,
> Mark.
>
> >
> > Signed-off-by: Alexander Potapenko <glider@...gle.com>
> > ---
> >  arch/arm64/include/asm/page.h | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/arch/arm64/include/asm/page.h b/arch/arm64/include/asm/page.h
> > index d39ddb258a049..75d6cd23a6790 100644
> > --- a/arch/arm64/include/asm/page.h
> > +++ b/arch/arm64/include/asm/page.h
> > @@ -21,6 +21,10 @@ extern void __cpu_copy_user_page(void *to, const void *from,
> >  extern void copy_page(void *to, const void *from);
> >  extern void clear_page(void *to);
> >
> > +#define __alloc_zeroed_user_highpage(movableflags, vma, vaddr) \
> > +     alloc_page_vma(GFP_HIGHUSER | __GFP_ZERO | movableflags, vma, vaddr)
> > +#define __HAVE_ARCH_ALLOC_ZEROED_USER_HIGHPAGE
> > +
> >  #define clear_user_page(addr,vaddr,pg)  __cpu_clear_user_page(addr, vaddr)
> >  #define copy_user_page(to,from,vaddr,pg) __cpu_copy_user_page(to, from, vaddr)
> >
> > --
> > 2.25.1.481.gfbce0eb801-goog
> >



-- 
Alexander Potapenko
Software Engineer

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