linux-kernel - Re: [PATCH] arm: Extend the check for RAM in /dev/mem

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Date:   Fri, 12 Jul 2019 16:34:23 +0100
From:   Will Deacon <will@...nel.org>
To:     "Raslan, KarimAllah" <karahmed@...zon.de>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "yaojun8558363@...il.com" <yaojun8558363@...il.com>,
        "ard.biesheuvel@...aro.org" <ard.biesheuvel@...aro.org>,
        "linux@...linux.org.uk" <linux@...linux.org.uk>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "james.morse@....com" <james.morse@....com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "info@...ux.net" <info@...ux.net>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "yuzhao@...gle.com" <yuzhao@...gle.com>,
        "rppt@...ux.ibm.com" <rppt@...ux.ibm.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "anders.roxell@...aro.org" <anders.roxell@...aro.org>,
        "anshuman.khandual@....com" <anshuman.khandual@....com>
Subject: Re: [PATCH] arm: Extend the check for RAM in /dev/mem

On Fri, Jul 12, 2019 at 03:13:38PM +0000, Raslan, KarimAllah wrote:
> On Fri, 2019-07-12 at 15:57 +0100, Will Deacon wrote:
> > On Fri, Jul 12, 2019 at 12:21:21AM +0200, KarimAllah Ahmed wrote:
> > > 
> > > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > > index 3645f29..cdc3e8e 100644
> > > --- a/arch/arm64/mm/mmu.c
> > > +++ b/arch/arm64/mm/mmu.c
> > > @@ -78,7 +78,7 @@ void set_swapper_pgd(pgd_t *pgdp, pgd_t pgd)
> > >  pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
> > >  			      unsigned long size, pgprot_t vma_prot)
> > >  {
> > > -	if (!pfn_valid(pfn))
> > > +	if (!memblock_is_memory(__pfn_to_phys(pfn)))
> > 
> > This looks broken to me, since it will end up returning 'true' for nomap
> > memory and we really don't want to map that using writeback attributes.
> 
> True, I will fix this by using memblock_is_map_memory instead. That said, do
> you have any concerns about this approach in general?

If you do that, I don't understand why you need the patch at all given our
implementation of pfn_valid() in arch/arm64/mm/init.c.

Will