linux-kernel - Re: [RFC/RFT PATCH 2/3] arm64: decouple check whether pfn is normal memory from pfn

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Message-ID: <YHdQtmuxpqi4wCE/@kernel.org>
Date:   Wed, 14 Apr 2021 23:29:42 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Anshuman Khandual <anshuman.khandual@....com>,
        linux-arm-kernel@...ts.infradead.org,
        Ard Biesheuvel <ardb@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Will Deacon <will@...nel.org>, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC/RFT PATCH 2/3] arm64: decouple check whether pfn is normal
 memory from pfn_valid()

On Wed, Apr 14, 2021 at 05:58:26PM +0200, David Hildenbrand wrote:
> On 08.04.21 07:14, Anshuman Khandual wrote:
> > 
> > On 4/7/21 10:56 PM, Mike Rapoport wrote:
> > > From: Mike Rapoport <rppt@...ux.ibm.com>
> > > 
> > > The intended semantics of pfn_valid() is to verify whether there is a
> > > struct page for the pfn in question and nothing else.
> > 
> > Should there be a comment affirming this semantics interpretation, above the
> > generic pfn_valid() in include/linux/mmzone.h ?
> > 
> > > 
> > > Yet, on arm64 it is used to distinguish memory areas that are mapped in the
> > > linear map vs those that require ioremap() to access them.
> > > 
> > > Introduce a dedicated pfn_is_memory() to perform such check and use it
> > > where appropriate.
> > > 
> > > Signed-off-by: Mike Rapoport <rppt@...ux.ibm.com>
> > > ---
> > >   arch/arm64/include/asm/memory.h | 2 +-
> > >   arch/arm64/include/asm/page.h   | 1 +
> > >   arch/arm64/kvm/mmu.c            | 2 +-
> > >   arch/arm64/mm/init.c            | 6 ++++++
> > >   arch/arm64/mm/ioremap.c         | 4 ++--
> > >   arch/arm64/mm/mmu.c             | 2 +-
> > >   6 files changed, 12 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> > > index 0aabc3be9a75..7e77fdf71b9d 100644
> > > --- a/arch/arm64/include/asm/memory.h
> > > +++ b/arch/arm64/include/asm/memory.h
> > > @@ -351,7 +351,7 @@ static inline void *phys_to_virt(phys_addr_t x)
> > >   #define virt_addr_valid(addr)	({					\
> > >   	__typeof__(addr) __addr = __tag_reset(addr);			\
> > > -	__is_lm_address(__addr) && pfn_valid(virt_to_pfn(__addr));	\
> > > +	__is_lm_address(__addr) && pfn_is_memory(virt_to_pfn(__addr));	\
> > >   })
> > >   void dump_mem_limit(void);
> > > diff --git a/arch/arm64/include/asm/page.h b/arch/arm64/include/asm/page.h
> > > index 012cffc574e8..32b485bcc6ff 100644
> > > --- a/arch/arm64/include/asm/page.h
> > > +++ b/arch/arm64/include/asm/page.h
> > > @@ -38,6 +38,7 @@ void copy_highpage(struct page *to, struct page *from);
> > >   typedef struct page *pgtable_t;
> > >   extern int pfn_valid(unsigned long);
> > > +extern int pfn_is_memory(unsigned long);
> > >   #include <asm/memory.h>
> > > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > > index 8711894db8c2..ad2ea65a3937 100644
> > > --- a/arch/arm64/kvm/mmu.c
> > > +++ b/arch/arm64/kvm/mmu.c
> > > @@ -85,7 +85,7 @@ void kvm_flush_remote_tlbs(struct kvm *kvm)
> > >   static bool kvm_is_device_pfn(unsigned long pfn)
> > >   {
> > > -	return !pfn_valid(pfn);
> > > +	return !pfn_is_memory(pfn);
> > >   }
> > >   /*
> > > diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> > > index 3685e12aba9b..258b1905ed4a 100644
> > > --- a/arch/arm64/mm/init.c
> > > +++ b/arch/arm64/mm/init.c
> > > @@ -258,6 +258,12 @@ int pfn_valid(unsigned long pfn)
> > >   }
> > >   EXPORT_SYMBOL(pfn_valid);
> > > +int pfn_is_memory(unsigned long pfn)
> > > +{
> > > +	return memblock_is_map_memory(PFN_PHYS(pfn));
> > > +}
> > > +EXPORT_SYMBOL(pfn_is_memory);> +
> > 
> > Should not this be generic though ? There is nothing platform or arm64
> > specific in here. Wondering as pfn_is_memory() just indicates that the
> > pfn is linear mapped, should not it be renamed as pfn_is_linear_memory()
> > instead ? Regardless, it's fine either way.
> 
> TBH, I dislike (generic) pfn_is_memory(). It feels like we're mixing
> concepts.

Yeah, at the moment NOMAP is very much arm specific so I'd keep it this way
for now.

>  NOMAP memory vs !NOMAP memory; even NOMAP is some kind of memory
> after all. pfn_is_map_memory() would be more expressive, although still
> sub-optimal.
>
> We'd actually want some kind of arm64-specific pfn_is_system_memory() or the
> inverse pfn_is_device_memory() -- to be improved.

In my current version (to be posted soon) I've started with
pfn_lineary_mapped() but then ended up with pfn_mapped() to make it
"upward" compatible with architectures that use direct rather than linear
map :)

-- 
Sincerely yours,
Mike.