Subject: Re: [PATCH v1 3/4] arm64: decouple check whether pfn is in linear map from pfn_valid()

From: Mike Rapoport <[email-masked]>
Date: Wed, 21 Apr 2021 08:32:55 +0300

On Tue, Apr 20, 2021 at 05:57:57PM +0200, David Hildenbrand wrote:
> On 20.04.21 11:09, Mike Rapoport wrote:
> > From: Mike Rapoport <[email-masked]>
> > 
> > The intended semantics of pfn_valid() is to verify whether there is a
> > struct page for the pfn in question and nothing else.
> > 
> > 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_map_memory() wrapper for
> > memblock_is_map_memory() to perform such check and use it where
> > appropriate.
> > 
> > Using a wrapper allows to avoid cyclic include dependencies.
> > 
> > Signed-off-by: Mike Rapoport <[email-masked]>
> > ---
> >   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..194f9f993d30 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_map_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..99a6da91f870 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_map_memory(unsigned long);
> >   #include <asm/memory.h>
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > index 8711894db8c2..23dd99e29b23 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_map_memory(pfn);
> >   }
> >   /*
> > diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> > index 3685e12aba9b..c54e329aca15 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_map_memory(unsigned long pfn)
> > +{
> 
> I think you might have to add (see pfn_valid())
> 
> if (PHYS_PFN(PFN_PHYS(pfn)) != pfn)
> 	return 0;
> 
> to catch false positives.
 
Yeah, makes sense. 

> > +	return memblock_is_map_memory(PFN_PHYS(pfn));
> > +}
> > +EXPORT_SYMBOL(pfn_is_map_memory);
> > +
> >   static phys_addr_t memory_limit = PHYS_ADDR_MAX;
> >   /*
> > diff --git a/arch/arm64/mm/ioremap.c b/arch/arm64/mm/ioremap.c
> > index b5e83c46b23e..b7c81dacabf0 100644
> > --- a/arch/arm64/mm/ioremap.c
> > +++ b/arch/arm64/mm/ioremap.c
> > @@ -43,7 +43,7 @@ static void __iomem *__ioremap_caller(phys_addr_t phys_addr, size_t size,
> >   	/*
> >   	 * Don't allow RAM to be mapped.
> >   	 */
> > -	if (WARN_ON(pfn_valid(__phys_to_pfn(phys_addr))))
> > +	if (WARN_ON(pfn_is_map_memory(__phys_to_pfn(phys_addr))))
> >   		return NULL;
> >   	area = get_vm_area_caller(size, VM_IOREMAP, caller);
> > @@ -84,7 +84,7 @@ EXPORT_SYMBOL(iounmap);
> >   void __iomem *ioremap_cache(phys_addr_t phys_addr, size_t size)
> >   {
> >   	/* For normal memory we already have a cacheable mapping. */
> > -	if (pfn_valid(__phys_to_pfn(phys_addr)))
> > +	if (pfn_is_map_memory(__phys_to_pfn(phys_addr)))
> >   		return (void __iomem *)__phys_to_virt(phys_addr);
> >   	return __ioremap_caller(phys_addr, size, __pgprot(PROT_NORMAL),
> > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > index 5d9550fdb9cf..26045e9adbd7 100644
> > --- a/arch/arm64/mm/mmu.c
> > +++ b/arch/arm64/mm/mmu.c
> > @@ -81,7 +81,7 @@ 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 (!pfn_is_map_memory(pfn))
> >   		return pgprot_noncached(vma_prot);
> >   	else if (file->f_flags & O_SYNC)
> >   		return pgprot_writecombine(vma_prot);
> > 
> 
> As discussed, in the future it would be nice if we could just rely on the
> memmap state. There are cases where pfn_is_map_memory() will now be slower
> than pfn_valid() -- e.g., we don't check for valid_section() in case of
> CONFIG_SPARSEMEM. This would apply where pfn_valid() would have returned
> "0".
>
> As we're not creating the direct map, kern_addr_valid() shouldn't need love.
> It'd be some kind of ugly if some generic code used by arm64 would be
> relying in case of arm64 on pfn_valid() to return the expected result; I
> doubt it.

No doubt there is a room for further improvement in this area.
 
> Acked-by: David Hildenbrand <[email-masked]>

Thanks!

-- 
Sincerely yours,
Mike.

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