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

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Message-ID: <62161846-4f03-e4b1-ae0b-fdf96f78d97c@redhat.com>
Date:   Thu, 15 Apr 2021 11:31:26 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Mike Rapoport <rppt@...nel.org>
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 14.04.21 22:29, Mike Rapoport wrote:
> 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 :)

And even that is moot. It doesn't tell you if a PFN is *actually* mapped 
(hello secretmem).

I'd suggest to just use memblock_is_map_memory() in arch specific code. 
Then it's clear what we are querying exactly and what the semantics 
might be.

-- 
Thanks,

David / dhildenb