linux-kernel - Re: [RFC PATCH 1/2] arm64: mm: Make virt_addr_valid to check for pfn

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Message-ID: <f9e1c4a0-a19f-6a56-7641-b4a9973c0807@gmail.com>
Date:   Mon, 2 Aug 2021 18:52:56 +0300
From:   Oleksandr <olekstysh@...il.com>
To:     Mike Rapoport <rppt@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Will Deacon <will@...nel.org>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Joey Gouly <joey.gouly@....com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Juergen Gross <jgross@...e.com>, Julien Grall <julien@....org>,
        Wei Chen <Wei.Chen@....com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [RFC PATCH 1/2] arm64: mm: Make virt_addr_valid to check for
 pfn_valid again


Hello, all.


On 02.08.21 18:08, Mike Rapoport wrote:
> Hi,
>
> On Mon, Aug 02, 2021 at 01:19:48PM +0100, Catalin Marinas wrote:
>> Adding Mike and Anshuman,
>>
>> On Wed, Jul 28, 2021 at 07:44:15PM +0300, Oleksandr Tyshchenko wrote:
>>> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
>>>
>>> The problem is that Arm's implementation of virt_addr_valid()
>>> leads to memblock_is_map_memory() check, which will fail for
>>> ZONE_DEVICE based addresses. But, the pfn_valid() check in turn
>>> is able to cope with ZONE_DEVICE based memory.
>>>
>>> You can find a good explanation of that problem at:
>>> https://lore.kernel.org/lkml/1614921898-4099-2-git-send-email-anshuman.khandual@arm.com
>>>
>>> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
>>> ---
>>> I am not quite sure whether it is a "correct" place and
>>> the change itself, I just partially restored a behaviour before:
>>> https://lore.kernel.org/lkml/20210511100550.28178-4-rppt@kernel.org
>>> So, the target of this patch is to get a feedback how to resolve
>>> this properly if, of course, this really needs to be resolved
>>> (I might miss important bits here).
>>>
>>> It is worth mentioning that patch doesn't fix the current code base
>>> (if I am not mistaken, no one calls virt_addr_valid() on Arm64 for
>>> ZONE_DEVICE based addresses at the moment, so it seems that nothing
>>> is broken), the fix is intended for the subsequent patch in this
>>> series that will try to enable Xen's "unpopulated-alloc" usage
>>> on Arm (it was enabled on x86 so far).
>>> Please see:
>>> [RFC PATCH 2/2] xen/unpopulated-alloc: Query hypervisor to provide
>>> unallocated space
>>>
>>> The subsequent patch will enable the code where virt_addr_valid()
>>> is used in drivers/xen/unpopulated-alloc.c:fill_list() to check that
>>> a virtual address returned by memremap_pages() is valid.
>   
>> I wonder what the point of calling virt_addr_valid() in fill_list() is?
>> If memremap_pages() succeeded, the pages were mapped at the returned
>> vaddr, there's no need for an additional virt_addr_valid() check.
> The virt_addr_valid() check in fill_list() looks bogus to me as well. If
> memremap_pages() succeeds the range is guaranteed to have proper page
> table.
>
> I believe the first patch should be rather removal of the virt_addr_valid()
> check in fill_list().
Thank you for the clarification, I will send a patch to remove 
virt_addr_valid()
check in fill_list() for the non-RFC version.


>   
>>> ---
>>>   arch/arm64/include/asm/memory.h | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
>>> index 824a365..1a35a44 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_is_map_memory(virt_to_pfn(__addr));	\
>>> +	__is_lm_address(__addr) && pfn_valid(virt_to_pfn(__addr));	\
>>>   })
>> pfn_valid() only guarantees the presence of a struct page but not
>> necessarily that the virtual address is accessible (valid). So this
>> change would break the NOMAP ranges case.
> +1
Oh, I got it.

-- 
Regards,

Oleksandr Tyshchenko