Re: [PATCH V3 1/2] arm64/mm: Fix pfn_valid() for ZONE_DEVICE based memory

[Catalin Marinas]

On Fri, Mar 05, 2021 at 10:54:57AM +0530, Anshuman Khandual wrote:
> pfn_valid() validates a pfn but basically it checks for a valid struct page
> backing for that pfn. It should always return positive for memory ranges
> backed with struct page mapping. But currently pfn_valid() fails for all
> ZONE_DEVICE based memory types even though they have struct page mapping.
> 
> pfn_valid() asserts that there is a memblock entry for a given pfn without
> MEMBLOCK_NOMAP flag being set. The problem with ZONE_DEVICE based memory is
> that they do not have memblock entries. Hence memblock_is_map_memory() will
> invariably fail via memblock_search() for a ZONE_DEVICE based address. This
> eventually fails pfn_valid() which is wrong. memblock_is_map_memory() needs
> to be skipped for such memory ranges. As ZONE_DEVICE memory gets hotplugged
> into the system via memremap_pages() called from a driver, their respective
> memory sections will not have SECTION_IS_EARLY set.
> 
> Normal hotplug memory will never have MEMBLOCK_NOMAP set in their memblock
> regions. Because the flag MEMBLOCK_NOMAP was specifically designed and set
> for firmware reserved memory regions. memblock_is_map_memory() can just be
> skipped as its always going to be positive and that will be an optimization
> for the normal hotplug memory. Like ZONE_DEVICE based memory, all normal
> hotplugged memory too will not have SECTION_IS_EARLY set for their sections
> 
> Skipping memblock_is_map_memory() for all non early memory sections would
> fix pfn_valid() problem for ZONE_DEVICE based memory and also improve its
> performance for normal hotplug memory as well.
> 
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Ard Biesheuvel <ardb@xxxxxxxxxx>
> Cc: Robin Murphy <robin.murphy@xxxxxxx>
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
> Fixes: 73b20c84d42d ("arm64: mm: implement pte_devmap support")
> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> ---
>  arch/arm64/mm/init.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 0ace5e68efba..5920c527845a 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -230,6 +230,18 @@ int pfn_valid(unsigned long pfn)
>  
>  	if (!valid_section(__pfn_to_section(pfn)))
>  		return 0;
> +
> +	/*
> +	 * ZONE_DEVICE memory does not have the memblock entries.
> +	 * memblock_is_map_memory() check for ZONE_DEVICE based
> +	 * addresses will always fail. Even the normal hotplugged
> +	 * memory will never have MEMBLOCK_NOMAP flag set in their
> +	 * memblock entries. Skip memblock search for all non early
> +	 * memory sections covering all of hotplug memory including
> +	 * both normal and ZONE_DEVICE based.
> +	 */
> +	if (!early_section(__pfn_to_section(pfn)))
> +		return pfn_section_valid(__pfn_to_section(pfn), pfn);

Would something like this work instead:

	if (online_device_section(ms))
		return 1;

to avoid the assumptions around early_section()?

-- 
Catalin