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Subject: Re: [RFC 1/2] arm64/mm: Fix pfn_valid() for ZONE_DEVICE based memory
To:     linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Cc:     catalin.marinas@arm.com, will@kernel.org, ardb@kernel.org,
        Mark Rutland <mark.rutland@arm.com>,
        James Morse <james.morse@arm.com>,
        Robin Murphy <robin.murphy@arm.com>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        Dan Williams <dan.j.williams@intel.com>,
        David Hildenbrand <david@redhat.com>,
        Mike Rapoport <rppt@linux.ibm.com>
References: <1608621144-4001-1-git-send-email-anshuman.khandual@arm.com>
 <1608621144-4001-2-git-send-email-anshuman.khandual@arm.com>
From:   Anshuman Khandual <anshuman.khandual@arm.com>
Message-ID: <bb5b9c39-d25b-6170-68ea-5b2bf297c1fd@arm.com>
Date:   Mon, 25 Jan 2021 11:52:32 +0530
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On 12/22/20 12:42 PM, Anshuman Khandual wrote:
> 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_DEVIE based memory, all hotplugged
> normal 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@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: Robin Murphy <robin.murphy@arm.com>
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Fixes: 73b20c84d42d ("arm64: mm: implement pte_devmap support")
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>

Hello David/Mike,

Given that we would need to rework early sections, memblock semantics via a
new config i.e EARLY_SECTION_MEMMAP_HOLES and also some possible changes to
ARCH_KEEP_MEMBLOCK and HAVE_ARCH_PFN_VALID, wondering if these patches here
which fixes a problem (and improves performance) can be merged first. After
that, I could start working on the proposed rework. Could you please let me
know your thoughts on this. Thank you.

- Anshuman