linux-kernel - Re: [RFC/RFT PATCH 0/3] arm64: drop pfn_valid_within() and simplify pfn

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Date:   Thu, 8 Apr 2021 10:49:02 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Mike Rapoport <rppt@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        David Hildenbrand <david@...hat.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,
        James Morse <james.morse@....com>
Subject: Re: [RFC/RFT PATCH 0/3] arm64: drop pfn_valid_within() and simplify
 pfn_valid()

Adding James here.

+ James Morse <james.morse@....com>

On 4/7/21 10:56 PM, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@...ux.ibm.com>
> 
> Hi,
> 
> These patches aim to remove CONFIG_HOLES_IN_ZONE and essentially hardwire
> pfn_valid_within() to 1. 

That would be really great for arm64 platform as it will save CPU cycles on
many generic MM paths, given that our pfn_valid() has been expensive.

> 
> The idea is to mark NOMAP pages as reserved in the memory map and restore

Though I am not really sure, would that possibly be problematic for UEFI/EFI
use cases as it might have just treated them as normal struct pages till now.

> the intended semantics of pfn_valid() to designate availability of struct
> page for a pfn.

Right, that would be better as the current semantics is not ideal.

> 
> With this the core mm will be able to cope with the fact that it cannot use
> NOMAP pages and the holes created by NOMAP ranges within MAX_ORDER blocks
> will be treated correctly even without the need for pfn_valid_within.
> 
> The patches are only boot tested on qemu-system-aarch64 so I'd really
> appreciate memory stress tests on real hardware.

Did some preliminary memory stress tests on a guest with portions of memory
marked as MEMBLOCK_NOMAP and did not find any obvious problem. But this might
require some testing on real UEFI environment with firmware using MEMBLOCK_NOMAP
memory to make sure that changing these struct pages to PageReserved() is safe.

> 
> If this actually works we'll be one step closer to drop custom pfn_valid()
> on arm64 altogether.

Right, planning to rework and respin the RFC originally sent last month.

https://patchwork.kernel.org/project/linux-mm/patch/1615174073-10520-1-git-send-email-anshuman.khandual@arm.com/