linux-kernel - Re: [PATCH v2 0/4] arm64: drop pfn_valid_within() and simplify pfn

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Message-ID: <9aa68d26-d736-3b75-4828-f148964eb7f0@huawei.com>
Date:   Thu, 22 Apr 2021 15:00:20 +0800
From:   Kefeng Wang <wangkefeng.wang@...wei.com>
To:     Mike Rapoport <rppt@...nel.org>,
        <linux-arm-kernel@...ts.infradead.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        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>
Subject: Re: [PATCH v2 0/4] arm64: drop pfn_valid_within() and simplify
 pfn_valid()


On 2021/4/21 14:51, 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.
>
> The idea is to mark NOMAP pages as reserved in the memory map and restore
> the intended semantics of pfn_valid() to designate availability of struct
> page for a pfn.
>
> 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.
>
> If this actually works we'll be one step closer to drop custom pfn_valid()
> on arm64 altogether.

Hi Mike，I have a question, without HOLES_IN_ZONE, the pfn_valid_within() 
in move_freepages_block()->move_freepages()
will be optimized, if there are holes in zone, the 'struce page'(memory 
map) for pfn range of hole will be free by
free_memmap(), and then the page traverse in the zone(with holes) from 
move_freepages() will meet the wrong page，
then it could panic at PageLRU(page) test, check link[1],

"The idea is to mark NOMAP pages as reserved in the memory map", I see 
the patch2 check memblock_is_nomap() in memory region
of memblock, but it seems that memblock_mark_nomap() is not called(maybe 
I missed), then memmap_init_reserved_pages() won't
work, so should the HOLES_IN_ZONE still be needed for generic mm code?

[1] 
https://lore.kernel.org/linux-arm-kernel/541193a6-2bce-f042-5bb2-88913d5f1047@arm.com/