linux-kernel - Re: [RFC] arm64: mm: update max

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Message-ID: <20210924205258.GA17966@hu-cgoldswo-sd.qualcomm.com>
Date:   Fri, 24 Sep 2021 13:52:58 -0700
From:   Chris Goldsworthy <quic_cgoldswo@...cinc.com>
To:     David Hildenbrand <david@...hat.com>
CC:     Florian Fainelli <f.fainelli@...il.com>,
        Chris Goldsworthy <quic_cgoldswo@...cinc.com>,
        Catalin Marinas <catalin.marinas@....com>,
        "Will Deacon" <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        Sudarshan Rajagopalan <quic_sudaraja@...cinc.com>,
        "Doug Berger" <opendmb@...il.com>
Subject: Re: [RFC] arm64: mm: update max_pfn after memory hotplug


Thanks for the response David.

On Fri, Sep 24, 2021 at 10:17:46AM +0200, David Hildenbrand wrote:
> no-map means that no direct mapping is to be created, right? We would still
> have a memmap IIRC, and the pages are PG_reserved.
> 
> Again, I think this is very similar to just having no-map regions like
> random memory holes within the existing memory layout.

For those curious, see __reserved_mem_alloc_size() >
early_init_dt_alloc_reserved_memory_arch() > memblock_mark_nomap() - the
'no-map' attribute is read in __reserved_mem_alloc_size() and the pre-requisite
steps need to have the relevant struct pages marked as PG_reserved 
are taken in memblock_mark_nomap().

> What Chris proposes here is very similar to
> arch/x86/mm/init_64.c:update_end_of_memory_vars() called during
> arch_add_memory()->add_pages() on x86-64.
> 

For other's reference, the patch was derived from what x86 is doing with max_pfn
(such that we also set max_low_pfn as is done in arm64's mm/init.c.