linux-kernel - Re: [RFC] mm: Enable generic pfn_valid() to handle early sections with memmap holes

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Message-ID: <20210311093302.GA30603@willie-the-truck>
Date:   Thu, 11 Mar 2021 09:33:02 +0000
From:   Will Deacon <will@...nel.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     Mike Rapoport <rppt@...nel.org>, linux-mm@...ck.org,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] mm: Enable generic pfn_valid() to handle early sections
 with memmap holes

On Thu, Mar 11, 2021 at 01:22:53PM +0530, Anshuman Khandual wrote:
> On 3/8/21 2:25 PM, Mike Rapoport wrote:
> > On Mon, Mar 08, 2021 at 08:57:53AM +0530, Anshuman Khandual wrote:
> >> Platforms like arm and arm64 have redefined pfn_valid() because their early
> >> memory sections might have contained memmap holes caused by memblock areas
> >> tagged with MEMBLOCK_NOMAP, which should be skipped while validating a pfn
> >> for struct page backing. This scenario could be captured with a new option
> >> CONFIG_HAVE_EARLY_SECTION_MEMMAP_HOLES and then generic pfn_valid() can be
> >> improved to accommodate such platforms. This reduces overall code footprint
> >> and also improves maintainability.
> > 
> > I wonder whether arm64 would still need to free parts of its memmap after
> 
> free_unused_memmap() is applicable when CONFIG_SPARSEMEM_VMEMMAP is not enabled.
> I am not sure whether there still might be some platforms or boards which would
> benefit from this. Hence lets just keep this unchanged for now.

In my opinion, unless there's a compelling reason for us to offer all of
these different implementations of the memmap on arm64 then we shouldn't
bother -- it's not like it's fun to maintain! Just use sparsemem vmemmap
and be done with it. Is there some reason we can't do that?

Will