-----Original Message-----
From: Catalin Marinas [mailto:catalin.marinas@arm.com]
Sent: Friday, September 4, 2020 12:06 AM
To: Mike Rapoport <redacted>
Cc: liwei (CM) <redacted>; will@kernel.org; Xiaqing (A)
[off-list ref]; Chenfeng (puck) [off-list ref];
butao [off-list ref]; fengbaopeng [off-list ref];
nsaenzjulienne@suse.de; steve.capper@arm.com; Song Bao Hua (Barry Song)
[off-list ref]; linux-arm-kernel@lists.infradead.org;
linux-kernel@vger.kernel.org; sujunfei [off-list ref]
Subject: Re: [PATCH v2] arm64: mm: free unused memmap for sparse memory
model that define VMEMMAP

On Mon, Aug 17, 2020 at 11:04:05AM +0300, Mike Rapoport wrote:

quoted

On Wed, Aug 12, 2020 at 09:06:55AM +0800, Wei Li wrote:

quoted

For the memory hole, sparse memory model that define

SPARSEMEM_VMEMMAP

quoted

quoted

do not free the reserved memory for the page map, this patch do it.

I've been thinking about it a bit more and it seems that instead of
freeing unused memory map it would be better to allocate the exact
memory map from the beginning.

In sparse_init_nid() we can replace PAGES_PER_SECTION parameter to
__populate_section_memmap() with the calculated value for architectures
that define HAVE_ARCH_PFN_VALID.

Or just use a smaller PAGES_PER_SECTION and reduce the waste ;).

Just to be clear, are you suggesting that we should use pfn_valid() on
the pages within a section to calculate the actual range? The
pfn_valid() implementation on arm64 checks for the validity of a sparse
section, so this would be called from within the sparse_init() code
path. I hope there's no dependency but I haven't checked. If it works,
it's fine by me, it solves the FLATMEM mem_map freeing as well.

With 4KB pages on arm64, vmemmap_populate() stops at the pmd level, so
it always allocates PMD_SIZE. Wei's patch also only frees in PMD_SIZE
amounts. So, with a sizeof(struct page) of 64 (2^6), a PMD_SIZE mem_map
section would cover 2^(21-6) pages, so that's equivalent to a
SECTION_SIZE_BITS of 21-6+12 = 27.

If we reduce SECTION_SIZE_BITS to 27 or less, this patch is a no-op.

It would be the simplest way to fix this issue. It seems X86_64 is
also using 27.

@wei, has you ever tried to send a patch to change SECTION_SIZE_BITS
to 27 for ARM64?

--
Catalin

Thanks
Barry

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