Hi, Barry

I have changed SECTION_SIZE_BITS to 27 in our products, but I don't have to submit it.

-----邮件原件-----
发件人: Song Bao Hua (Barry Song) 
发送时间: 2020年11月16日 16:34
收件人: Catalin Marinas [off-list ref]; Mike Rapoport [off-list ref]; liwei (CM) [off-list ref]
抄送: 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; linux-arm-kernel@lists.infradead.org; linux-kernel@vger.kernel.org; sujunfei [off-list ref]; Linuxarm [off-list ref]
主题: RE: [PATCH v2] arm64: mm: free unused memmap for sparse memory model that define VMEMMAP

-----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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