答复: [PATCH v2] arm64: mm: free unused memmap for sparse memory model that define VMEMMAP

["liwei (CM)" <liwei213@huawei.com>]

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 <catalin.marinas@arm.com>; Mike Rapoport <rppt@linux.ibm.com>; liwei (CM) <liwei213@huawei.com>
抄送: will@kernel.org; Xiaqing (A) <saberlily.xia@hisilicon.com>; Chenfeng (puck) <puck.chen@hisilicon.com>; butao <butao@hisilicon.com>; fengbaopeng <fengbaopeng2@hisilicon.com>; nsaenzjulienne@suse.de; steve.capper@arm.com; linux-arm-kernel@lists.infradead.org; linux-kernel@vger.kernel.org; sujunfei <sujunfei2@hisilicon.com>; Linuxarm <linuxarm@huawei.com>
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 <rppt@linux.ibm.com>
> Cc: liwei (CM) <liwei213@huawei.com>; will@kernel.org; Xiaqing (A) 
> <saberlily.xia@hisilicon.com>; Chenfeng (puck) 
> <puck.chen@hisilicon.com>; butao <butao@hisilicon.com>; fengbaopeng 
> <fengbaopeng2@hisilicon.com>; nsaenzjulienne@suse.de; 
> steve.capper@arm.com; Song Bao Hua (Barry Song) 
> <song.bao.hua@hisilicon.com>; linux-arm-kernel@lists.infradead.org;
> linux-kernel@vger.kernel.org; sujunfei <sujunfei2@hisilicon.com>
> 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:
> > On Wed, Aug 12, 2020 at 09:06:55AM +0800, Wei Li wrote:
> > > For the memory hole, sparse memory model that define
> SPARSEMEM_VMEMMAP
> > > 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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