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

[liwei (CM)]

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@xxxxxxx>; Mike Rapoport <rppt@xxxxxxxxxxxxx>; liwei (CM) <liwei213@xxxxxxxxxx>
抄送: will@xxxxxxxxxx; Xiaqing (A) <saberlily.xia@xxxxxxxxxxxxx>; Chenfeng (puck) <puck.chen@xxxxxxxxxxxxx>; butao <butao@xxxxxxxxxxxxx>; fengbaopeng <fengbaopeng2@xxxxxxxxxxxxx>; nsaenzjulienne@xxxxxxx; steve.capper@xxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; sujunfei <sujunfei2@xxxxxxxxxxxxx>; Linuxarm <linuxarm@xxxxxxxxxx>
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@xxxxxxx]
> Sent: Friday, September 4, 2020 12:06 AM
> To: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> Cc: liwei (CM) <liwei213@xxxxxxxxxx>; will@xxxxxxxxxx; Xiaqing (A) 
> <saberlily.xia@xxxxxxxxxxxxx>; Chenfeng (puck) 
> <puck.chen@xxxxxxxxxxxxx>; butao <butao@xxxxxxxxxxxxx>; fengbaopeng 
> <fengbaopeng2@xxxxxxxxxxxxx>; nsaenzjulienne@xxxxxxx; 
> steve.capper@xxxxxxx; Song Bao Hua (Barry Song) 
> <song.bao.hua@xxxxxxxxxxxxx>; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; sujunfei <sujunfei2@xxxxxxxxxxxxx>
> 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