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

[liwei (CM)]

-----邮件原件-----
发件人: Catalin Marinas [mailto:catalin.marinas at arm.com] 
发送时间: 2020年7月22日 20:49
收件人: liwei (CM) <liwei213 at huawei.com>
抄送: Mike Rapoport <rppt at linux.ibm.com>; will at kernel.org; Xiaqing (A) <saberlily.xia at hisilicon.com>; Chenfeng (puck) <puck.chen at hisilicon.com>; butao <butao at hisilicon.com>; fengbaopeng <fengbaopeng2 at hisilicon.com>; nsaenzjulienne at suse.de; steve.capper at arm.com; Song Bao Hua (Barry Song) <song.bao.hua at hisilicon.com>; linux-arm-kernel at lists.infradead.org; linux-kernel at vger.kernel.org; sujunfei <sujunfei2 at hisilicon.com>; zhaojiapeng <zhaojiapeng at huawei.com>
主题: Re: 答复: [PATCH] arm64: mm: free unused memmap for sparse memory model that define VMEMMAP

On Wed, Jul 22, 2020 at 08:41:17AM +0000, liwei (CM) wrote:
> Mike Rapoport wrote:
> > On Tue, Jul 21, 2020 at 03:32:03PM +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.
> > 
> > Are there numbers showing how much memory is actually freed?
> > 
> > The freeing of empty memmap would become rather complex with these 
> > changes, do the memory savings justify it?
> 
> In the sparse memory model, the size of a section is 1 GB 
> (SECTION_SIZE_BITS 30) by default.

Can we reduce SECTION_SIZE_BITS instead? Say 26?

Hi, Catalin

Yes, you are right, reduce SECTION_SIZE_BITS to 26 can save almost the same memory as the patch. 

1) However, it is not clear whether changing the section size has any other impact.

2) Just like the flat memory model and the sparse memory model that does not define VMEMMAP, both of them have their own ways to free unused memmap. I think we've given a similar way for sparse memory define VMEMMAP.

3) This explicit free unused memmap method does reduce unnecessary memory waste for users who do not notice the section size modification.

Hope you will reconsider the purpose and significance of this patch, thanks.

--
Catalin