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Date:   Thu, 23 Jul 2020 12:29:26 +0100
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     "liwei (CM)" <liwei213@huawei.com>
Cc:     Mike Rapoport <rppt@linux.ibm.com>,
        "will@kernel.org" <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" <nsaenzjulienne@suse.de>,
        "steve.capper@arm.com" <steve.capper@arm.com>,
        "Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>,
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        <linux-arm-kernel@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        sujunfei <sujunfei2@hisilicon.com>,
        zhaojiapeng <zhaojiapeng@huawei.com>
Subject: Re: =?utf-8?B?562U5aSNOiDnrZTlpI06IFtQQVRD?=
 =?utf-8?Q?H=5D_arm64=3A_mm=3A_fre?= =?utf-8?Q?e?= unused memmap for sparse
 memory model that define VMEMMAP
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On Wed, Jul 22, 2020 at 01:40:34PM +0000, liwei (CM) wrote:
> Catalin Marinas wrote:
> > 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?
> 
> 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.

Well, we should analyse this.

> 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.

I think we did it for flatmem initially (on arm32) and added support for
sparsemem later on, so free_unused_memmap() had to cope with sparse
sections. On arm64 we introduced vmemmap support and didn't bother with
the freeing at all because of the added complexity of the vmemmap page
tables.

I wonder whether we should just disallow flatmem and non-vmemmap
sparsemem on arm64. Is there any value in keeping them around?

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

But if we changed SECTION_SIZE_BITS in the mainline kernel, then we
wouldn't need additional code to free the unused memmap.

-- 
Catalin