On Thu, Jul 23, 2020 at 12:29:26PM +0100, Catalin Marinas wrote:

On Wed, Jul 22, 2020 at 01:40:34PM +0000, liwei (CM) wrote:

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Catalin Marinas wrote:

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On Wed, Jul 22, 2020 at 08:41:17AM +0000, liwei (CM) wrote:

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Mike Rapoport wrote:

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On Tue, Jul 21, 2020 at 03:32:03PM +0800, Wei Li wrote:

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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.

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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?

FLATMEM is useful for UMA systems with a single memory bank, so probably
it's worth keeping it for low end machines.

Non-vmemmap sparsemem is essentially disable in arch/arm64/Kconfig, so
for NUMA configurations SPARSEMEM_VMEMMAP is the only choice.
 

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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.

Moreover if we reduce SECTION_SIZE_BITS, we can drop
free_unused_memmap() and since the arm64 memory map for sparse will not
differ from other arches we can drop custom pfn_valid() as well.
 

-- 
Catalin

-- 
Sincerely yours,
Mike.

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