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

[Catalin Marinas <catalin.marinas@arm.com>]

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.

-- 
Catalin