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
