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Date: Thu, 3 Sep 2020 15:27:22 +0300 From: Mike Rapoport <rppt@...ux.ibm.com> To: Catalin Marinas <catalin.marinas@....com> Cc: Wei Li <liwei213@...wei.com>, will@...nel.org, saberlily.xia@...ilicon.com, puck.chen@...ilicon.com, butao@...ilicon.com, fengbaopeng2@...ilicon.com, nsaenzjulienne@...e.de, steve.capper@....com, song.bao.hua@...ilicon.com, linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, sujunfei2@...ilicon.com Subject: Re: [PATCH v2] arm64: mm: free unused memmap for sparse memory model that define VMEMMAP On Thu, Sep 03, 2020 at 01:05:58PM +0100, Catalin Marinas wrote: > 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. What I meant was that sparse_init()->__populate_section_memmap() would not blindly presume that the entire section is valid, but would take into account The actual DRAM banks listed in memblock.memory. For that to work we'll need a version of pfn_valid() that does not rely on the validity of sparse section, but uses some other means, e.g. memblock. Apparently, I've looked at arm32 version of pfn_valid() and missed the section validity check :) I was thinking about doing something like this for 32-bit systems (non-ARM) that cannot affort small sections because of the limited space in the page->flags. > 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 -- Sincerely yours, Mike.
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