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

[Song Bao Hua (Barry Song)]

> -----Original Message-----
> From: Catalin Marinas [mailto:catalin.marinas at arm.com]
> Sent: Friday, September 4, 2020 12:06 AM
> To: Mike Rapoport <rppt at linux.ibm.com>
> Cc: liwei (CM) <liwei213 at huawei.com>; will at kernel.org; Xiaqing (A)
> <saberlily.xia at hisilicon.com>; Chenfeng (puck) <puck.chen at hisilicon.com>;
> butao <butao at hisilicon.com>; fengbaopeng <fengbaopeng2 at hisilicon.com>;
> nsaenzjulienne at suse.de; steve.capper at arm.com; Song Bao Hua (Barry Song)
> <song.bao.hua at hisilicon.com>; linux-arm-kernel at lists.infradead.org;
> linux-kernel at vger.kernel.org; sujunfei <sujunfei2 at hisilicon.com>
> Subject: Re: [PATCH v2] arm64: mm: free unused memmap for sparse memory
> model that define VMEMMAP
> 
> 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.

It would be the simplest way to fix this issue. It seems X86_64 is
also using 27.

@wei, has you ever tried to send a patch to change SECTION_SIZE_BITS
to 27 for ARM64?

> 
> --
> Catalin

Thanks
Barry