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

[Mike Rapoport <rppt@linux.ibm.com>]

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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