[PATCH] powerpc/mm/book3s/64: Rework page table geometry for lower memory usage

[Balbir Singh]

On Tue, 2017-05-09 at 18:05 +1000, Michael Ellerman wrote:
> Recently in commit f6eedbba7a26 ("powerpc/mm/hash: Increase VA range to 128TB")
> we increased the virtual address space for user processes to 128TB by default,
> and up to 512TB if user space opts in.
> 
> This obviously required expanding the range of the Linux page tables. For Book3s
> 64-bit using hash and with PAGE_SIZE=64K, we increased the PGD to 2^15 entries.
> This meant we could cover the full address range, while still being able to
> insert a 16G hugepage at the PGD level and a 16M hugepage in the PMD.
> 
> The downside of that geometry is that it uses a lot of memory for the PGD, and
> in particular makes the PGD a 4-page allocation, which means it's much more
> likely to fail under memory pressure.
> 
> Instead we can make the PMD larger, so that a single PUD entry maps 16G,
> allowing the 16G hugepages to sit at that level in the tree. We're then able to
> split the remaining bits between the PUG and PGD. We make the PGD slightly
> larger as that results in lower memory usage for typical programs.
> 
> When THP is enabled the PMD actually doubles in size, to 2^11 entries, or 2^14
> bytes, which is large but still < PAGE_SIZE.
> 
> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
> ---

Reviewed-by: Balbir Singh <bsingharora at gmail.com>