On Tue, 2017-05-09 at 08:05:48 UTC, 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 <m...@ellerman.id.au>
> Reviewed-by: Aneesh Kumar K.V <aneesh.ku...@linux.vnet.ibm.com>
> Reviewed-by: Balbir Singh <bsinghar...@gmail.com>
Applied to powerpc next.
https://git.kernel.org/powerpc/c/ba95b5d0359609b4ec8010f77c40ab
cheers
