Re: [PATCH 2/2] powerpc/nvdimm: Update vmemmap_populated to check sub-section range

[Dan Williams <dan.j.williams@intel.com>]

On Mon, Sep 9, 2019 at 11:29 PM Aneesh Kumar K.V
<aneesh.kumar@linux.ibm.com> wrote:
>
> With commit: 7cc7867fb061 ("mm/devm_memremap_pages: enable sub-section remap")
> pmem namespaces are remapped in 2M chunks. On architectures like ppc64 we
> can map the memmap area using 16MB hugepage size and that can cover
> a memory range of 16G.
>
> While enabling new pmem namespaces, since memory is added in sub-section chunks,
> before creating a new memmap mapping, kernel should check whether there is an
> existing memmap mapping covering the new pmem namespace. Currently, this is
> validated by checking whether the section covering the range is already
> initialized or not. Considering there can be multiple namespaces in the same
> section this can result in wrong validation. Update this to check for
> sub-sections in the range. This is done by checking for all pfns in the range we
> are mapping.
>
> We could optimize this by checking only just one pfn in each sub-section. But
> since this is not fast-path we keep this simple.
>
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> ---
>  arch/powerpc/mm/init_64.c | 45 ++++++++++++++++++++-------------------
>  1 file changed, 23 insertions(+), 22 deletions(-)
>
> diff --git a/arch/powerpc/mm/init_64.c b/arch/powerpc/mm/init_64.c
> index 4e08246acd79..7710ccdc19a2 100644
> --- a/arch/powerpc/mm/init_64.c
> +++ b/arch/powerpc/mm/init_64.c
> @@ -70,30 +70,24 @@ EXPORT_SYMBOL_GPL(kernstart_addr);
>
>  #ifdef CONFIG_SPARSEMEM_VMEMMAP
>  /*
> - * Given an address within the vmemmap, determine the pfn of the page that
> - * represents the start of the section it is within.  Note that we have to
> - * do this by hand as the proffered address may not be correctly aligned.
> - * Subtraction of non-aligned pointers produces undefined results.
> - */
> -static unsigned long __meminit vmemmap_section_start(unsigned long page)
> -{
> -       unsigned long offset = page - ((unsigned long)(vmemmap));
> -
> -       /* Return the pfn of the start of the section. */
> -       return (offset / sizeof(struct page)) & PAGE_SECTION_MASK;
> -}
> -
> -/*
> - * Check if this vmemmap page is already initialised.  If any section
> + * Check if this vmemmap page is already initialised.  If any sub section
>   * which overlaps this vmemmap page is initialised then this page is
>   * initialised already.
>   */
> -static int __meminit vmemmap_populated(unsigned long start, int page_size)
> +
> +static int __meminit vmemmap_populated(unsigned long start, int size)
>  {
> -       unsigned long end = start + page_size;
> -       start = (unsigned long)(pfn_to_page(vmemmap_section_start(start)));
> +       unsigned long end = start + size;
>
> -       for (; start < end; start += (PAGES_PER_SECTION * sizeof(struct page)))
> +       /* start is size aligned and it is always > sizeof(struct page) */
> +       VM_BUG_ON(start & sizeof(struct page));

If start is size aligned why not include that assumption in the VM_BUG_ON()?

Otherwise it seems this patch could be reduced simply by:

s/PAGE_SECTION_MASK/PAGE_SUBSECTION_MASK/
s/PAGES_PER_SECTION/PAGES_PER_SUBSECTION/

...and leave the vmemmap_section_start() function in place? In other
words this path used to guarantee that 'start' was aligned to the
minimum mem-hotplug granularity, the change looks ok on the surface,
but it seems a subtle change in semantics.

Can you get an ack from a powerpc maintainer, or maybe this patch
should route through the powerpc tree?

I'll take patch1 through the nvdimm tree.