Re: [PATCH v9 2/8] mm,memory_hotplug: Relax fully spanned sections check

Michal Hocko
2021-04-20T09:40:55Z

On Fri 16-04-21 13:24:05, Oscar Salvador wrote:
> When using self-hosted vmemmap pages, the number of pages passed to
> {online,offline}_pages might not fully span sections, but they always
> fully span pageblocks.
> Relax the check account for that case.

It would be good to call those out explicitly. It would be also
great to explain why pageblock_nr_pages is an actual constrain. There
shouldn't be any real reason for that except for "we want online_pages
to operate on whole memblocks and memmap_on_memory will poke
pageblock_nr_pages aligned holes in the beginning which is a special
case we want to allow."

> Signed-off-by: Oscar Salvador <osalvador@suse.de>
> Reviewed-by: David Hildenbrand <david@redhat.com>

With the changelog extended and the comment clarification (se below)
feel free to add
Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> mm/memory_hotplug.c | 18 ++++++++++++++----
> 1 file changed, 14 insertions(+), 4 deletions(-)
>
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 0cdbbfbc5757..25e59d5dc13c 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -838,9 +838,14 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages,
> int ret;
> struct memory_notify arg;
>
> - /* We can only online full sections (e.g., SECTION_IS_ONLINE) */
> + /* We can only offline full sections (e.g., SECTION_IS_ONLINE).
> + * However, when using e.g: memmap_on_memory, some pages are initialized
> + * prior to calling in here. The remaining amount of pages must be
> + * pageblock aligned.

I would rephrase (and also note that multi line comment usually have a
leading line without any content - not that I care much though).

/*
* {on,off}lining is constrained to full memory sections (or
* more precisly to memory blocks from the user space POV).
* memmap_on_memory is an exception because it reserves initial
* part of the physical memory space for vmemmaps. That space is
* pageblock aligned.
> + */

Same comment would apply to oofline_pages.

> if (WARN_ON_ONCE(!nr_pages ||
> - !IS_ALIGNED(pfn | nr_pages, PAGES_PER_SECTION)))
> + !IS_ALIGNED(pfn, pageblock_nr_pages) ||
> + !IS_ALIGNED(pfn + nr_pages, PAGES_PER_SECTION)))
> return -EINVAL;
>
> mem_hotplug_begin();
> @@ -1573,9 +1578,14 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages)
> int ret, node;
> char *reason;
>
> - /* We can only offline full sections (e.g., SECTION_IS_ONLINE) */
> + /* We can only offline full sections (e.g., SECTION_IS_ONLINE).
> + * However, when using e.g: memmap_on_memory, some pages are initialized
> + * prior to calling in here. The remaining amount of pages must be
> + * pageblock aligned.
> + */
> if (WARN_ON_ONCE(!nr_pages ||
> - !IS_ALIGNED(start_pfn | nr_pages, PAGES_PER_SECTION)))
> + !IS_ALIGNED(start_pfn, pageblock_nr_pages) ||
> + !IS_ALIGNED(start_pfn + nr_pages, PAGES_PER_SECTION)))
> return -EINVAL;
>
> mem_hotplug_begin();
> --
> 2.16.3

--
Michal Hocko
SUSE Labs