Re: [PATCH v1 1/6] mm/page_alloc: tweak comments in has_unmovable_pages()

David Hildenbrand
2020-07-29T12:29:43Z

On 29.07.20 12:47, Baoquan He wrote:
> On 07/28/20 at 04:07pm, David Hildenbrand wrote:
>> On 28.07.20 15:48, Baoquan He wrote:
>>> On 06/30/20 at 04:26pm, David Hildenbrand wrote:
>>>> Let's move the split comment regarding bootmem allocations and memory
>>>> holes, especially in the context of ZONE_MOVABLE, to the PageReserved()
>>>> check.
>>>>
>>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>>> Cc: Michal Hocko <mhocko@suse.com>
>>>> Cc: Michael S. Tsirkin <mst@redhat.com>
>>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>>> ---
>>>> mm/page_alloc.c | 22 ++++++----------------
>>>> 1 file changed, 6 insertions(+), 16 deletions(-)
>>>>
>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>> index 48eb0f1410d47..bd3ebf08f09b9 100644
>>>> --- a/mm/page_alloc.c
>>>> +++ b/mm/page_alloc.c
>>>> @@ -8207,14 +8207,6 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
>>>> unsigned long iter = 0;
>>>> unsigned long pfn = page_to_pfn(page);
>>>>
>>>> - /*
>>>> - * TODO we could make this much more efficient by not checking every
>>>> - * page in the range if we know all of them are in MOVABLE_ZONE and
>>>> - * that the movable zone guarantees that pages are migratable but
>>>> - * the later is not the case right now unfortunatelly. E.g. movablecore
>>>> - * can still lead to having bootmem allocations in zone_movable.
>>>> - */
>>>> -
>>>> if (is_migrate_cma_page(page)) {
>>>> /*
>>>> * CMA allocations (alloc_contig_range) really need to mark
>>>> @@ -8233,6 +8225,12 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
>>>>
>>>> page = pfn_to_page(pfn + iter);
>>>>
>>>> + /*
>>>> + * Both, bootmem allocations and memory holes are marked
>>>> + * PG_reserved and are unmovable. We can even have unmovable
>>>> + * allocations inside ZONE_MOVABLE, for example when
>>>> + * specifying "movable_core".
>>> ~~~~ should be 'movablecore', we don't
>>> have kernel parameter 'movable_core'.
>>
>> Agreed!
>>
>>>
>>> Otherwise, this looks good to me. Esp the code comment at below had been
>>> added very long time ago and obsolete.
>>>
>>> Reviewed-by: Baoquan He <bhe@redhat.com>
>>>
>>> By the way, David, do you know what is the situation of having unmovable
>>> allocations inside ZONE_MOVABLE when specifying 'movablecore'? I quickly
>>> went through find_zone_movable_pfns_for_nodes(), but didn't get why.
>>> Could you tell a little more detail about it?
>>
>> As far as I understand, it can happen that we have memblock allocations
>> during boot that fall into an area the kernel later configures to span
>> the movable zone (via movable_core).
>
> Seems yes, thanks a lot. Wondering who is still using
> movablecore|kernelcore in what use case.
>

AFAIK, it's the only (guaranteed) way to get ZONE_MOVABLE without
involving memory hotplug. As I learned, the movable zone is also
interesting beyond memory hotunplug. It limits unmovable fragmentation
and e.g., makes THP/huge pages more likely/easier to allocate.

--
Thanks,

David / dhildenb