Re: [External] Re: [PATCH v3 1/6] mm: migrate: do not migrate HugeTLB page whose refcount is one

[David Hildenbrand]

On 12.01.21 14:40, Muchun Song wrote:
> On Tue, Jan 12, 2021 at 7:11 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>>
>> On 12.01.21 12:00, David Hildenbrand wrote:
>>> On 10.01.21 13:40, Muchun Song wrote:
>>>> If the refcount is one when it is migrated, it means that the page
>>>> was freed from under us. So we are done and do not need to migrate.
>>>>
>>>> This optimization is consistent with the regular pages, just like
>>>> unmap_and_move() does.
>>>>
>>>> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
>>>> Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>>>> Acked-by: Yang Shi <shy828301@xxxxxxxxx>
>>>> ---
>>>>  mm/migrate.c | 6 ++++++
>>>>  1 file changed, 6 insertions(+)
>>>>
>>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>>> index 4385f2fb5d18..a6631c4eb6a6 100644
>>>> --- a/mm/migrate.c
>>>> +++ b/mm/migrate.c
>>>> @@ -1279,6 +1279,12 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
>>>>              return -ENOSYS;
>>>>      }
>>>>
>>>> +    if (page_count(hpage) == 1) {
>>>> +            /* page was freed from under us. So we are done. */
>>>> +            putback_active_hugepage(hpage);
>>>> +            return MIGRATEPAGE_SUCCESS;
>>>> +    }
>>>> +
>>>>      new_hpage = get_new_page(hpage, private);
>>>>      if (!new_hpage)
>>>>              return -ENOMEM;
>>>>
>>>
>>> Question: What if called via alloc_contig_range() where we even want to
>>> "migrate" free pages, meaning, relocate it?
>>>
>>
>> To be more precise:
>>
>> a) We don't have dissolve_free_huge_pages() calls on the
>> alloc_contig_range() path. So we *need* migration IIUC.
> 
> Without this patch, if you want to migrate a HUgeTLB page,
> the page is freed to the hugepage pool. With this patch,
> the page is also freed to the hugepage pool.
> I didn't see any different. I am missing something?

I am definitely not an expert on hugetlb pools, that's why I am asking.

Isn't it, that with your code, no new page is allocated - so
dissolve_free_huge_pages() might just refuse to dissolve due to
reservations, bailing out, no?

(as discussed, looks like alloc_contig_range() needs to be fixed to
handle this correctly)

-- 
Thanks,

David / dhildenb