Re: [PATCH v4 1/6] mm: migrate: do not migrate HugeTLB page whose refcount is one

[David Hildenbrand]

On 13.01.21 06:22, Muchun Song wrote:
> All pages isolated for the migration have an elevated reference count
> and therefore seeing a reference count equal to 1 means that the last
> user of the page has dropped the reference and the page has became
> unused and there doesn't make much sense to migrate it anymore. This has
> been done for regular pages and this patch does the same for hugetlb
> pages. Although the likelyhood of the race is rather small for hugetlb
> pages it makes sense the two code paths in sync.
> 
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Acked-by: Yang Shi <shy828301@xxxxxxxxx>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
>  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;
> 

Happy to say that I now know understand what's going on here

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>

-- 
Thanks,

David / dhildenb