Re: [PATCH v3 2/5] hugetlb: convert page_huge_active() HPageMigratable flag

[Michal Hocko]

On Fri 22-01-21 11:52:28, Mike Kravetz wrote:
> Use the new hugetlb page specific flag HPageMigratable to replace the
> page_huge_active interfaces.  By it's name, page_huge_active implied
> that a huge page was on the active list.  However, that is not really
> what code checking the flag wanted to know.  It really wanted to determine
> if the huge page could be migrated.  This happens when the page is actually
> added to the page cache and/or task page table.  This is the reasoning
> behind the name change.

yeah, definitely less confusing!

> The VM_BUG_ON_PAGE() calls in the *_huge_active() interfaces are not
> really necessary as we KNOW the page is a hugetlb page.  Therefore, they
> are removed.
> 
> The routine page_huge_active checked for PageHeadHuge before testing the
> active bit.  This is unnecessary in the case where we hold a reference or
> lock and know it is a hugetlb head page.  page_huge_active is also called
> without holding a reference or lock (scan_movable_pages), and can race with
> code freeing the page.  The extra check in page_huge_active shortened the
> race window, but did not prevent the race.  Offline code calling
> scan_movable_pages already deals with these races, so removing the check
> is acceptable.  Add comment to racy code.
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
> Reviewed-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> Reviewed-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

One nit below
[...]
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index a7eb05315c6e..58be44a915d1 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -480,9 +480,13 @@ unsigned long hugetlb_get_unmapped_area(struct file *file, unsigned long addr,
>   * HPG_restore_reserve - Set when a hugetlb page consumes a reservation at
>   *	allocation time.  Cleared when page is fully instantiated.  Free
>   *	routine checks flag to restore a reservation on error paths.
> + * HPG_migratable  - Set after a newly allocated page is added to the page
> + *	cache and/or page tables.  Indicates the page is a candidate for
> + *	migration.

The state change is synchronized by hugetlb_lock.

Pls.

>   */
>  enum hugetlb_page_flags {
>  	HPG_restore_reserve = 0,
> +	HPG_migratable,
>  	__NR_HPAGEFLAGS,
>  };
-- 
Michal Hocko
SUSE Labs