Re: [PATCH] mm, thp: relax migration wait when failed to get tail page

[Hugh Dickins]

On Wed, 2 Jun 2021, Xu Yu wrote:

> We notice that hung task happens in a conner but practical scenario when
> CONFIG_PREEMPT_NONE is enabled, as follows.
> 
> Process 0                       Process 1                     Process 2..Inf
> split_huge_page_to_list
>     unmap_page
>         split_huge_pmd_address
>                                 __migration_entry_wait(head)
>                                                               __migration_entry_wait(tail)
>     remap_page (roll back)
>         remove_migration_ptes
>             rmap_walk_anon
>                 cond_resched
> 
> Where __migration_entry_wait(tail) is occurred in kernel space, e.g.,
> copy_to_user, which will immediately fault again without rescheduling,
> and thus occupy the cpu fully.
> 
> When there are too many processes performing __migration_entry_wait on
> tail page, remap_page will never be done after cond_resched.
> 
> This relaxes __migration_entry_wait on tail page, thus gives remap_page
> a chance to complete.
> 
> Signed-off-by: Gang Deng <gavin.dg@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Xu Yu <xuyu@xxxxxxxxxxxxxxxxx>

Well caught: you're absolutely right that there's a bug there.
But isn't cond_resched() just papering over the real bug, and
what it should do is a "page = compound_head(page);" before the
get_page_unless_zero()? How does that work out in your testing?

Hugh

> ---
>  mm/migrate.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index b234c3f3acb7..df2dc39fe566 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -301,8 +301,11 @@ void __migration_entry_wait(struct mm_struct *mm, pte_t *ptep,
>  	 * is zero; but we must not call put_and_wait_on_page_locked() without
>  	 * a ref. Use get_page_unless_zero(), and just fault again if it fails.
>  	 */
> -	if (!get_page_unless_zero(page))
> -		goto out;
> +	if (!get_page_unless_zero(page)) {
> +		pte_unmap_unlock(ptep, ptl);
> +		cond_resched();
> +		return;
> +	}
>  	pte_unmap_unlock(ptep, ptl);
>  	put_and_wait_on_page_locked(page, TASK_UNINTERRUPTIBLE);
>  	return;
> -- 
> 2.20.1.2432.ga663e714
> 
> 
>