On Tue, 8 Jun 2021, Xu Yu wrote:
> We notice that hung task happens in a conner but practical scenario when
But I still don't understand what you mean by "conner":
common, corner, something else? Maybe just delete "conner but ".
> 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 in fstat, 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 makes __migration_entry_wait operate on the compound head page,
> thus waits for remap_page to complete, whether the THP is split
> successfully or roll back.
>
> Note that put_and_wait_on_page_locked helps to drop the page reference
> acquired with get_page_unless_zero, as soon as the page is on the wait
> queue, before actually waiting. So splitting the THP is only prevented
> for a brief interval.
>
> Fixes: ba98828088ad ("thp: add option to setup migration entries during PMD split")
> Suggested-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Signed-off-by: Gang Deng <gavin.dg@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Xu Yu <xuyu@xxxxxxxxxxxxxxxxx>
Thanks:
Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
And I hope Andrew will add Cc stable when it goes into his tree.
I'll leave the (independent) discussion of optimal wakeup strategy
to Kirill and Matthew: no strong opinion from me, it works as it is.
> ---
> mm/migrate.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index b234c3f3acb7..41ff2c9896c4 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -295,6 +295,7 @@ void __migration_entry_wait(struct mm_struct *mm, pte_t *ptep,
> goto out;
>
> page = migration_entry_to_page(entry);
> + page = compound_head(page);
>
> /*
> * Once page cache replacement of page migration started, page_count
> --
> 2.20.1.2432.ga663e714
>
>
