Re: [PATCH v2] mm, thp: use head page in __migration_entry_wait

[Kirill A. Shutemov]

On Tue, Jun 08, 2021 at 05:22:39PM +0800, 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 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>

Looks good to me:

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

But there's one quirk: if split succeed we effectively wait on wrong
page to be unlocked. And it may take indefinite time if split_huge_page()
was called on the head page.

Maybe we should consider waking up head waiter on head page, even if it is
still locked after split?

Something like this (untested):

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 63ed6b25deaa..f79a38e21e53 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2535,6 +2535,9 @@ static void __split_huge_page(struct page *page, struct list_head *list,
 		 */
 		put_page(subpage);
 	}
+
+	if (page == head)
+		wake_up_page_bit(page, PG_locked);
 }

 int total_mapcount(struct page *page)
-- 
 Kirill A. Shutemov