Re: Memory hotplug softlock issue

[Vlastimil Babka]

On 11/19/18 3:10 PM, Michal Hocko wrote:
> On Mon 19-11-18 13:51:21, Michal Hocko wrote:
>> On Mon 19-11-18 13:40:33, Michal Hocko wrote:
>>> How are
>>> we supposed to converge when the swapin code waits for the migration to
>>> finish with the reference count elevated?

Indeed this looks wrong. How comes we only found this out now? I guess
the race window where refcounts matter is only a part of the whole
migration, where we update the mapping (migrate_page_move_mapping()).
That's before copying contents, flags etc.

>> Just to clarify. This is not only about swapin obviously. Any caller of
>> __migration_entry_wait is affected the same way AFAICS.
> 
> In other words. Why cannot we do the following?
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index f7e4bfdc13b7..7ccab29bcf9a 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -324,19 +324,9 @@ void __migration_entry_wait(struct mm_struct *mm, pte_t *ptep,
>  		goto out;
>  
>  	page = migration_entry_to_page(entry);
> -
> -	/*
> -	 * Once page cache replacement of page migration started, page_count
> -	 * *must* be zero. And, we don't want to call wait_on_page_locked()
> -	 * against a page without get_page().
> -	 * So, we use get_page_unless_zero(), here. Even failed, page fault
> -	 * will occur again.
> -	 */
> -	if (!get_page_unless_zero(page))
> -		goto out;
>  	pte_unmap_unlock(ptep, ptl);
> -	wait_on_page_locked(page);
> -	put_page(page);
> +	page_lock(page);
> +	page_unlock(page);

So what protects us from locking a page whose refcount dropped to zero?
and is being freed? The checks in freeing path won't be happy about a
stray lock.

I suspect it's not that simple to fix this. Perhaps migration code could
set some flag/bit in the page during the part where it stabilizes
refcounts, and __migration_entry_wait() would just spin until the bit is
cleared, and only then proceed with the current get_page+wait? Or we
could maybe wait on the pte itself and not page?

>  	return;
>  out:
>  	pte_unmap_unlock(ptep, ptl);
>