Re: [PATCH 10/16] mm/migration: remove PG_writeback handle in folio_migrate_flags

[Andrew Morton]

On Fri, 4 Mar 2022 17:34:03 +0800 Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:

> When newfolio reaches here, it's guaranteed that PG_writeback is not set
> because caller ensures writeback must have been completed. Remove this
> unneeded check and cleanup the relevant comment.

What guarantees that writeback cannot start after the caller has checked?

I see no such check in iomap_migrate_page()?

> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -549,18 +549,6 @@ void folio_migrate_flags(struct folio *newfolio, struct folio *folio)
>  	if (!folio_test_hugetlb(folio))
>  		folio->private = NULL;
>  
> -	/*
> -	 * If any waiters have accumulated on the new page then
> -	 * wake them up.
> -	 */
> -	if (folio_test_writeback(newfolio))
> -		folio_end_writeback(newfolio);
> -
> -	/*
> -	 * PG_readahead shares the same bit with PG_reclaim.  The above
> -	 * end_page_writeback() may clear PG_readahead mistakenly, so set the
> -	 * bit after that.
> -	 */
>  	if (folio_test_readahead(folio))
>  		folio_set_readahead(newfolio);
>  

folio_migrate_flags() and folio_migrate_copy() are global,
export-to-modules functions but have no interface documentation.  That
was bad of us.  

I wonder why those two functions are exported to modules anyway.