linux-kernel - Re: [PATCH 10/16] mm/migration: remove PG_writeback handle in folio_migrate

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Date:   Sun, 6 Mar 2022 17:21:21 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     <mike.kravetz@...cle.com>, <shy828301@...il.com>,
        <willy@...radead.org>, <ying.huang@...el.com>, <ziy@...dia.com>,
        <minchan@...nel.org>, <apopple@...dia.com>,
        <ave.hansen@...ux.intel.com>, <o451686892@...il.com>,
        <almasrymina@...gle.com>, <jhubbard@...dia.com>,
        <rcampbell@...dia.com>, <peterx@...hat.com>,
        <naoya.horiguchi@....com>, <mhocko@...e.com>, <riel@...hat.com>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 10/16] mm/migration: remove PG_writeback handle in
 folio_migrate_flags

On Fri, 4 Mar 2022 17:34:03 +0800 Miaohe Lin <linmiaohe@...wei.com> 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.