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SubjectRe: [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@huawei.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.

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