On 2022/3/7 9:21, Andrew Morton wrote: > 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? Drivers should set writeback flag under page lock. But I'am not sure this is always obeyed. > > I see no such check in iomap_migrate_page()? iomap_migrate_page acts as a migratepage callback. The check is actually done in __unmap_and_move. > >> --- 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); IIUC, this should also be broken too. If a page can be flagged writeback under page lock, we can't simply do folio_end_writeback here as page might still be under the use of drivers. More works should be done to handle the case correctly. >> - >> - /* >> - * 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. That's really a pity. > . > Many thanks for your comment and reply!
