On Wed, Jul 15, 2020 at 10:45:39AM +0800, Robbie Ko wrote: > > Kirill A. Shutemov 於 2020/7/14 下午6:19 寫道: > > On Tue, Jul 14, 2020 at 11:46:12AM +0200, Vlastimil Babka wrote: > > > On 7/13/20 3:57 AM, Robbie Ko wrote: > > > > Vlastimil Babka 於 2020/7/10 下午11:31 寫道: > > > > > On 7/9/20 4:48 AM, robbieko wrote: > > > > > > From: Robbie Ko <robbieko@xxxxxxxxxxxx> > > > > > > > > > > > > When a migrate page occurs, we first create a migration entry > > > > > > to replace the original pte, and then go to fallback_migrate_page > > > > > > to execute a writeout if the migratepage is not supported. > > > > > > > > > > > > In the writeout, we will clear the dirty bit of the page and use > > > > > > page_mkclean to clear the dirty bit along with the corresponding pte, > > > > > > but page_mkclean does not support migration entry. > > I don't follow the scenario. > > > > When we establish migration entries with try_to_unmap(), it transfers > > dirty bit from PTE to the page. > > Sorry, I mean is _PAGE_RW with pte_write > > When we establish migration entries with try_to_unmap(), > we create a migration entry, and if pte_write we set it to SWP_MIGRATION_WRITE, > which will replace the migration entry with the original pte. > > When migratepage, we go to fallback_migrate_page to execute a writeout > if the migratepage is not supported. > > In the writeout, we call clear_page_dirty_for_io to clear the dirty bit of the page > and use page_mkclean to clear pte _PAGE_RW with pte_wrprotect in page_mkclean_one. > > However, page_mkclean_one does not support migration entries, so the > migration entry is still SWP_MIGRATION_WRITE. > > In writeout, then we call remove_migration_ptes to remove the migration entry, > because it is still SWP_MIGRATION_WRITE so set _PAGE_RW to pte via pte_mkwrite. > > Therefore, subsequent mmap wirte will not trigger page_mkwrite to cause data loss. Hm, okay. Folks, is there any good reason why try_to_unmap(TTU_MIGRATION) should not clear PTE (make the PTE none) for file page? -- Kirill A. Shutemov
