Messages in this thread | | | Subject | Re: [PATCH] mm : fix pte _PAGE_DIRTY bit when fallback migrate page | From | Robbie Ko <> | Date | Thu, 16 Jul 2020 18:15:23 +0800 |
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Kirill A. Shutemov 於 2020/7/15 下午4:11 寫道: > 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@synology.com> >>>>>>> >>>>>>> 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? > This, I'm not sure. But I think that for the fs that support migratepage, when migratepage is finished, the page should still be dirty, and the pte should still have _PAGE_RW, when the next mmap write occurs, we don't need to trigger the page_mkwrite again.
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