Re: [PATCH] mm : fix pte _PAGE_DIRTY bit when fallback migrate page

[Robbie Ko]

Kirill A. Shutemov æ 2020/7/15 äå4:11 åé:
> On Wed, Jul 15, 2020 at 10:45:39AM +0800, Robbie Ko wrote:
> > Kirill A. Shutemov æ 2020/7/14 äå6:19 åé:
> > > 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 æ 2020/7/10 äå11:31 åé:
> > > > > > 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?
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.