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

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Date:   Wed, 15 Jul 2020 11:11:48 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Robbie Ko <robbieko@...ology.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>, linux-btrfs@...r.kernel.org,
        Roman Gushchin <guro@...com>, David Sterba <dsterba@...e.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH] mm : fix pte _PAGE_DIRTY bit when fallback migrate page

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@...ology.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?

-- 
 Kirill A. Shutemov