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

[Robbie Ko <robbieko@synology.com>]

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.