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

["Chris Mason" <clm@fb.com>]

On 16 Jul 2020, at 6:15, Robbie Ko wrote:

> 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.

I don’t know the page migration code well, but you’ll need this one 
as well on the 4.4 kernel you mentioned:

commit 25f3c5021985e885292980d04a1423fd83c967bb
Author: Chris Mason <clm@fb.com>
Date:   Tue Jan 21 11:51:42 2020 -0500

     Btrfs: keep pages dirty when using btrfs_writepage_fixup_worker

And this one as well:

commit 7703bdd8d23e6ef057af3253958a793ec6066b28
Author: Chris Mason <clm@fb.com>
Date:   Wed Jun 20 07:56:11 2018 -0700

     Btrfs: don't clean dirty pages during buffered writes

With those two in place, we haven’t found lost data from the migration 
code, but we did see the fallback migration helper dirtying pages 
without going through page_mkwrite, which triggers the suboptimal btrfs 
fixup worker code path.  This isn’t a yea or nay on the patch, just 
additional info.

-chris