Re: [PATCH 1/5] mm: migrate: Move the page count validation to the proper place

[Baolin Wang <baolin.wang@linux.alibaba.com>]

Hi,

> On Fri, Aug 06, 2021 at 11:07:18AM +0800, Baolin Wang wrote:
>> Hi Matthew,
>>
>>> On Thu, Aug 05, 2021 at 11:05:56PM +0800, Baolin Wang wrote:
>>>> We've got the expected count for anonymous page or file page by
>>>> expected_page_refs() at the beginning of migrate_page_move_mapping(),
>>>> thus we should move the page count validation a little forward to
>>>> reduce duplicated code.
>>>
>>> Please add an explanation to the changelog for why it's safe to pull
>>> this out from under the i_pages lock.
>>
>> Sure. In folio_migrate_mapping(), we are sure that the migration page was
>> isolated from lru list and locked, so I think there are no race to get the
>> page count without i_pages lock. Please correct me if I missed something
>> else. Thanks.
> 
> Unless the page has been removed from i_pages, this isn't a correct
> explanation.  Even if it has been removed from i_pages, unless an
> RCU grace period has passed, another CPU may still be able to inc the
> refcount on it (temporarily).  The same is true for the page tables,
> by the way; if someone is using get_user_pages_fast(), they may still
> be able to see the page.

I don't think this is an issue, cause now we've established a migration 
pte for this migration page under page lock. If the user want to get 
page by get_user_pages_fast(), it will wait for the page miggration 
finished by migration_entry_wait(). So I still think there is no need to 
check the migration page count under the i_pages lock.