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

[Baolin Wang]

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.