Re: [Patch v4] mm: thp: remove the defer list related code since this will not happen

[David Rientjes]

On Mon, 20 Jan 2020, Michal Hocko wrote:

> > When migrating memcg charges of thp memory, there are two possibilities:
> > 
> >  (1) The underlying compound page is mapped by a pmd and thus does is not 
> >      on a deferred split queue (it's mapped), or
> > 
> >  (2) The compound page is not mapped by a pmd and is awaiting split on a
> >      deferred split queue.
> > 
> > The current charge migration implementation does *not* migrate charges for 
> > thp memory on the deferred split queue, it only migrates charges for pages 
> > that are mapped by a pmd.
> > 
> > Thus, to migrate charges, the underlying compound page cannot be on a 
> > deferred split queue; no list manipulation needs to be done in 
> > mem_cgroup_move_account().
> > 
> > With the current code, the underlying compound page is moved to the 
> > deferred split queue of the memcg its memory is not charged to, so 
> > susbequent reclaim will consider these pages for the wrong memcg.  Remove 
> > the deferred split queue handling in mem_cgroup_move_account() entirely.
> 
> I believe this still doesn't describe the underlying problem to the full
> extent. What happens with the page on the deferred list when it
> shouldn't be there in fact? Unless I am missing something deferred_split_scan
> will simply split that huge page. Which is a bit unfortunate but nothing
> really critical. This should be mentioned in the changelog.
> 

Are you referring to a compound page on the deferred split queue before a 
task is moved?  I'm not sure this is within the scope of Wei's patch.. 
this is simply preventing a page from being moved to the deferred split
queue of a memcg that it is not charged to.  Is there a concern about why 
this code can be removed or a suggestion on something else it should be 
doing instead?