On Mon 20-01-20 13:10:56, David Rientjes wrote:

On Mon, 20 Jan 2020, Michal Hocko wrote:

quoted

quoted

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?

No, I do not have any concern about the patch itslef. It is that the
changelog doesn't decribe the user visible effect. All I am saying is
that the current code splits THPs of moved pages under memory pressure
even if that is not needed. And that is a clear bug.
-- 
Michal Hocko
SUSE Labs