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

[Michal Hocko <mhocko@kernel.org>]

On Sat 18-01-20 15:36:06, David Rientjes wrote:
> On Sat, 18 Jan 2020, Andrew Morton wrote:
> 
> > On Sat, 18 Jan 2020 07:38:36 +0800 Wei Yang <richardw.yang@linux.intel.com> wrote:
> > 
> > > If compound is true, this means it is a PMD mapped THP. Which implies
> > > the page is not linked to any defer list. So the first code chunk will
> > > not be executed.
> > > 
> > > Also with this reason, it would not be proper to add this page to a
> > > defer list. So the second code chunk is not correct.
> > > 
> > > Based on this, we should remove the defer list related code.
> > > 
> > > Fixes: 87eaceb3faa5 ("mm: thp: make deferred split shrinker memcg aware")
> > > 
> > > Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
> > > Suggested-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > > Cc: <stable@vger.kernel.org>    [5.4+]
> > 
> > This patch is identical to "mm: thp: grab the lock before manipulating
> > defer list", which is rather confusing.  Please let people know when
> > this sort of thing is done.
> > 
> > The earlier changelog mentioned a possible race condition.  This
> > changelog does not.  In fact this changelog fails to provide any
> > description of any userspace-visible runtime effects of the bug. 
> > Please send along such a description for inclusion, as always.
> > 
> 
> The locking concern that Wei was originally looking at is no longer an 
> issue because we determined that the code in question could simply be 
> removed.
> 
> I think the following can be added to the changelog:
> 
> ----->o-----
> 
> 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.

With that clarified, feel free to add

Acked-by: Michal Hocko <mhocko@suse.com>

-- 
Michal Hocko
SUSE Labs