Re: [PATCH] mm/memcg: set memcg when split pages

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Mar 02, 2021 at 12:24:41PM -0800, Hugh Dickins wrote:
> On Tue, 2 Mar 2021, Michal Hocko wrote:
> > [Cc Johannes for awareness and fixup Nick's email]
> > 
> > On Tue 02-03-21 01:34:51, Zhou Guanghui wrote:
> > > When split page, the memory cgroup info recorded in first page is
> > > not copied to tail pages. In this case, when the tail pages are
> > > freed, the uncharge operation is not performed. As a result, the
> > > usage of this memcg keeps increasing, and the OOM may occur.
> > > 
> > > So, the copying of first page's memory cgroup info to tail pages
> > > is needed when split page.
> > 
> > I was not aware that alloc_pages_exact is used for accounted allocations
> > but git grep told me otherwise so this is not a theoretical one. Both
> > users (arm64 and s390 kvm) are quite recent AFAICS. split_page is also
> > used in dma allocator but I got lost in indirection so I have no idea
> > whether there are any users there.
> 
> Yes, it's a bit worrying that such a low-level thing as split_page()
> can now get caught up in memcg accounting, but I suppose that's okay.
> 
> I feel rather strongly that whichever way it is done, THP splitting
> and split_page() should use the same interface to memcg.
> 
> And a look at mem_cgroup_split_huge_fixup() suggests that nowadays
> there need to be css_get()s too - or better, a css_get_many().
> 
> Its #ifdef CONFIG_TRANSPARENT_HUGEPAGE should be removed, rename
> it mem_cgroup_split_page_fixup(), and take order from caller.

+1

There is already a split_page_owner() in both these places as well
which does a similar thing. Mabye we can match that by calling it
split_page_memcg() and having it take a nr of pages?

> Though I've never much liked that separate pass: would it be
> better page by page, like this copy_page_memcg() does?  Though
> mem_cgroup_disabled() and css_getting make that less appealing.

Agreed on both counts. mem_cgroup_disabled() is a jump label and would
be okay, IMO, but the refcounting - though it is (usually) per-cpu -
adds at least two branches and rcu read locking.