Re: [PATCH hotfix 2/2] mm/thp: fix deferred split unqueue naming and locking

[Hugh Dickins <hughd@google.com>]

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On Fri, 25 Oct 2024, Yang Shi wrote:
> On Thu, Oct 24, 2024 at 11:57 PM Hugh Dickins <hughd@google.com> wrote:
> > On Thu, 24 Oct 2024, Yang Shi wrote:
> > > On Wed, Oct 23, 2024 at 9:13 PM Hugh Dickins <hughd@google.com> wrote:
> > > > --- a/mm/page_alloc.c
> > > > +++ b/mm/page_alloc.c
> > > > @@ -2681,7 +2681,9 @@ void free_unref_folios(struct folio_batch *folios)
> > > >                 unsigned long pfn = folio_pfn(folio);
> > > >                 unsigned int order = folio_order(folio);
> > > >
> > > > -               folio_undo_large_rmappable(folio);
> > > > +               if (mem_cgroup_disabled())
> > > > +                       folio_unqueue_deferred_split(folio);
> > >
> > > This looks confusing. It looks all callsites of free_unref_folios()
> > > have folio_unqueue_deferred_split() and memcg uncharge called before
> > > it. If there is any problem, memcg uncharge should catch it. Did I
> > > miss something?
> >
> > I don't understand what you're suggesting there.  But David remarked
> > on it too, so it seems that I do need at least to add some comment.
> >
> > I'd better re-examine the memcg/non-memcg forking paths again: but by
> > strange coincidence (or suggestion?), I'm suddenly now too tired here,
> > precisely where David stopped too.  I'll have to come back to this
> > tomorrow, sorry.
> 
> I perhaps misunderstood this code. Just feel free to correct me if it
> doesn't make sense to you. But, yes, some comments are definitely
> welcome and helpful for understanding the code and review.

Thanks a lot for challenging that: it was me who misunderstood, not you.

I might just be inventing this excuse, but I think what happened was,
I'd been staring at an earlier release tree, and in that earlier tree
the prior unqueueing was tucked away inside a memcg function, but not
done in the #ifndef CONFIG_MEMCG stub: so I thought that this
folio_unqueue_deferred_split() in free_unref_folios() was needed just
to do it when mem_cgroup_disabled() (either by CONFIG or bootoption).

And I thought the "if (mem_cgroup_disabled())" was comment enough:
except it made no sense to you and David who saw what I was blind to
(and what you describe perfectly clearly above - it depresses me
sometimes, how I cannot even read what someone wrote, until I've
arrived at the same conclusion myself!).

If my story about !memcg stubs is true, then I think Matthew has
been cleaning all that up recently.  Except for put_pages_list()
(where I now see he wanted to insert a VM_BUG_ON(folio_memcg) in
April, but was forced to retreat): that one does not have a
folio_unqueue_deferred_split() in, but the good news is that
it no longer has any callers - I'll send a patch to delete it.

And instead of my misunderstood code above in free_unref_folios(),
just deleting the folio_undo_large_unmappable() line, with a
comment in the commit message.

Thanks!
Hugh