Re: [PATCH v2 2/2] mm: collect the number of anon large folios on split_deferred list

[Barry Song <21cnbao@gmail.com>]

On Thu, Aug 22, 2024 at 5:39 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 12.08.24 00:49, Barry Song wrote:
> > From: Barry Song <v-songbaohua@oppo.com>
> >
> > When an mTHP is added to the deferred_list, its partial pages
> > are unused, leading to wasted memory and potentially increasing
> > memory reclamation pressure.
> >
> > Detailing the specifics of how unmapping occurs is quite difficult
> > and not that useful, so we adopt a simple approach: each time an
> > mTHP enters the deferred_list, we increment the count by 1; whenever
> > it leaves for any reason, we decrement the count by 1.
> >
> > Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> > ---
> >   Documentation/admin-guide/mm/transhuge.rst | 5 +++++
> >   include/linux/huge_mm.h                    | 1 +
> >   mm/huge_memory.c                           | 6 ++++++
> >   3 files changed, 12 insertions(+)
> >
> > diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> > index 9fdfb46e4560..7072469de8a8 100644
> > --- a/Documentation/admin-guide/mm/transhuge.rst
> > +++ b/Documentation/admin-guide/mm/transhuge.rst
> > @@ -532,6 +532,11 @@ nr_anon
> >          These huge pages could be entirely mapped or have partially
> >          unmapped/unused subpages.
> >
> > +nr_split_deferred
> > +       the number of anon huge pages which have been partially unmapped
> > +       and put onto split queue. Those unmapped subpages are also unused
> > +       and temporarily wasting memory.
>
> The name suggests something else ... like a counter of how many have
> been deferred split :)
>
> Would "nr_anon_partially_mapped" "nr_anon_split_pending" (or something
> less mouthful) be clearer?
>
> (likely "anon" really should be part of the name in any case)
>
> The name we chose (and the implied semantics) will likely have
> implications on the handling of Usamas series.
>

Hi David,

Your point is quite similar to my V1, though not exactly the same. I aimed to
make the name more meaningful for users.
https://lore.kernel.org/all/20240808010457.228753-3-21cnbao@gmail.com/

Ryan felt that the name should be consistent with the existing split_deferred.
https://lore.kernel.org/all/36e8f1be-868d-4bce-8f32-e2d96b8b7af3@arm.com/#t

It seems that the existing split_deferred may now be less optimal with
Usama's series, as entirely_mapped folios might also be on the list.

Ryan is out right now, but I suppose he will be convinced that
"nr_anon_partially_mapped" is probably a better name once
he returns and reviews Usama's series. :-)

> --
> Cheers,
>
> David / dhildenb
>

Thanks
Barry