Re: [v2 PATCH 7/9] mm: vmscan: don't need allocate shrinker->nr_deferred for memcg aware shrinkers

Yang Shi
2020-12-18T00:57:05Z

On Tue, Dec 15, 2020 at 3:07 PM Yang Shi <shy828301@gmail.com> wrote:
>
> On Mon, Dec 14, 2020 at 7:05 PM Dave Chinner <david@fromorbit.com> wrote:
> >
> > On Mon, Dec 14, 2020 at 02:37:20PM -0800, Yang Shi wrote:
> > > Now nr_deferred is available on per memcg level for memcg aware shrinkers, so don't need
> > > allocate shrinker->nr_deferred for such shrinkers anymore.
> > >
> > > Signed-off-by: Yang Shi <shy828301@gmail.com>
> > > ---
> > > mm/vmscan.c | 28 ++++++++++++++--------------
> > > 1 file changed, 14 insertions(+), 14 deletions(-)
> > >
> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > index bce8cf44eca2..8d5bfd818acd 100644
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -420,7 +420,15 @@ unsigned long lruvec_lru_size(struct lruvec *lruvec, enum lru_list lru, int zone
> > > */
> > > int prealloc_shrinker(struct shrinker *shrinker)
> > > {
> > > - unsigned int size = sizeof(*shrinker->nr_deferred);
> > > + unsigned int size;
> > > +
> > > + if (is_deferred_memcg_aware(shrinker)) {
> > > + if (prealloc_memcg_shrinker(shrinker))
> > > + return -ENOMEM;
> > > + return 0;
> > > + }
> > > +
> > > + size = sizeof(*shrinker->nr_deferred);
> > >
> > > if (shrinker->flags & SHRINKER_NUMA_AWARE)
> > > size *= nr_node_ids;
> > > @@ -429,26 +437,18 @@ int prealloc_shrinker(struct shrinker *shrinker)
> > > if (!shrinker->nr_deferred)
> > > return -ENOMEM;
> > >
> > > - if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
> > > - if (prealloc_memcg_shrinker(shrinker))
> > > - goto free_deferred;
> > > - }
> > > -
> > > return 0;
> > > -
> > > -free_deferred:
> > > - kfree(shrinker->nr_deferred);
> > > - shrinker->nr_deferred = NULL;
> > > - return -ENOMEM;
> > > }
> >
> > I'm trying to put my finger on it, but this seems wrong to me. If
> > memcgs are disabled, then prealloc_memcg_shrinker() needs to fail.
> > The preallocation code should not care about internal memcg details
> > like this.
> >
> > /*
> > * If the shrinker is memcg aware and memcgs are not
> > * enabled, clear the MEMCG flag and fall back to non-memcg
> > * behaviour for the shrinker.
> > */
> > if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
> > error = prealloc_memcg_shrinker(shrinker);
> > if (!error)
> > return 0;
> > if (error != -ENOSYS)
> > return error;
> >
> > /* memcgs not enabled! */
> > shrinker->flags &= ~SHRINKER_MEMCG_AWARE;
> > }
> >
> > size = sizeof(*shrinker->nr_deferred);
> > ....
> > return 0;
> > }
> >
> > This guarantees that only the shrinker instances taht have a
> > correctly set up memcg attached to them will have the
> > SHRINKER_MEMCG_AWARE flag set. Hence in all the rest of the shrinker
> > code, we only ever need to check for SHRINKER_MEMCG_AWARE to
> > determine what we should do....
>
> Thanks. I see your point. We could move the memcg specific details
> into prealloc_memcg_shrinker().
>
> It seems we have to acquire shrinker_rwsem before we check and modify
> SHIRNKER_MEMCG_AWARE bit if we may clear it.

Hi Dave,

Is it possible that shrinker register races with shrinker unregister?
It seems impossible to me by a quick visual code inspection. But I'm
not a VFS expert so I'm not quite sure.

If it is impossible the implementation would be quite simple otherwise
we need move shrinker_rwsem acquire/release to
prealloc_shrinker/free_prealloced_shrinker/unregister_shrinker to
protect SHRINKER_MEMCG_AWARE update.

>
> >
> > > void free_prealloced_shrinker(struct shrinker *shrinker)
> > > {
> > > - if (!shrinker->nr_deferred)
> > > + if (is_deferred_memcg_aware(shrinker)) {
> > > + unregister_memcg_shrinker(shrinker);
> > > return;
> > > + }
> > >
> > > - if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> > > - unregister_memcg_shrinker(shrinker);
> > > + if (!shrinker->nr_deferred)
> > > + return;
> > >
> > > kfree(shrinker->nr_deferred);
> > > shrinker->nr_deferred = NULL;
> >
> > e.g. then this function can simply do:
> >
> > {
> > if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> > return unregister_memcg_shrinker(shrinker);
> > kfree(shrinker->nr_deferred);
> > shrinker->nr_deferred = NULL;
> > }
> >
> > Cheers,
> >
> > Dave.
> > --
> > Dave Chinner
> > david@fromorbit.com