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

Yang Shi
2020-12-15T23:08:00Z

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.

>
> > 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