Re: [v6 PATCH 09/11] mm: vmscan: don't need allocate shrinker->nr_deferred for memcg aware shrinkers

[Yang Shi <shy828301@gmail.com>]

On Thu, Feb 4, 2021 at 2:14 AM Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
>
> On 04.02.2021 12:29, Kirill Tkhai wrote:
> > On 03.02.2021 20:20, 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.
> >>
> >> The prealloc_memcg_shrinker() would return -ENOSYS if !CONFIG_MEMCG or memcg is disabled
> >> by kernel command line, then shrinker's SHRINKER_MEMCG_AWARE flag would be cleared.
> >> This makes the implementation of this patch simpler.
> >>
> >> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> >> Signed-off-by: Yang Shi <shy828301@gmail.com>
> >> ---
> >>  mm/vmscan.c | 31 ++++++++++++++++---------------
> >>  1 file changed, 16 insertions(+), 15 deletions(-)
> >>
> >> diff --git a/mm/vmscan.c b/mm/vmscan.c
> >> index 545422d2aeec..20a35d26ae12 100644
> >> --- a/mm/vmscan.c
> >> +++ b/mm/vmscan.c
> >> @@ -334,6 +334,9 @@ static int prealloc_memcg_shrinker(struct shrinker *shrinker)
> >>  {
> >>      int id, ret = -ENOMEM;
> >>
> >> +    if (mem_cgroup_disabled())
> >> +            return -ENOSYS;
> >> +
> >>      down_write(&shrinker_rwsem);
> >>      /* This may call shrinker, so it must use down_read_trylock() */
> >>      id = idr_alloc(&shrinker_idr, shrinker, 0, 0, GFP_KERNEL);
> >> @@ -414,7 +417,7 @@ static bool writeback_throttling_sane(struct scan_control *sc)
> >>  #else
> >>  static int prealloc_memcg_shrinker(struct shrinker *shrinker)
> >>  {
> >> -    return 0;
> >> +    return -ENOSYS;
> >>  }
> >>
> >>  static void unregister_memcg_shrinker(struct shrinker *shrinker)
> >> @@ -525,8 +528,18 @@ 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;
> >> +    int err;
> >> +
> >> +    if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
> >> +            err = prealloc_memcg_shrinker(shrinker);
> >> +            if (err != -ENOSYS)
> >> +                    return err;
> >>
> >> +            shrinker->flags &= ~SHRINKER_MEMCG_AWARE;
> >> +    }
> >> +
> >> +    size = sizeof(*shrinker->nr_deferred);
> >>      if (shrinker->flags & SHRINKER_NUMA_AWARE)
> >>              size *= nr_node_ids;
> >
> > This may sound surprisingly, but IIRC do_shrink_slab() may be called on early boot
> > *even before* root_mem_cgroup is allocated. AFAIR, I received syzcaller crash report
> > because of this, when I was implementing shrinker_maps.
> >
> > This is a reason why we don't use shrinker_maps even in case of mem cgroup is not
> > disabled: we iterate every shrinker of shrinker_list. See check in shrink_slab():
> >
> >       if (!mem_cgroup_disabled() && !mem_cgroup_is_root(memcg))
> >
> > Possible, we should do the same for nr_deferred: 1)always allocate shrinker->nr_deferred,
> > 2)use shrinker->nr_deferred in count_nr_deferred() and set_nr_deferred().
>
> I looked over my mail box, and I can't find that crash report and conditions to reproduce.
>
> Hm, let's remain this as is, and we rework this in case of such early shrinker call is still
> possible, and there will be a report...

Sure. But I'm wondering how that could happen. On a very small machine?

>
> Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
>
> With only nit:
>
> >>
> >> @@ -534,26 +547,14 @@ 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;
> >>  }
> >>
> >>  void free_prealloced_shrinker(struct shrinker *shrinker)
> >>  {
> >> -    if (!shrinker->nr_deferred)
> >> -            return;
> >> -
> >>      if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> >> -            unregister_memcg_shrinker(shrinker);
> >> +            return unregister_memcg_shrinker(shrinker);
>
> I've never seen return of void function in linux kernel. I'm not sure this won't confuse people.

Will fix in v7.

>
> >>
> >>      kfree(shrinker->nr_deferred);
> >>      shrinker->nr_deferred = NULL;
> >>
> >
>
>