On Mon, Jan 11, 2021 at 1:38 PM Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote:
>
> On 11.01.2021 21:17, Yang Shi wrote:
> > On Wed, Jan 6, 2021 at 2:22 AM Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote:
> >>
> >> On 06.01.2021 01:58, Yang Shi wrote:
> >>> Currently registered shrinker is indicated by non-NULL shrinker->nr_deferred.
> >>> This approach is fine with nr_deferred at the shrinker level, but the following
> >>> patches will move MEMCG_AWARE shrinkers' nr_deferred to memcg level, so their
> >>> shrinker->nr_deferred would always be NULL. This would prevent the shrinkers
> >>> from unregistering correctly.
> >>>
> >>> Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
> >>> ---
> >>> include/linux/shrinker.h | 7 ++++---
> >>> mm/vmscan.c | 13 +++++++++----
> >>> 2 files changed, 13 insertions(+), 7 deletions(-)
> >>>
> >>> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
> >>> index 0f80123650e2..1eac79ce57d4 100644
> >>> --- a/include/linux/shrinker.h
> >>> +++ b/include/linux/shrinker.h
> >>> @@ -79,13 +79,14 @@ struct shrinker {
> >>> #define DEFAULT_SEEKS 2 /* A good number if you don't know better. */
> >>>
> >>> /* Flags */
> >>> -#define SHRINKER_NUMA_AWARE (1 << 0)
> >>> -#define SHRINKER_MEMCG_AWARE (1 << 1)
> >>> +#define SHRINKER_REGISTERED (1 << 0)
> >>> +#define SHRINKER_NUMA_AWARE (1 << 1)
> >>> +#define SHRINKER_MEMCG_AWARE (1 << 2)
> >>> /*
> >>> * It just makes sense when the shrinker is also MEMCG_AWARE for now,
> >>> * non-MEMCG_AWARE shrinker should not have this flag set.
> >>> */
> >>> -#define SHRINKER_NONSLAB (1 << 2)
> >>> +#define SHRINKER_NONSLAB (1 << 3)
> >>>
> >>> extern int prealloc_shrinker(struct shrinker *shrinker);
> >>> extern void register_shrinker_prepared(struct shrinker *shrinker);
> >>> diff --git a/mm/vmscan.c b/mm/vmscan.c
> >>> index 8da765a85569..9761c7c27412 100644
> >>> --- a/mm/vmscan.c
> >>> +++ b/mm/vmscan.c
> >>> @@ -494,6 +494,7 @@ void register_shrinker_prepared(struct shrinker *shrinker)
> >>> if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> >>> idr_replace(&shrinker_idr, shrinker, shrinker->id);
> >>> #endif
> >>> + shrinker->flags |= SHRINKER_REGISTERED;
> >>
> >> In case of we introduce this new flag, we should kill old flag SHRINKER_REGISTERING,
> >> which are not needed anymore (we should you the new flag instead of that).
> >
> > The only think that I'm confused with is the check in
> > shrink_slab_memcg, it does:
> >
> > shrinker = idr_find(&shrinker_idr, i);
> > if (unlikely(!shrinker || shrinker == SHRINKER_REGISTERING)) {
> >
> > When allocating idr, the shrinker is associated with
> > SHRINKER_REGISTERING. But, shrink_slab_memcg does acquire read
> > shrinker_rwsem, and idr_alloc is called with holding write
> > shrinker_rwsem, so I'm supposed shrink_slab_memcg should never see
> > shrinker is registering.
>
> After prealloc_shrinker() shrinker is visible for shrink_slab_memcg().
> This is the moment shrink_slab_memcg() sees SHRINKER_REGISTERED.
Yes, this exactly is what I'm supposed.
>
> > If so it seems easy to remove
> > SHRINKER_REGISTERING.
> >
> > We just need change that check to:
> > !shrinker || !(shrinker->flags & SHRINKER_REGISTERED)
> >
> >>> up_write(&shrinker_rwsem);
> >>> }
> >>>
> >>> @@ -513,13 +514,17 @@ EXPORT_SYMBOL(register_shrinker);
> >>> */
> >>> void unregister_shrinker(struct shrinker *shrinker)
> >>> {
> >>> - if (!shrinker->nr_deferred)
> >>> - return;
> >>> - if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> >>> - unregister_memcg_shrinker(shrinker);
> >>> down_write(&shrinker_rwsem);
> >>
> >> I do not think there are some users which registration may race with unregistration.
> >> So, I think we should check SHRINKER_REGISTERED unlocked similar to we used to check
> >> shrinker->nr_deferred unlocked.
> >
> > Yes, I agree.
> >
> >>
> >>> + if (!(shrinker->flags & SHRINKER_REGISTERED)) {
> >>> + up_write(&shrinker_rwsem);
> >>> + return;
> >>> + }
> >>> list_del(&shrinker->list);
> >>> + shrinker->flags &= ~SHRINKER_REGISTERED;
> >>> up_write(&shrinker_rwsem);
> >>> +
> >>> + if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> >>> + unregister_memcg_shrinker(shrinker);
> >>> kfree(shrinker->nr_deferred);
> >>> shrinker->nr_deferred = NULL;
> >>> }
> >>>
> >>
> >>
>
>
