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

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

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