Re: Re: [PATCH 1/1] mm/shrinker: add NULL checks after rcu_dereference() in shrinker bit functions

[傅清爽]

Hi Qi,

 

Thank you very much for your detailed analysis and patient explanation.

I fully understand your point that the existing caller constraints guarantee shrinker_info cannot be NULL here, and this NULL check is unnecessary defensive code.

 

I will drop this patch and close this thread.

 

Thanks again for your review guidance.

 

Best regards,

Qingshuang Fu

 

主   题：Re: [PATCH 1/1] mm/shrinker: add NULL checks after rcu_dereference() in shrinker bit functions
日   期：2026年06月24日18:49
发件人：Qi Zheng
收件人：fffsqian,Andrew Morton,Dave Chinner,Roman Gushchin,Muchun Song,Qi Zheng
抄送人：linux-kernel,linux-mm

Hi Qingshuang, On 6/24/26 5:55 PM, fffsqian@xxxxxxx wrote: > From: Qingshuang Fu <fuqingshuang@xxxxxxxxxx> > > The functions set_shrinker_bit(), xchg_nr_deferred_memcg(), and > add_nr_deferred_memcg() access shrinker_info fields immediately > after rcu_dereference() without checking for NULL. > > This is inconsistent with shrink_slab_memcg() which properly checks > "if (unlikely(!info)) goto unlock;" before accessing info fields. > > The shrinker_info can be NULL during memcg initialization or after > shrinker_info expansion failure. Directly accessing info->map_nr_max > or info->unit[] without NULL validation could cause kernel NULL > pointer dereference and panic. Really? Did you actually hit this issue, or are you able to reproduce it? Or is it just spotted via code inspection? The callers in all three of these places should guarantee that `info` can not possibly be NULL. :( Thanks, Qi > > Fix this by adding proper NULL checks in all three functions to > ensure consistent RCU protection and prevent potential crashes in > the shrinker subsystem. > > Fixes: 307bececcd1205bcb ("mm: shrinker: add a secondary array for shrinker_info::{map, nr_deferred}") > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > Cc: Dave Chinner <david@xxxxxxxxxxxxx> > Cc: Qi Zheng <qi.zheng@xxxxxxxxx> > Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx> > Cc: Muchun Song <muchun.song@xxxxxxxxx> > Cc: linux-mm@xxxxxxxxx > Signed-off-by: Qingshuang Fu <fuqingshuang@xxxxxxxxxx> > --- > mm/shrinker.c | 11 +++++++++++ > 1 file changed, 11 insertions(+) > > diff --git a/mm/shrinker.c b/mm/shrinker.c > index 7082d01c8c9d..ecde3cc44459 100644 > --- a/mm/shrinker.c > +++ b/mm/shrinker.c > @@ -200,6 +200,8 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id) > > rcu_read_lock(); > info = rcu_dereference(memcg->nodeinfo[nid]->shrinker_info); > + if (unlikely(!info)) > + goto unlock; > if (!WARN_ON_ONCE(shrinker_id >= info->map_nr_max)) { > struct shrinker_info_unit *unit; > > @@ -208,6 +210,7 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id) > smp_mb__before_atomic(); > set_bit(shrinker_id_to_offset(shrinker_id), unit->map); > } > +unlock: > rcu_read_unlock(); > } > } > @@ -258,6 +261,10 @@ static long xchg_nr_deferred_memcg(int nid, struct shrinker *shrinker, > > rcu_read_lock(); > info = rcu_dereference(memcg->nodeinfo[nid]->shrinker_info); > + if (unlikely(!info)) { > + rcu_read_unlock(); > + return 0; > + } > unit = info->unit[shrinker_id_to_index(shrinker->id)]; > nr_deferred = atomic_long_xchg(&unit->nr_deferred[shrinker_id_to_offset(shrinker->id)], 0); > rcu_read_unlock(); > @@ -274,6 +281,10 @@ static long add_nr_deferred_memcg(long nr, int nid, struct shrinker *shrinker, > > rcu_read_lock(); > info = rcu_dereference(memcg->nodeinfo[nid]->shrinker_info); > + if (unlikely(!info)) { > + rcu_read_unlock(); > + return 0; > + } > unit = info->unit[shrinker_id_to_index(shrinker->id)]; > nr_deferred = > atomic_long_add_return(nr, &unit->nr_deferred[shrinker_id_to_offset(shrinker->id)]); > > base-commit: 840ef6c78e6a2f694b578ecb9063241c992aaa9e </fuqingshuang@xxxxxxxxxx></muchun.song@xxxxxxxxx></roman.gushchin@xxxxxxxxx></qi.zheng@xxxxxxxxx></david@xxxxxxxxxxxxx></akpm@xxxxxxxxxxxxxxxxxxxx></fuqingshuang@xxxxxxxxxx>

 

 

 

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