Re: [PATCH v2 1/7] mm: vmscan: add a map_nr_max field to shrinker_info

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On 2023/2/25 23:14, Kirill Tkhai wrote:
Hi Qi,

On 25.02.2023 11:18, Qi Zheng wrote:


On 2023/2/23 21:27, Qi Zheng wrote:
To prepare for the subsequent lockless memcg slab shrink,
add a map_nr_max field to struct shrinker_info to records
its own real shrinker_nr_max.

No functional changes.

Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>

I missed Suggested-by here, hi Kirill, can I add it?

Suggested-by: Kirill Tkhai <tkhai@xxxxx>

Yes, feel free to add this tag.

There is a comment below.

---
   include/linux/memcontrol.h |  1 +
   mm/vmscan.c                | 29 ++++++++++++++++++-----------
   2 files changed, 19 insertions(+), 11 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index b6eda2ab205d..aa69ea98e2d8 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -97,6 +97,7 @@ struct shrinker_info {
       struct rcu_head rcu;
       atomic_long_t *nr_deferred;
       unsigned long *map;
+    int map_nr_max;
   };
     struct lruvec_stats_percpu {
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 9c1c5e8b24b8..9f895ca6216c 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -224,9 +224,16 @@ static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg,
                        lockdep_is_held(&shrinker_rwsem));
   }
   +static inline bool need_expand(int new_nr_max, int old_nr_max)
+{
+    return round_up(new_nr_max, BITS_PER_LONG) >
+           round_up(old_nr_max, BITS_PER_LONG);
+}
+
   static int expand_one_shrinker_info(struct mem_cgroup *memcg,
                       int map_size, int defer_size,
-                    int old_map_size, int old_defer_size)
+                    int old_map_size, int old_defer_size,
+                    int new_nr_max)
   {
       struct shrinker_info *new, *old;
       struct mem_cgroup_per_node *pn;
@@ -240,12 +247,16 @@ static int expand_one_shrinker_info(struct mem_cgroup *memcg,
           if (!old)
               return 0;
   +        if (!need_expand(new_nr_max, old->map_nr_max))
+            return 0;
+
           new = kvmalloc_node(sizeof(*new) + size, GFP_KERNEL, nid);
           if (!new)
               return -ENOMEM;
             new->nr_deferred = (atomic_long_t *)(new + 1);
           new->map = (void *)new->nr_deferred + defer_size;
+        new->map_nr_max = new_nr_max;
             /* map: set all old bits, clear all new bits */
           memset(new->map, (int)0xff, old_map_size);
@@ -295,6 +306,7 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
           }
           info->nr_deferred = (atomic_long_t *)(info + 1);
           info->map = (void *)info->nr_deferred + defer_size;
+        info->map_nr_max = shrinker_nr_max;
           rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
       }
       up_write(&shrinker_rwsem);
@@ -302,12 +314,6 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
       return ret;
   }
   -static inline bool need_expand(int nr_max)
-{
-    return round_up(nr_max, BITS_PER_LONG) >
-           round_up(shrinker_nr_max, BITS_PER_LONG);
-}
-
   static int expand_shrinker_info(int new_id)
   {
       int ret = 0;
@@ -316,7 +322,7 @@ static int expand_shrinker_info(int new_id)
       int old_map_size, old_defer_size = 0;
       struct mem_cgroup *memcg;
   -    if (!need_expand(new_nr_max))
+    if (!need_expand(new_nr_max, shrinker_nr_max))
           goto out;
         if (!root_mem_cgroup)
@@ -332,7 +338,8 @@ static int expand_shrinker_info(int new_id)
       memcg = mem_cgroup_iter(NULL, NULL, NULL);
       do {
           ret = expand_one_shrinker_info(memcg, map_size, defer_size,
-                           old_map_size, old_defer_size);
+                           old_map_size, old_defer_size,
+                           new_nr_max);
           if (ret) {
               mem_cgroup_iter_break(NULL, memcg);
               goto out;
@@ -432,7 +439,7 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg)
       for_each_node(nid) {
           child_info = shrinker_info_protected(memcg, nid);
           parent_info = shrinker_info_protected(parent, nid);
-        for (i = 0; i < shrinker_nr_max; i++) {
+        for (i = 0; i < child_info->map_nr_max; i++) {
               nr = atomic_long_read(&child_info->nr_deferred[i]);
               atomic_long_add(nr, &parent_info->nr_deferred[i]);
           }
@@ -899,7 +906,7 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
       if (unlikely(!info))
           goto unlock;
   -    for_each_set_bit(i, info->map, shrinker_nr_max) {
+    for_each_set_bit(i, info->map, info->map_nr_max) {
           struct shrink_control sc = {
               .gfp_mask = gfp_mask,
               .nid = nid,

The patch as whole thing won't work as expected. It won't ever call shrinker with ids from [round_down(shrinker_nr_max, sizeof(unsigned long)) + 1, shrinker_nr_max - 1]

Just replay the sequence we add new shrinkers:

1)We add shrinker #0:
    shrinker_nr_max = 0;

    prealloc_memcg_shrinker()
       id = 0;
       expand_shrinker_info(0)
         new_nr_max = 1;
         expand_one_shrinker_info(new_nr_max = 1)
           new->map_nr_max = 1;
         shrinker_nr_max = 1;

2)We add shrinker #1:
    prealloc_memcg_shrinker()
      id = 1;
      expand_shrinker_info(1)
        new_nr_max = 2;
        need_expand(2, 1) => false => ignore expand
        shrinker_nr_max = 2;

3)Then we call shrinker:
   shrink_slab_memcg()
     for_each_set_bit(i, info->map, 1/* info->map_nr_max */ ) {
     } => ignore shrinker #1

I'd fixed this patch by something like the below:

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 9f895ca6216c..bb617a3871f1 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -224,12 +224,6 @@ static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg,
  					 lockdep_is_held(&shrinker_rwsem));
  }
-static inline bool need_expand(int new_nr_max, int old_nr_max)
-{
-	return round_up(new_nr_max, BITS_PER_LONG) >
-	       round_up(old_nr_max, BITS_PER_LONG);
-}
-
  static int expand_one_shrinker_info(struct mem_cgroup *memcg,
  				    int map_size, int defer_size,
  				    int old_map_size, int old_defer_size,
@@ -247,9 +241,6 @@ static int expand_one_shrinker_info(struct mem_cgroup *memcg,
  		if (!old)
  			return 0;
- if (!need_expand(new_nr_max, old->map_nr_max))
-			return 0;
-

Maybe we can keep this. For example, when we failed to allocate memory by calling kvmalloc_node() last time, some shrinker_info may have been
expanded, and these shrinker_info do not need to be expanded again.

  		new = kvmalloc_node(sizeof(*new) + size, GFP_KERNEL, nid);
  		if (!new)
  			return -ENOMEM;
@@ -317,14 +308,11 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
  static int expand_shrinker_info(int new_id)
  {
  	int ret = 0;
-	int new_nr_max = new_id + 1;
+	int new_nr_max = round_up(new_id + 1, BITS_PER_LONG);
  	int map_size, defer_size = 0;
  	int old_map_size, old_defer_size = 0;
  	struct mem_cgroup *memcg;
- if (!need_expand(new_nr_max, shrinker_nr_max))
-		goto out;
-
  	if (!root_mem_cgroup)
  		goto out;
@@ -359,9 +347,11 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id) rcu_read_lock();
  		info = rcu_dereference(memcg->nodeinfo[nid]->shrinker_info);
-		/* Pairs with smp mb in shrink_slab() */
-		smp_mb__before_atomic();
-		set_bit(shrinker_id, info->map);
+		if (!WARN_ON_ONCE(shrinker_id >= info->map_nr_max)) {
+			/* Pairs with smp mb in shrink_slab() */
+			smp_mb__before_atomic();
+			set_bit(shrinker_id, info->map);
+		}
  		rcu_read_unlock();
  	}
  }

(I also added a new check into set_shrinker_bit() for safety).

Kirill

--
Thanks,
Qi




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