Re: [PATCH 2/2] mm: oom: show unreclaimable slab info when kernel panic

[David Rientjes]

On Thu, 21 Sep 2017, Yang Shi wrote:

> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 99736e0..173c423 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -43,6 +43,7 @@
>  
>  #include <asm/tlb.h>
>  #include "internal.h"
> +#include "slab.h"
>  
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/oom.h>
> @@ -427,6 +428,14 @@ static void dump_header(struct oom_control *oc, struct task_struct *p)
>  		dump_tasks(oc->memcg, oc->nodemask);
>  }
>  
> +static void dump_header_with_slabinfo(struct oom_control *oc, struct task_struct *p)
> +{
> +	dump_header(oc, p);
> +
> +	if (IS_ENABLED(CONFIG_SLABINFO))
> +		show_unreclaimable_slab();
> +}
> +
>  /*
>   * Number of OOM victims in flight
>   */

I don't think we need a new function for this.  Where you want to dump 
unreclaimable slab before panic, just call a new dump_unreclaimable_slab() 
function that gets declared in slab.h that is a no-op when CONFIG_SLABINFO 
is disabled.  We just want to do

	dump_header(...);
	dump_unreclaimable_slab(...);
	panic(...);

> diff --git a/mm/slab.c b/mm/slab.c
> index 04dec48..4f4971c 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -4132,6 +4132,7 @@ void get_slabinfo(struct kmem_cache *cachep, struct slabinfo *sinfo)
>  	sinfo->shared = cachep->shared;
>  	sinfo->objects_per_slab = cachep->num;
>  	sinfo->cache_order = cachep->gfporder;
> +	sinfo->reclaim = is_reclaimable(cachep);

We don't need a new field, we already have cachep->flags accessible.

>  }
>  
>  void slabinfo_show_stats(struct seq_file *m, struct kmem_cache *cachep)
> diff --git a/mm/slab.h b/mm/slab.h
> index 0733628..2f1ebce 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -186,6 +186,7 @@ struct slabinfo {
>  	unsigned int shared;
>  	unsigned int objects_per_slab;
>  	unsigned int cache_order;
> +	unsigned int reclaim;

Not needed.

>  };
>  
>  void get_slabinfo(struct kmem_cache *s, struct slabinfo *sinfo);
> @@ -352,6 +353,11 @@ static inline void memcg_link_cache(struct kmem_cache *s)
>  
>  #endif /* CONFIG_MEMCG && !CONFIG_SLOB */
>  
> +static inline bool is_reclaimable(struct kmem_cache *s)
> +{
> +	return (s->flags & SLAB_RECLAIM_ACCOUNT) ? true : false;
> +}
> +

I don't think we need this.

>  static inline struct kmem_cache *cache_from_obj(struct kmem_cache *s, void *x)
>  {
>  	struct kmem_cache *cachep;
> @@ -504,6 +510,7 @@ static inline struct kmem_cache_node *get_node(struct kmem_cache *s, int node)
>  void *memcg_slab_next(struct seq_file *m, void *p, loff_t *pos);
>  void memcg_slab_stop(struct seq_file *m, void *p);
>  int memcg_slab_show(struct seq_file *m, void *p);
> +void show_unreclaimable_slab(void);
>  
>  void ___cache_free(struct kmem_cache *cache, void *x, unsigned long addr);
>  
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 904a83b..f2c6200 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -35,6 +35,8 @@
>  static DECLARE_WORK(slab_caches_to_rcu_destroy_work,
>  		    slab_caches_to_rcu_destroy_workfn);
>  
> +#define K(x) ((x)/1024)
> +

I don't think we need this.

>  /*
>   * Set of flags that will prevent slab merging
>   */
> @@ -1272,6 +1274,35 @@ static int slab_show(struct seq_file *m, void *p)
>  	return 0;
>  }
>  
> +void show_unreclaimable_slab()

void show_unreclaimable_slab(void)

> +{
> +	struct kmem_cache *s = NULL;

No initialization needed.

> +	struct slabinfo sinfo;
> +
> +	memset(&sinfo, 0, sizeof(sinfo));
> +
> +	printk("Unreclaimable slab info:\n");
> +	printk("Name                      Used          Total\n");
> +
> +	/*
> +	 * Here acquiring slab_mutex is unnecessary since we don't prefer to
> +	 * get sleep in oom path right before kernel panic, and avoid race condition.
> +	 * Since it is already oom, so there should be not any big allocation
> +	 * which could change the statistics significantly.
> +	 */
> +	list_for_each_entry(s, &slab_caches, list) {
> +		if (!is_root_cache(s))
> +			continue;
> +

We need to do the memset() here.

> +		get_slabinfo(s, &sinfo);
> +
> +		if (!is_reclaimable(s) && sinfo.num_objs > 0)
> +			printk("%-17s %10luKB %10luKB\n", cache_name(s), K(sinfo.active_objs * s->size), K(sinfo.num_objs * s->size));

I think you can just check for SLAB_RECLAIM_ACCOUNT here.

Everything in this function should be pr_info().

> +	}
> +}
> +EXPORT_SYMBOL(show_unreclaimable_slab);
> +#undef K
> +
>  #if defined(CONFIG_MEMCG) && !defined(CONFIG_SLOB)
>  void *memcg_slab_start(struct seq_file *m, loff_t *pos)
>  {
> diff --git a/mm/slub.c b/mm/slub.c
> index 163352c..5c17c0a 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -5872,6 +5872,7 @@ void get_slabinfo(struct kmem_cache *s, struct slabinfo *sinfo)
>  	sinfo->num_slabs = nr_slabs;
>  	sinfo->objects_per_slab = oo_objects(s->oo);
>  	sinfo->cache_order = oo_order(s->oo);
> +	sinfo->reclaim = is_reclaimable(s);

Not needed.

>  }
>  
>  void slabinfo_show_stats(struct seq_file *m, struct kmem_cache *s)