Re: [PATCH 3/3] mm: oom: show unreclaimable slab info when unreclaimable slabs > user memory

[Yang Shi]

On 10/6/17 2:37 AM, Michal Hocko wrote:
> On Thu 05-10-17 05:29:10, Yang Shi wrote:
> > Kernel may panic when oom happens without killable process sometimes it
> > is caused by huge unreclaimable slabs used by kernel.
> >
> > Although kdump could help debug such problem, however, kdump is not
> > available on all architectures and it might be malfunction sometime.
> > And, since kernel already panic it is worthy capturing such information
> > in dmesg to aid touble shooting.
> >
> > Print out unreclaimable slab info (used size and total size) which
> > actual memory usage is not zero (num_objs * size != 0) when
> > unreclaimable slabs amount is greater than total user memory (LRU
> > pages).
> >
> > The output looks like:
> >
> > Unreclaimable slab info:
> > Name                      Used          Total
> > rpc_buffers               31KB         31KB
> > rpc_tasks                  7KB          7KB
> > ebitmap_node            1964KB       1964KB
> > avtab_node              5024KB       5024KB
> > xfs_buf                 1402KB       1402KB
> > xfs_ili                  134KB        134KB
> > xfs_efi_item             115KB        115KB
> > xfs_efd_item             115KB        115KB
> > xfs_buf_item             134KB        134KB
> > xfs_log_item_desc        342KB        342KB
> > xfs_trans               1412KB       1412KB
> > xfs_ifork                212KB        212KB
>
> OK this looks better. The naming is not the greatest but I will not
> nitpick on this. I have one question though
>
> > Signed-off-by: Yang Shi <yang.s@xxxxxxxxxxxxxxx>
> [...]
> > +void dump_unreclaimable_slab(void)
> > +{
> > +	struct kmem_cache *s, *s2;
> > +	struct slabinfo sinfo;
> > +
> > +	/*
> > +	 * Here acquiring slab_mutex is risky since we don't prefer to get
> > +	 * sleep in oom path. But, without mutex hold, it may introduce a
> > +	 * risk of crash.
> > +	 * Use mutex_trylock to protect the list traverse, dump nothing
> > +	 * without acquiring the mutex.
> > +	 */
> > +	if (!mutex_trylock(&slab_mutex)) {
> > +		pr_warn("excessive unreclaimable slab but cannot dump stats\n");
> > +		return;
> > +	}
> > +
> > +	pr_info("Unreclaimable slab info:\n");
> > +	pr_info("Name                      Used          Total\n");
> > +
> > +	list_for_each_entry_safe(s, s2, &slab_caches, list) {
> > +		if (!is_root_cache(s) || (s->flags & SLAB_RECLAIM_ACCOUNT))
> > +			continue;
> > +
> > +		memset(&sinfo, 0, sizeof(sinfo));
>
> why do you zero out the structure. All the fields you are printing are
> filled out in get_slabinfo.

No special reason, just wipe out the potential stale data on the stack.

Yang

> > +		get_slabinfo(s, &sinfo);
> > +
> > +		if (sinfo.num_objs > 0)
> > +			pr_info("%-17s %10luKB %10luKB\n", cache_name(s),
> > +				(sinfo.active_objs * s->size) / 1024,
> > +				(sinfo.num_objs * s->size) / 1024);
> > +	}
> > +	mutex_unlock(&slab_mutex);
> > +}
> > +
> >   #if defined(CONFIG_MEMCG) && !defined(CONFIG_SLOB)
> >   void *memcg_slab_start(struct seq_file *m, loff_t *pos)
> >   {
> > --
> > 1.8.3.1