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SubjectRe: [PATCH 3/3] mm: oom: show unreclaimable slab info when unreclaimable slabs > user memory
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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@alibaba-inc.com>
> [...]
>> +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
>

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