Re: [PATCH 0/2 v8] oom: capture unreclaimable slab info in oom message

From: Yang Shi
Date: Mon Oct 02 2017 - 11:40:42 EST

On 9/30/17 4:00 AM, Tetsuo Handa wrote:
Yang Shi wrote:
On 9/28/17 1:45 PM, Tetsuo Handa wrote:
Yang Shi wrote:
On 9/28/17 12:57 PM, Tetsuo Handa wrote:
Yang Shi wrote:
On 9/27/17 9:36 PM, Tetsuo Handa wrote:
On 2017/09/28 6:46, Yang Shi wrote:
Changelog v7 -> v8:
* Adopted Michal’s suggestion to dump unreclaim slab info when unreclaimable slabs amount > total user memory. Not only in oom panic path.

Holding slab_mutex inside dump_unreclaimable_slab() was refrained since V2
because there are


users. If we call dump_unreclaimable_slab() for non OOM panic path, aren't we
introducing a risk of crash (i.e. kernel panic) for regular OOM path?

I don't see the difference between regular oom path and oom path other
than calling panic() at last.

And, the slab dump may be called by panic path too, it is for both
regular and panic path.

Calling a function that might cause kerneloops immediately before calling panic()
would be tolerable, for the kernel will panic after all. But calling a function
that might cause kerneloops when there is no plan to call panic() is a bug.

I got your point. slab_mutex is used to protect the list of all the
slabs, since we are already in oom, there should be not kmem cache
destroy happen during the list traverse. And, list_for_each_entry() has
been replaced to list_for_each_entry_safe() to make the traverse more

I consider that OOM event and kmem chache destroy event can run concurrently
because slab_mutex is not held by OOM event (and unfortunately cannot be held
due to possibility of deadlock) in order to protect the list of all the slabs.

I don't think replacing list_for_each_entry() with list_for_each_entry_safe()
makes the traverse more robust, for list_for_each_entry_safe() does not defer
freeing of memory used by list element. Rather, replacing list_for_each_entry()
with list_for_each_entry_rcu() (and making relevant changes such as
rcu_read_lock()/rcu_read_unlock()/synchronize_rcu()) will make the traverse safe.

I'm not sure if rcu could satisfy this case. rcu just can protect
slab_caches_to_rcu_destroy list, which is used by SLAB_TYPESAFE_BY_RCU

I'm not sure why you are talking about SLAB_TYPESAFE_BY_RCU.
What I meant is that

Upon registration:

// do initialize/setup stuff here
synchronize_rcu(); // <= for dump_unreclaimable_slab()
list_add_rcu(&kmem_cache->list, &slab_caches);

Upon unregistration:

synchronize_rcu(); // <= for dump_unreclaimable_slab()
// do finalize/cleanup stuff here

then (if my understanding is correct)

list_for_each_entry_rcu(s, &slab_caches, list) {
if (!is_root_cache(s) || (s->flags & SLAB_RECLAIM_ACCOUNT))

memset(&sinfo, 0, sizeof(sinfo));
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);

will make dump_unreclaimable_slab() safe.

Thanks for the detailed description. However, it sounds this change is too much for slub, I'm not sure if this may change the subtle behavior of slub.

trylock sounds like a good alternative.