On Fri 12-10-12 11:45:46, Glauber Costa wrote:

On 10/11/2012 04:42 PM, Michal Hocko wrote:

quoted

On Mon 08-10-12 14:06:12, Glauber Costa wrote:

[...]

quoted

quoted

+	/*
+	 * Conditions under which we can wait for the oom_killer.
+	 * __GFP_NORETRY should be masked by __mem_cgroup_try_charge,
+	 * but there is no harm in being explicit here
+	 */
+	may_oom = (gfp & __GFP_WAIT) && !(gfp & __GFP_NORETRY);

Well we _have to_ check __GFP_NORETRY here because if we don't then we
can end up in OOM. mem_cgroup_do_charge returns CHARGE_NOMEM for
__GFP_NORETRY (without doing any reclaim) and of oom==true we decrement
oom retries counter and eventually hit OOM killer. So the comment is
misleading.

I will update. What i understood from your last message is that we don't
really need to, because try_charge will do it.

IIRC I just said it couldn't happen before because migration doesn't go
through charge and thp disable oom by default.

quoted

quoted

+
+	_memcg = memcg;
+	ret = __mem_cgroup_try_charge(NULL, gfp, size >> PAGE_SHIFT,
+				      &_memcg, may_oom);
+
+	if (!ret) {
+		ret = res_counter_charge(&memcg->kmem, size, &fail_res);

Now that I'm thinking about the charging ordering we should charge the
kmem first because we would like to hit kmem limit before we hit u+k
limit, don't we. 
Say that you have kmem limit 10M and the total limit 50M. Current `u'
would be 40M and this charge would cause kmem to hit the `k' limit. I
think we should fail to charge kmem before we go to u+k and potentially
reclaim/oom.
Or has this been alredy discussed and I just do not remember?

This has never been discussed as far as I remember. We charged u first
since day0, and you are so far the first one to raise it...

One of the things in favor of charging 'u' first is that
mem_cgroup_try_charge is already equipped to make a lot of decisions,
like when to allow reclaim, when to bypass charges, and it would be good
if we can reuse all that.

Hmm, I think that we should prevent from those decisions if kmem charge
would fail anyway (especially now when we do not have targeted slab
reclaim).

You oom-based argument makes some sense, if all other scenarios are
unchanged by this, I can change it. I will give this some more
consideration.

[...]

quoted

	/*
	 * Keep reference on memcg while the page is charged to prevent
	 * group from vanishing because allocation can outlive their
	 * tasks. The reference is dropped in __memcg_kmem_uncharge_page
	 */

please

I can do that, but keep in mind this piece of code is going away soon =)

Yes I have noticed that and replied to myself that it is not necessary.

-- 
Michal Hocko
SUSE Labs

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>