On Fri, 28 Jan 2011 17:25:58 +0900
Minchan Kim [off-list ref] wrote:

Hi Hannes,

On Fri, Jan 28, 2011 at 5:17 PM, Johannes Weiner [off-list ref] wrote:

quoted

On Fri, Jan 28, 2011 at 05:04:16PM +0900, Minchan Kim wrote:

quoted

Hi Kame,

On Fri, Jan 28, 2011 at 1:58 PM, KAMEZAWA Hiroyuki
[off-list ref] wrote:

quoted

How about this ?
==
From: KAMEZAWA Hiroyuki <redacted>

Current memory cgroup's code tends to assume page_size == PAGE_SIZE
and arrangement for THP is not enough yet.

This is one of fixes for supporing THP. This adds
mem_cgroup_check_margin() and checks whether there are required amount of
free resource after memory reclaim. By this, THP page allocation
can know whether it really succeeded or not and avoid infinite-loop
and hangup.

Total fixes for do_charge()/reclaim memory will follow this patch.

If this patch is only related to THP, I think patch order isn't good.
Before applying [2/4], huge page allocation will retry without
reclaiming and loop forever by below part.

@@ -1854,9 +1858,6 @@ static int __mem_cgroup_do_charge(struct

A  A  A  } else
A  A  A  A  A  A  A  mem_over_limit = mem_cgroup_from_res_counter(fail_res, res);

- A  A  if (csize > PAGE_SIZE) /* change csize and retry */
- A  A  A  A  A  A  return CHARGE_RETRY;
-
A  A  A  if (!(gfp_mask & __GFP_WAIT))
A  A  A  A  A  A  A  return CHARGE_WOULDBLOCK;

Am I missing something?

No, you are correct. A But I am not sure the order really matters in
theory: you have two endless loops that need independent fixing.

That's why I ask a question.
Two endless loop?

One is what I mentioned. The other is what?
Maybe this patch solve the other.
But I can't guess it by only this description. Stupid..

Please open my eyes.

One is.

  if (csize > PAGE_SIZE)
	return CHARGE_RETRY;

By this, reclaim will never be called.


Another is a check after memory reclaim.
==
       ret = mem_cgroup_hierarchical_reclaim(mem_over_limit, NULL,
                                        gfp_mask, flags);
        /*
         * try_to_free_mem_cgroup_pages() might not give us a full
         * picture of reclaim. Some pages are reclaimed and might be
         * moved to swap cache or just unmapped from the cgroup.
         * Check the limit again to see if the reclaim reduced the
         * current usage of the cgroup before giving up
         */
        if (ret || mem_cgroup_check_under_limit(mem_over_limit))
                return CHARGE_RETRY;
==

ret != 0 if one page is reclaimed. Then, khupaged will retry charge and 
cannot get enough room, reclaim, one page -> again. SO, in busy memcg,
HPAGE_SIZE allocation never fails.

Even if khupaged luckly allocates HPAGE_SIZE, because khugepaged walks vmas
one by one and try to collapse each pmd, under mmap_sem(), this seems a hang by
khugepaged, infinite loop.


Thanks,
-Kame





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