Re: [PATCH 12/12] vmscan: Do not writeback pages in direct reclaim

From: Mel Gorman
Date: Tue Jun 15 2010 - 10:16:28 EST

On Tue, Jun 15, 2010 at 10:04:24AM -0400, Rik van Riel wrote:
> On 06/15/2010 09:59 AM, Mel Gorman wrote:
>> On Tue, Jun 15, 2010 at 09:34:18AM -0400, Rik van Riel wrote:
>>> On 06/15/2010 07:45 AM, Mel Gorman wrote:
>>>>>> +/* kswapd and memcg can writeback as they are unlikely to overflow stack */
>>>>>> +static inline bool reclaim_can_writeback(struct scan_control *sc)
>>>>>> +{
>>>>>> + return current_is_kswapd() || sc->mem_cgroup != NULL;
>>>>>> +}
>>> If direct reclaim can overflow the stack, so can direct
>>> memcg reclaim. That means this patch does not solve the
>>> stack overflow, while admitting that we do need the
>>> ability to get specific pages flushed to disk from the
>>> pageout code.
>> What path is taken with memcg != NULL that could overflow the stack? I
>> couldn't spot one but mm/memcontrol.c is a bit tangled so finding all
>> its use cases is tricky. The critical path I had in mind though was
>> direct reclaim and for that path, memcg == NULL or did I miss something?
> mem_cgroup_hierarchical_reclaim -> try_to_free_mem_cgroup_pages

But in turn, where is mem_cgroup_hierarchical_reclaim called from direct
reclaim? It appears to be only called from the fault path or as a result
of the memcg changing size.

Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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