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