Re: [PATCH] slub: consider pfmemalloc_match() inget_partial_node()

[David Rientjes]

On Sat, 25 Aug 2012, Joonsoo Kim wrote:

> There is no consideration for pfmemalloc_match() in get_partial(). If we don't
> consider that, we can't restrict access to PFMEMALLOC page mostly.
> 
> We may encounter following scenario.
> 
> Assume there is a request from normal allocation
> and there is no objects in per cpu cache and no node partial slab.
> 
> In this case, slab_alloc go into slow-path and
> new_slab_objects() is invoked. It may return PFMEMALLOC page.
> Current user is not allowed to access PFMEMALLOC page,
> deactivate_slab() is called (commit 5091b74a95d447e34530e713a8971450a45498b3),
> then return object from PFMEMALLOC page.
> 
> Next time, when we meet another request from normal allocation,
> slab_alloc() go into slow-path and re-go new_slab_objects().
> In new_slab_objects(), we invoke get_partial() and we get a partial slab
> which we have been deactivated just before, that is, PFMEMALLOC page.
> We extract one object from it and re-deactivate.
> 
> "deactivate -> re-get in get_partial -> re-deactivate" occures repeatedly.
> 
> As a result, we can't restrict access to PFMEMALLOC page and
> moreover, it introduce much performance degration to normal allocation
> because of deactivation frequently.
> 
> Now, we need to consider pfmemalloc_match() in get_partial_node()
> It prevent "deactivate -> re-get in get_partial".
> Instead, new_slab() is called. It may return !PFMEMALLOC page,
> so above situation will be suspended sometime.
> 
> Signed-off-by: Joonsoo Kim <js1304@xxxxxxxxx>
> Cc: David Miller <davem@xxxxxxxxxxxxx>
> Cc: Neil Brown <neilb@xxxxxxx>
> Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> Cc: Mike Christie <michaelc@xxxxxxxxxxx>
> Cc: Eric B Munson <emunson@xxxxxxxxx>
> Cc: Eric Dumazet <eric.dumazet@xxxxxxxxx>
> Cc: Sebastian Andrzej Siewior <sebastian@xxxxxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>

Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
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