Re: [RFC 2/4] mm, page_alloc: fix fast-path race with cpuset update or removal

[Mel Gorman]

On Tue, Jan 17, 2017 at 11:16:08PM +0100, Vlastimil Babka wrote:
> Ganapatrao Kulkarni reported that the LTP test cpuset01 in stress mode triggers
> OOM killer in few seconds, despite lots of free memory. The test attemps to
> repeatedly fault in memory in one process in a cpuset, while changing allowed
> nodes of the cpuset between 0 and 1 in another process.
> 
> One possible cause is that in the fast path we find the preferred zoneref
> according to current mems_allowed, so that it points to the middle of the
> zonelist, skipping e.g. zones of node 1 completely. If the mems_allowed is
> updated to contain only node 1, we never reach it in the zonelist, and trigger
> OOM before checking the cpuset_mems_cookie.
> 
> This patch fixes the particular case by redoing the preferred zoneref search
> if we switch back to the original nodemask. The condition is also slightly
> changed so that when the last non-root cpuset is removed, we don't miss it.
> 
> Note that this is not a full fix, and more patches will follow.
> 
> Reported-by: Ganapatrao Kulkarni <gpkulkarni@xxxxxxxxx>
> Fixes: 682a3385e773 ("mm, page_alloc: inline the fast path of the zonelist iterator")
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

-- 
Mel Gorman
SUSE Labs