Messages in this thread | | | Subject | Re: [PATCH] mm,vmscan: fix divide by zero in get_scan_count | From | Rik van Riel <> | Date | Mon, 30 Aug 2021 09:24:22 -0400 |
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On Mon, 2021-08-30 at 13:33 +0200, Michal Hocko wrote: > On Thu 26-08-21 22:01:49, Rik van Riel wrote: > > Changeset f56ce412a59d ("mm: memcontrol: fix occasional OOMs due to > > proportional memory.low reclaim") introduced a divide by zero > > corner > > case when oomd is being used in combination with cgroup memory.low > > protection. > > > > When oomd decides to kill a cgroup, it will force the cgroup memory > > to be reclaimed after killing the tasks, by writing to the > > memory.max > > file for that cgroup, forcing the remaining page cache and > > reclaimable > > slab to be reclaimed down to zero. > > > > Previously, on cgroups with some memory.low protection that would > > result > > in the memory being reclaimed down to the memory.low limit, or > > likely not > > at all, having the page cache reclaimed asynchronously later. > > > > With f56ce412a59d the oomd write to memory.max tries to reclaim all > > the > > way down to zero, which may race with another reclaimer, to the > > point of > > ending up with the divide by zero below. > > > > This patch implements the obvious fix. > > I must be missing something but how can cgroup_size be ever 0 when it > is > max(cgroup_size, protection) and protection != 0?
Going into the condition we use if (low || min), where it is possible for low > 0 && min == 0.
Inside the conditional, we can end up testing against min.
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