Hello, Roman. The reclaim behavior is a bit worrisome. * It disables an entire swap area while reclaim is in progress. Most systems only have one swap area, so this would disable allocating new swap area for everyone. * The reclaim seems very inefficient. IIUC, it has to read every swap page to see whether the page belongs to the target memcg and for each matching page, which involves walking page mm's and page tables. An easy optimization would be walking swap_cgroup_ctrl so that it only reads swap entries which belong to the target cgroup and avoid disabling swap for others, but looking at the code, I wonder whether we need active reclaim at all. Swap already tries to aggressively reclaim swap entries when swap usage > 50% of the limit, so simply reducing the limit already triggers aggressive reclaim, and given that it's swap, just waiting it out could be the better behavior anyway, so how about something like the following? ------ 8< ------ From: Tejun Heo <tj@xxxxxxxxxx> Subject: mm: memcg: allow lowering memory.swap.max below the current usage Currently an attempt to set swap.max into a value lower than the actual swap usage fails, which causes configuration problems as there's no way of lowering the configuration below the current usage short of turning off swap entirely. This makes swap.max difficult to use and allows delegatees to lock the delegator out of reducing swap allocation. This patch updates swap_max_write() so that the limit can be lowered below the current usage. It doesn't implement active reclaiming of swap entries for the following reasons. * mem_cgroup_swap_full() already tells the swap machinary to aggressively reclaim swap entries if the usage is above 50% of limit, so simply lowering the limit automatically triggers gradual reclaim. * Forcing back swapped out pages is likely to heavily impact the workload and mess up the working set. Given that swap usually is a lot less valuable and less scarce, letting the existing usage dissipate over time through the above gradual reclaim and as they're falted back in is likely the better behavior. Signed-off-by: Tejun Heo <tj@xxxxxxxxxx> Cc: Roman Gushchin <guro@xxxxxx> Cc: Johannes Weiner <hannes@xxxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxxxx> Cc: Shaohua Li <shli@xxxxxx> Cc: Rik van Riel <riel@xxxxxxxxxxx> Cc: linux-kernel@xxxxxxxxxxxxxxx Cc: linux-mm@xxxxxxxxx Cc: cgroups@xxxxxxxxxxxxxxx --- Documentation/cgroup-v2.txt | 5 +++++ mm/memcontrol.c | 6 +----- 2 files changed, 6 insertions(+), 5 deletions(-) --- a/Documentation/cgroup-v2.txt +++ b/Documentation/cgroup-v2.txt @@ -1199,6 +1199,11 @@ PAGE_SIZE multiple when read back. Swap usage hard limit. If a cgroup's swap usage reaches this limit, anonymous memory of the cgroup will not be swapped out. + When reduced under the current usage, the existing swap + entries are reclaimed gradually and the swap usage may stay + higher than the limit for an extended period of time. This + reduces the impact on the workload and memory management. + Usage Guidelines ~~~~~~~~~~~~~~~~ --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -6144,11 +6144,7 @@ static ssize_t swap_max_write(struct ker if (err) return err; - mutex_lock(&memcg_limit_mutex); - err = page_counter_limit(&memcg->swap, max); - mutex_unlock(&memcg_limit_mutex); - if (err) - return err; + xchg(&memcg->swap.limit, max); return nbytes; }
