On Fri, Mar 31, 2023 at 1:08 AM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
...
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a3e38851b34ac..bf9d8e175e92a 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -533,7 +533,35 @@ EXPORT_SYMBOL(mm_account_reclaimed_pages);
> static void flush_reclaim_state(struct scan_control *sc,
> struct reclaim_state *rs)
> {
> - if (rs) {
> + /*
> + * Currently, reclaim_state->reclaimed includes three types of pages
> + * freed outside of vmscan:
> + * (1) Slab pages.
> + * (2) Clean file pages from pruned inodes.
> + * (3) XFS freed buffer pages.
> + *
> + * For all of these cases, we have no way of finding out whether these
> + * pages were related to the memcg under reclaim. For example, a freed
> + * slab page could have had only a single object charged to the memcg
> + * under reclaim. Also, populated inodes are not on shrinker LRUs
> + * anymore except on highmem systems.
> + *
> + * Instead of over-reporting the reclaimed pages in a memcg reclaim,
> + * only count such pages in system-wide reclaim. This prevents
> + * unnecessary retries during memcg charging and false positive from
> + * proactive reclaim (memory.reclaim).
What happens when writing to the root memory.reclaim?
> + *
> + * For uncommon cases were the freed pages were actually significantly
> + * charged to the memcg under reclaim, and we end up under-reporting, it
> + * should be fine. The freed pages will be uncharged anyway, even if
> + * they are not reported properly, and we will be able to make forward
> + * progress in charging (which is usually in a retry loop).
> + *
> + * We can go one step further, and report the uncharged objcg pages in
> + * memcg reclaim, to make reporting more accurate and reduce
> + * under-reporting, but it's probably not worth the complexity for now.
> + */
> + if (rs && !cgroup_reclaim(sc)) {
To answer the question above, global_reclaim() would be preferred.
