Re: [PATCH mmotm] vmscan: fix may_swap handling for memcg

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

Hi

> From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> 
> Commit 2e2e425989080cc534fc0fca154cae515f971cf5 ("vmscan,memcg: reintroduce
> sc->may_swap) add may_swap flag and handle it at get_scan_ratio().
> 
> But the result of get_scan_ratio() is ignored when priority == 0, and this
> means, when memcg hits the mem+swap limit, anon pages can be swapped
> just in vain. Especially when memcg causes oom by mem+swap limit,
> we can see many and many pages are swapped out.
> 
> Instead of not scanning anon lru completely when priority == 0, this patch adds
> a hook to handle may_swap flag in shrink_page_list() to avoid using useless swaps,
> and calls try_to_free_swap() if needed because it can reduce
> both mem.usage and memsw.usage if the page(SwapCache) is unused anymore.
> 
> Such unused-but-managed-under-memcg SwapCache can be made in some paths,
> for example trylock_page() failure in free_swap_cache().
> 
> Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>

I think root cause is following branch, right?
if so, Why can't we handle this issue on shrink_zone()?


---------------------------------------------------------------
static void shrink_zone(int priority, struct zone *zone,
                                struct scan_control *sc)
{
        get_scan_ratio(zone, sc, percent);

        for_each_evictable_lru(l) {
                int file = is_file_lru(l);
                unsigned long scan;

                scan = zone_nr_pages(zone, sc, l);
                if (priority) {				// !!here!!
                        scan >>= priority;
                        scan = (scan * percent[file]) / 100;
                }




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