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Date:   Thu, 23 Apr 2020 23:01:17 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Joonsoo Kim <js1304@gmail.com>,
        Alex Shi <alex.shi@linux.alibaba.com>
Cc:     Shakeel Butt <shakeelb@google.com>,
        Hugh Dickins <hughd@google.com>,
        Michal Hocko <mhocko@suse.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Roman Gushchin <guro@fb.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com
Subject: Re: [PATCH 15/18] mm: memcontrol: make swap tracking an integral
 part of memory control
Message-ID: <20200424030117.GC464082@cmpxchg.org>
References: <20200420221126.341272-1-hannes@cmpxchg.org>
 <20200420221126.341272-16-hannes@cmpxchg.org>
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On Mon, Apr 20, 2020 at 06:11:23PM -0400, Johannes Weiner wrote:
> @@ -6884,9 +6876,6 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
>  	VM_BUG_ON_PAGE(PageLRU(page), page);
>  	VM_BUG_ON_PAGE(page_count(page), page);
>  
> -	if (!do_memsw_account())
> -		return;
> -
>  	memcg = page->mem_cgroup;
>  
>  	/* Readahead page, never charged */

I messed up here.

mem_cgroup_swapout() must not run on cgroup2, because cgroup2 uses
mem_cgroup_try_charge_swap() instead. Both record a swap entry and
running them both will trigger a VM_BUG_ON() on an existing record.

I'm actually somewhat baffled why this didn't trigger in my
MADV_PAGEOUT -> MADV_WILLNEED swap test. memory.max driven swapout
triggered it right away.

!do_memsw_account() needs to be !cgroup_subsys_on_dfl(memory_cgrp_subsys)

> @@ -6913,7 +6902,7 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
>  	if (!mem_cgroup_is_root(memcg))
>  		page_counter_uncharge(&memcg->memory, nr_entries);
>  
> -	if (memcg != swap_memcg) {
> +	if (do_memsw_account() && memcg != swap_memcg) {
>  		if (!mem_cgroup_is_root(swap_memcg))
>  			page_counter_charge(&swap_memcg->memsw, nr_entries);
>  		page_counter_uncharge(&memcg->memsw, nr_entries);

And this can be !cgroup_memory_noswap instead. It'll do the same
thing, but will be clearer.

I'll have it fixed in version 2.