Re: [PATCH 15/18] mm: memcontrol: make swap tracking an integral part of memory control

[Johannes Weiner <hannes@cmpxchg.org>]

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.