Re: memcg uncharge page counter mismatch

From: Johannes Weiner <hannes@cmpxchg.org>
To: Michal Hocko <mhocko@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Kirill A. Shutemov" <kirill@shutemov.name>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: memcg uncharge page counter mismatch
Date: Fri, 4 Dec 2015 11:01:28 -0500	[thread overview]
Message-ID: <20151204160128.GA24431@cmpxchg.org> (raw)
In-Reply-To: <20151203154729.GI9264@dhcp22.suse.cz>

On Thu, Dec 03, 2015 at 04:47:29PM +0100, Michal Hocko wrote:
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 79a29d564bff..143c933f0b81 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4895,6 +4895,14 @@ static int mem_cgroup_move_charge_pte_range(pmd_t *pmd,
>  		switch (get_mctgt_type(vma, addr, ptent, &target)) {
>  		case MC_TARGET_PAGE:
>  			page = target.page;
> +			/*
> +			 * We can have a part of the split pmd here. Moving it
> +			 * can be done but it would be too convoluted so simply
> +			 * ignore such a partial THP and keep it in original
> +			 * memcg. There should be somebody mapping the head.
> +			 */
> +			if (PageCompound(page))
> +				goto put;
>  			if (isolate_lru_page(page))
>  				goto put;
>  			if (!mem_cgroup_move_account(page, false,

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

The charge moving concept is fundamentally flawed and its
implementation here is incomplete and races with reclaim.

Really, nobody should be using this. Absent any actual regression
reports, a minimal fix to stop this code from generating warnings
should be enough.