Re: memcg uncharge page counter mismatch

[Minchan Kim <minchan@kernel.org>]

On Thu, Dec 03, 2015 at 04:47:29PM +0100, Michal Hocko wrote:
> On Thu 03-12-15 15:58:50, Michal Hocko wrote:
> [....]
> > Warning, this looks ugly as hell.
> 
> I was thinking about it some more and it seems that we should rather not
> bother with partial thp at all and keep it in the original memcg
> instead. It is way much less code and I do not think this will be too
> disruptive. Somebody should be holding the thp head, right?
> 
> Minchan, does this fix the issue you are seeing.

This patch solves the issue but not sure it's right approach.
I think it could make regression that in old, we could charge
a THP page but we can't now. Whether it's trivial or not, it depends
on memcg guys.

Thanks.


> ---
> 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,
> -- 
> Michal Hocko
> SUSE Labs

-- 
Kind regards,
Minchan Kim