Re: [PATCH v2] mm/vmscan: get number of pages on the LRU list in memcgroup base on lru_zone_size

[Andrew Morton <akpm@linux-foundation.org>]

On Thu,  5 Sep 2019 15:10:34 +0800 Honglei Wang <honglei.wang@oracle.com> wrote:

> lruvec_lru_size() is involving lruvec_page_state_local() to get the
> lru_size in the current code. It's base on lruvec_stat_local.count[]
> of mem_cgroup_per_node. This counter is updated in batch. It won't
> do charge if the number of coming pages doesn't meet the needs of
> MEMCG_CHARGE_BATCH who's defined as 32 now.
> 
> The testcase in LTP madvise09[1] fails due to small block memory is
> not charged. It creates a new memcgroup and sets up 32 MADV_FREE
> pages. Then it forks child who will introduce memory pressure in the
> memcgroup. The MADV_FREE pages are expected to be released under the
> pressure, but 32 is not more than MEMCG_CHARGE_BATCH and these pages
> won't be charged in lruvec_stat_local.count[] until some more pages
> come in to satisfy the needs of batch charging. So these MADV_FREE
> pages can't be freed in memory pressure which is a bit conflicted
> with the definition of MADV_FREE.
> 
> Getting lru_size base on lru_zone_size of mem_cgroup_per_node which
> is not updated in batch can make it a bit more accurate in similar
> scenario.

I redid the changelog somewhat:

: lruvec_lru_size() is invokving lruvec_page_state_local() to get the
: lru_size.  It's base on lruvec_stat_local.count[] of mem_cgroup_per_node. 
: This counter is updated in a batched way.  It won't be charged if the
: number of incoming pages doesn't meet the needs of MEMCG_CHARGE_BATCH
: which is defined as 32.
: 
: The testcase in LTP madvise09[1] fails because small blocks of memory are
: not charged.  It creates a new memcgroup and sets up 32 MADV_FREE pages. 
: Then it forks a child who will introduce memory pressure in the memcgroup.
: The MADV_FREE pages are expected to be released under the pressure, but
: 32 is not more than MEMCG_CHARGE_BATCH and these pages won't be charged in
: lruvec_stat_local.count[] until some more pages come in to satisfy the
: needs of batch charging.  So these MADV_FREE pages can't be freed in
: memory pressure which is a bit conflicted with the definition of
: MADV_FREE.
: 
: Get the lru_size base on lru_zone_size of mem_cgroup_per_node which is not
: updated via batching can making it more accurate in this scenario.
: 
: This is effectively a partial reversion of 1a61ab8038e72 ("mm: memcontrol:
: replace zone summing with lruvec_page_state()").
: 
: [1] https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/syscalls/madvise/madvise09.c


> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -354,12 +354,13 @@ unsigned long zone_reclaimable_pages(struct zone *zone)
>   */
>  unsigned long lruvec_lru_size(struct lruvec *lruvec, enum lru_list lru, int zone_idx)
>  {
> -	unsigned long lru_size;
> +	unsigned long lru_size = 0;
>  	int zid;
>  
> -	if (!mem_cgroup_disabled())
> -		lru_size = lruvec_page_state_local(lruvec, NR_LRU_BASE + lru);
> -	else
> +	if (!mem_cgroup_disabled()) {
> +		for (zid = 0; zid < MAX_NR_ZONES; zid++)
> +			lru_size += mem_cgroup_get_zone_lru_size(lruvec, lru, zid);
> +	} else
>  		lru_size = node_page_state(lruvec_pgdat(lruvec), NR_LRU_BASE + lru);
>  
>  	for (zid = zone_idx + 1; zid < MAX_NR_ZONES; zid++) {

Do we think this problem is serious enough to warrant backporting into
earlier kernels?