Re: [PATCH 1/2 v2] mm: vmscan: do not pass reclaimed slab to vmpressure

[Minchan Kim]

On Fri, Jan 27, 2017 at 01:43:36PM +0530, Vinayak Menon wrote:
> It is noticed that during a global reclaim the memory
> reclaimed via shrinking the slabs can sometimes result
> in reclaimed pages being greater than the scanned pages
> in shrink_node. When this is passed to vmpressure, the
> unsigned arithmetic results in the pressure value to be
> huge, thus resulting in a critical event being sent to
> root cgroup. While this can be fixed by underflow checks
> in vmpressure, adding reclaimed slab without a corresponding
> increment of nr_scanned results in incorrect vmpressure
> reporting. So do not consider reclaimed slab pages in
> vmpressure calculation.

I belive we could enhance the description better.

problem

VM include nr_reclaimed of slab but not nr_scanned so pressure
calculation can be underflow.

solution

do not consider reclaimed slab pages for vmpressure

why

Freeing a page by slab shrinking depends on each slab's object
population so the cost model(i.e., scan:free) is not fair with
LRU pages. Also, every shrinker doesn't account reclaimed pages.
Lastly, this regression happens since 6b4f7799c6a5

> 
> Signed-off-by: Vinayak Menon <vinmenon@xxxxxxxxxxxxxx>
> ---
>  mm/vmscan.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 947ab6f..37c4486 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2594,16 +2594,16 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
>  				    sc->nr_scanned - nr_scanned,
>  				    node_lru_pages);
>  
> -		if (reclaim_state) {
> -			sc->nr_reclaimed += reclaim_state->reclaimed_slab;
> -			reclaim_state->reclaimed_slab = 0;
> -		}
> -
>  		/* Record the subtree's reclaim efficiency */
>  		vmpressure(sc->gfp_mask, sc->target_mem_cgroup, true,
>  			   sc->nr_scanned - nr_scanned,
>  			   sc->nr_reclaimed - nr_reclaimed);
>  

Please add comment about "vmpressure excludes reclaimed pages via slab
because blah blah blah" so upcoming patches doesn't make mistake again.

Thanks!

> +		if (reclaim_state) {
> +			sc->nr_reclaimed += reclaim_state->reclaimed_slab;
> +			reclaim_state->reclaimed_slab = 0;
> +		}
> +
>  		if (sc->nr_reclaimed - nr_reclaimed)
>  			reclaimable = true;
>  
> -- 
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