Re: [PATCH] vmscan: memcg needs may_swap (Re: [patch] vmscan:rename sc.may_swap to may_unmap)

[Daisuke Nishimura]

On Mon, 30 Mar 2009 08:45:28 +0900 (JST), KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> wrote:
> > On Fri, 27 Mar 2009 15:19:26 +0900
> > Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx> wrote:
> > 
> > > Added
> > >  Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> > >  Cc: Balbir Singh <balbir@xxxxxxxxxx>
> > > 
> > > I'm sorry for replying to a very old mail.
> > > 
> > > > @@ -1713,7 +1713,7 @@ unsigned long try_to_free_mem_cgroup_pag
> > > >  {
> > > >  	struct scan_control sc = {
> > > >  		.may_writepage = !laptop_mode,
> > > > -		.may_swap = 1,
> > > > +		.may_unmap = 1,
> > > >  		.swap_cluster_max = SWAP_CLUSTER_MAX,
> > > >  		.swappiness = swappiness,
> > > >  		.order = 0,
> > > > @@ -1723,7 +1723,7 @@ unsigned long try_to_free_mem_cgroup_pag
> > > >  	struct zonelist *zonelist;
> > > >  
> > > >  	if (noswap)
> > > > -		sc.may_swap = 0;
> > > > +		sc.may_unmap = 0;
> > > >  
> > > >  	sc.gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
> > > >  			(GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK);
> > > IIUC, memcg had used may_swap as a flag for "we need to use swap?" as the name indicate.
> > > 
> > > Because, when mem+swap hits the limit, trying to swapout pages is meaningless
> > > as it doesn't change mem+swap usage.
> > > 
> > Good catch...sigh, I missed this disussion.
> > 
> > 
> > 
> > > What do you think of this patch?
> > > ===
> > > From: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
> > > 
> > > vmscan-rename-scmay_swap-to-may_unmap.patch removed may_swap flag,
> > > but memcg had used it as a flag for "we need to use swap?", as the
> > > name indicate.
> > > 
> > > And in current implementation, memcg cannot reclaim mapped file caches
> > > when mem+swap hits the limit.
> > > 
> > When mem+swap hits the limit, swap-out anonymous page doesn't reduce the
> > amount of usage of mem+swap, so, swap-out should be avoided.
> > 
> > > re-introduce may_swap flag and handle it at shrink_page_list.
> > > 
> > > This patch doesn't influence any scan_control users other than memcg.
> > > 
> > 
> > 
> > > Signed-off-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
> > 
> > Seems good,
> > Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> > 
> > But hum....Maybe this lru scan work in the same way as the case
> > of !total_swap_pages. (means don't scan anon LRU.)
> > revisit this later.
> 
> Well, How about following patch?
> 
I think your patch looks better because vain scanning of anon list
is avoided.

Thanks,
Daisuke Nishimura.

> So, I have to agree my judgement of may_unmap was wrong.
> You explain memcg can use may_swap instead may_unmap. and I think
> other may_unmap user (zone_reclaim and shrink_all_list) can convert
> may_unmap code to may_swap.
> 
> IOW, Nishimura-san, you explain we can remove the branch of the may_unmap
> from shrink_page_list().
> it's really good job. thanks!
> 
> 
> ========
> Subject: vmswan: reintroduce sc->may_swap
> 
> vmscan-rename-scmay_swap-to-may_unmap.patch removed may_swap flag,
> but memcg had used it as a flag for "we need to use swap?", as the
> name indicate.
> 
> And in current implementation, memcg cannot reclaim mapped file caches
> when mem+swap hits the limit.
> 
> re-introduce may_swap flag and handle it at get_scan_ratio().
> This patch doesn't influence any scan_control users other than memcg.
> 
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> Signed-off-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
> --
>  mm/vmscan.c |   12 ++++++++++--
>  1 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 3be6157..00ea4a1 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -63,6 +63,9 @@ struct scan_control {
>  	/* Can mapped pages be reclaimed? */
>  	int may_unmap;
>  
> +	/* Can pages be swapped as part of reclaim? */
> +	int may_swap;
> +
>  	/* This context's SWAP_CLUSTER_MAX. If freeing memory for
>  	 * suspend, we effectively ignore SWAP_CLUSTER_MAX.
>  	 * In this context, it doesn't matter that we scan the
> @@ -1379,7 +1382,7 @@ static void get_scan_ratio(struct zone *zone, struct scan_control *sc,
>  	struct zone_reclaim_stat *reclaim_stat = get_reclaim_stat(zone, sc);
>  
>  	/* If we have no swap space, do not bother scanning anon pages. */
> -	if (nr_swap_pages <= 0) {
> +	if (!sc->may_swap || (nr_swap_pages <= 0)) {
>  		percent[0] = 0;
>  		percent[1] = 100;
>  		return;
> @@ -1695,6 +1698,7 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
>  		.may_writepage = !laptop_mode,
>  		.swap_cluster_max = SWAP_CLUSTER_MAX,
>  		.may_unmap = 1,
> +		.may_swap = 1,
>  		.swappiness = vm_swappiness,
>  		.order = order,
>  		.mem_cgroup = NULL,
> @@ -1714,6 +1718,7 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem_cont,
>  	struct scan_control sc = {
>  		.may_writepage = !laptop_mode,
>  		.may_unmap = 1,
> +		.may_swap = 1,
>  		.swap_cluster_max = SWAP_CLUSTER_MAX,
>  		.swappiness = swappiness,
>  		.order = 0,
> @@ -1723,7 +1728,7 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem_cont,
>  	struct zonelist *zonelist;
>  
>  	if (noswap)
> -		sc.may_unmap = 0;
> +		sc.may_swap = 0;
>  
>  	sc.gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
>  			(GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK);
> @@ -1763,6 +1768,7 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order)
>  	struct scan_control sc = {
>  		.gfp_mask = GFP_KERNEL,
>  		.may_unmap = 1,
> +		.may_swap = 1,
>  		.swap_cluster_max = SWAP_CLUSTER_MAX,
>  		.swappiness = vm_swappiness,
>  		.order = order,
> @@ -2109,6 +2115,7 @@ unsigned long shrink_all_memory(unsigned long nr_pages)
>  	struct scan_control sc = {
>  		.gfp_mask = GFP_KERNEL,
>  		.may_unmap = 0,
> +		.may_swap = 1,
>  		.swap_cluster_max = nr_pages,
>  		.may_writepage = 1,
>  		.isolate_pages = isolate_pages_global,
> @@ -2289,6 +2296,7 @@ static int __zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
>  	struct scan_control sc = {
>  		.may_writepage = !!(zone_reclaim_mode & RECLAIM_WRITE),
>  		.may_unmap = !!(zone_reclaim_mode & RECLAIM_SWAP),
> +		.may_swap = 1,
>  		.swap_cluster_max = max_t(unsigned long, nr_pages,
>  					SWAP_CLUSTER_MAX),
>  		.gfp_mask = gfp_mask,
> 
> 
> 
> 
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