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

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Fri, 27 Mar 2009 15:19:26 +0900
Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

> Added
>  Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>  Cc: Balbir Singh <balbir@in.ibm.com>
> 
> 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@mxp.nes.nec.co.jp>
> 
> 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@mxp.nes.nec.co.jp>

Seems good,
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

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.

-Kame

> ---
>  mm/vmscan.c |   15 ++++++++++++++-
>  1 files changed, 14 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index c815653..86118d9 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -64,6 +64,9 @@ 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
> @@ -616,6 +619,11 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  		if (!sc->may_unmap && page_mapped(page))
>  			goto keep_locked;
>  
> +		if (!sc->may_swap && PageSwapBacked(page)
> +			/* SwapCache uses 'swap' already */
> +			&& !PageSwapCache(page))
> +			goto keep_locked;
> +
>  		/* Double the slab pressure for mapped and swapcache pages */
>  		if (page_mapped(page) || PageSwapCache(page))
>  			sc->nr_scanned++;
> @@ -1696,6 +1704,7 @@ 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,
> @@ -1715,6 +1724,7 @@ 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,
> @@ -1724,7 +1734,7 @@ 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);
> @@ -1764,6 +1774,7 @@ 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,
> @@ -2110,6 +2121,7 @@ unsigned long shrink_all_memory(unsigned long nr_pages)
>  	struct scan_control sc = {
>  		.gfp_mask = GFP_KERNEL,
>  		.may_unmap = 0,
> +		.may_swap = 1,
>  		.may_writepage = 1,
>  		.isolate_pages = isolate_pages_global,
>  	};
> @@ -2292,6 +2304,7 @@ 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,
>