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

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.

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>
---
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,

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