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Subject[RFC][PATCH 7/8] mm/vmscan: never demote for memcg reclaim
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From: Dave Hansen <dave.hansen@linux.intel.com>

Global reclaim aims to reduce the amount of memory used on
a given node or set of nodes. Migrating pages to another
node serves this purpose.

memcg reclaim is different. Its goal is to reduce the
total memory consumption of the entire memcg, across all
nodes. Migration does not assist memcg reclaim because
it just moves page contents between nodes rather than
actually reducing memory consumption.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Suggested-by: Yang Shi <yang.shi@linux.alibaba.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
---

b/mm/vmscan.c | 61 +++++++++++++++++++++++++++++++++++++++-------------------
1 file changed, 42 insertions(+), 19 deletions(-)

diff -puN mm/vmscan.c~never-demote-for-memcg-reclaim mm/vmscan.c
--- a/mm/vmscan.c~never-demote-for-memcg-reclaim 2020-06-29 16:34:44.018312591 -0700
+++ b/mm/vmscan.c 2020-06-29 16:34:44.023312591 -0700
@@ -289,7 +289,8 @@ static bool writeback_throttling_sane(st
#endif

static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg,
- int node_id)
+ int node_id,
+ struct scan_control *sc)
{
/* Always age anon pages when we have swap */
if (memcg == NULL) {
@@ -300,8 +301,14 @@ static inline bool can_reclaim_anon_page
return true;
}

- /* Also age anon pages if we can auto-migrate them */
- if (next_demotion_node(node_id) >= 0)
+ /*
+ * Also age anon pages if we can auto-migrate them.
+ *
+ * Migrating a page does not reduce comsumption of a
+ * memcg so should not be performed when in memcg
+ * reclaim.
+ */
+ if ((sc && cgroup_reclaim(sc)) && (next_demotion_node(node_id) >= 0))
return true;

/* No way to reclaim anon pages */
@@ -319,7 +326,7 @@ unsigned long zone_reclaimable_pages(str

nr = zone_page_state_snapshot(zone, NR_ZONE_INACTIVE_FILE) +
zone_page_state_snapshot(zone, NR_ZONE_ACTIVE_FILE);
- if (can_reclaim_anon_pages(NULL, zone_to_nid(zone)))
+ if (can_reclaim_anon_pages(NULL, zone_to_nid(zone), NULL))
nr += zone_page_state_snapshot(zone, NR_ZONE_INACTIVE_ANON) +
zone_page_state_snapshot(zone, NR_ZONE_ACTIVE_ANON);

@@ -1084,6 +1091,32 @@ static void page_check_dirty_writeback(s
mapping->a_ops->is_dirty_writeback(page, dirty, writeback);
}

+
+static int shrink_do_demote_mapping(struct page *page,
+ struct list_head *page_list,
+ struct scan_control *sc)
+{
+ int rc;
+
+ /* It is pointless to do demotion in memcg reclaim */
+ if (cgroup_reclaim(sc))
+ return -ENOTSUPP;
+
+ rc = migrate_demote_mapping(page);
+ /*
+ * -ENOMEM on a THP may indicate either migration is
+ * unsupported or there was not enough contiguous
+ * space. Split the THP into base pages and retry the
+ * head immediately. The tail pages will be considered
+ * individually within the current loop's page list.
+ */
+ if (rc == -ENOMEM && PageTransHuge(page) &&
+ !split_huge_page_to_list(page, page_list))
+ rc = migrate_demote_mapping(page);
+
+ return rc;
+}
+
/*
* shrink_page_list() returns the number of reclaimed pages
*/
@@ -1251,17 +1284,7 @@ static unsigned long shrink_page_list(st
; /* try to reclaim the page below */
}

- rc = migrate_demote_mapping(page);
- /*
- * -ENOMEM on a THP may indicate either migration is
- * unsupported or there was not enough contiguous
- * space. Split the THP into base pages and retry the
- * head immediately. The tail pages will be considered
- * individually within the current loop's page list.
- */
- if (rc == -ENOMEM && PageTransHuge(page) &&
- !split_huge_page_to_list(page, page_list))
- rc = migrate_demote_mapping(page);
+ rc = shrink_do_demote_mapping(page, page_list, sc);

if (rc == MIGRATEPAGE_SUCCESS) {
unlock_page(page);
@@ -2287,7 +2310,7 @@ static void get_scan_count(struct lruvec
enum lru_list lru;

/* If we have no swap space, do not bother scanning anon pages. */
- if (!sc->may_swap || !can_reclaim_anon_pages(memcg, pgdat->node_id)) {
+ if (!sc->may_swap || !can_reclaim_anon_pages(memcg, pgdat->node_id, sc)) {
scan_balance = SCAN_FILE;
goto out;
}
@@ -2593,7 +2616,7 @@ static void shrink_lruvec(struct lruvec
* rebalance the anon lru active/inactive ratio.
*/
if (can_reclaim_anon_pages(lruvec_memcg(lruvec),
- lruvec_pgdat(lruvec)->node_id) &&
+ lruvec_pgdat(lruvec)->node_id, sc) &&
inactive_is_low(lruvec, LRU_INACTIVE_ANON))
shrink_active_list(SWAP_CLUSTER_MAX, lruvec,
sc, LRU_ACTIVE_ANON);
@@ -2664,7 +2687,7 @@ static inline bool should_continue_recla
*/
pages_for_compaction = compact_gap(sc->order);
inactive_lru_pages = node_page_state(pgdat, NR_INACTIVE_FILE);
- if (can_reclaim_anon_pages(NULL, pgdat->node_id))
+ if (can_reclaim_anon_pages(NULL, pgdat->node_id, sc))
inactive_lru_pages += node_page_state(pgdat, NR_INACTIVE_ANON);

return inactive_lru_pages > pages_for_compaction;
@@ -3417,7 +3440,7 @@ static void age_active_anon(struct pglis
struct mem_cgroup *memcg;
struct lruvec *lruvec;

- if (!can_reclaim_anon_pages(NULL, pgdat->node_id))
+ if (!can_reclaim_anon_pages(NULL, pgdat->node_id, sc))
return;

lruvec = mem_cgroup_lruvec(NULL, pgdat);
_
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