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Subject[PATCH 2/4] memcg: simplify corner case handling of LRU.
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

This patch simplifies LRU handling of racy case (memcg+SwapCache).
At charging, SwapCache tend to be on LRU already. So, before
overwriting pc->mem_cgroup, the page must be removed from LRU and
added to LRU later.

This patch does
spin_lock(zone->lru_lock);
if (PageLRU(page))
remove from LRU
overwrite pc->mem_cgroup
if (PageLRU(page))
add to new LRU.
spin_unlock(zone->lru_lock);

And guarantee all pages are not on LRU at modifying pc->mem_cgroup.
This patch also unfies lru handling of replace_page_cache() and
swapin.

Changelog:
- modify PageLRU flag correctly.
- handle replace_page_cache.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
mm/memcontrol.c | 109 ++++++++-----------------------------------------------
1 files changed, 16 insertions(+), 93 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 947c62c..7a857e8 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1071,86 +1071,6 @@ struct lruvec *mem_cgroup_lru_move_lists(struct zone *zone,
}

/*
- * At handling SwapCache and other FUSE stuff, pc->mem_cgroup may be changed
- * while it's linked to lru because the page may be reused after it's fully
- * uncharged. To handle that, unlink page_cgroup from LRU when charge it again.
- * It's done under lock_page and expected that zone->lru_lock isnever held.
- */
-static void mem_cgroup_lru_del_before_commit(struct page *page)
-{
- enum lru_list lru;
- unsigned long flags;
- struct zone *zone = page_zone(page);
- struct page_cgroup *pc = lookup_page_cgroup(page);
-
- /*
- * Doing this check without taking ->lru_lock seems wrong but this
- * is safe. Because if page_cgroup's USED bit is unset, the page
- * will not be added to any memcg's LRU. If page_cgroup's USED bit is
- * set, the commit after this will fail, anyway.
- * This all charge/uncharge is done under some mutual execustion.
- * So, we don't need to taking care of changes in USED bit.
- */
- if (likely(!PageLRU(page)))
- return;
-
- spin_lock_irqsave(&zone->lru_lock, flags);
- lru = page_lru(page);
- /*
- * The uncharged page could still be registered to the LRU of
- * the stale pc->mem_cgroup.
- *
- * As pc->mem_cgroup is about to get overwritten, the old LRU
- * accounting needs to be taken care of. Let root_mem_cgroup
- * babysit the page until the new memcg is responsible for it.
- *
- * The PCG_USED bit is guarded by lock_page() as the page is
- * swapcache/pagecache.
- */
- if (PageLRU(page) && PageCgroupAcctLRU(pc) && !PageCgroupUsed(pc)) {
- del_page_from_lru_list(zone, page, lru);
- add_page_to_lru_list(zone, page, lru);
- }
- spin_unlock_irqrestore(&zone->lru_lock, flags);
-}
-
-static void mem_cgroup_lru_add_after_commit(struct page *page)
-{
- enum lru_list lru;
- unsigned long flags;
- struct zone *zone = page_zone(page);
- struct page_cgroup *pc = lookup_page_cgroup(page);
- /*
- * putback: charge:
- * SetPageLRU SetPageCgroupUsed
- * smp_mb smp_mb
- * PageCgroupUsed && add to memcg LRU PageLRU && add to memcg LRU
- *
- * Ensure that one of the two sides adds the page to the memcg
- * LRU during a race.
- */
- smp_mb();
- /* taking care of that the page is added to LRU while we commit it */
- if (likely(!PageLRU(page)))
- return;
- spin_lock_irqsave(&zone->lru_lock, flags);
- lru = page_lru(page);
- /*
- * If the page is not on the LRU, someone will soon put it
- * there. If it is, and also already accounted for on the
- * memcg-side, it must be on the right lruvec as setting
- * pc->mem_cgroup and PageCgroupUsed is properly ordered.
- * Otherwise, root_mem_cgroup has been babysitting the page
- * during the charge. Move it to the new memcg now.
- */
- if (PageLRU(page) && !PageCgroupAcctLRU(pc)) {
- del_page_from_lru_list(zone, page, lru);
- add_page_to_lru_list(zone, page, lru);
- }
- spin_unlock_irqrestore(&zone->lru_lock, flags);
-}
-
-/*
* Checks whether given mem is same or in the root_mem_cgroup's
* hierarchy subtree
*/
@@ -2695,14 +2615,27 @@ __mem_cgroup_commit_charge_lrucare(struct page *page, struct mem_cgroup *memcg,
enum charge_type ctype)
{
struct page_cgroup *pc = lookup_page_cgroup(page);
+ struct zone *zone = page_zone(page);
+ unsigned long flags;
+ bool removed = false;
+
/*
* In some case, SwapCache, FUSE(splice_buf->radixtree), the page
* is already on LRU. It means the page may on some other page_cgroup's
* LRU. Take care of it.
*/
- mem_cgroup_lru_del_before_commit(page);
+ spin_lock_irqsave(&zone->lru_lock, flags);
+ if (PageLRU(page)) {
+ del_page_from_lru_list(zone, page, page_lru(page));
+ ClearPageLRU(page);
+ removed = true;
+ }
__mem_cgroup_commit_charge(memcg, page, 1, pc, ctype);
- mem_cgroup_lru_add_after_commit(page);
+ if (removed) {
+ add_page_to_lru_list(zone, page, page_lru(page));
+ SetPageLRU(page);
+ }
+ spin_unlock_irqrestore(&zone->lru_lock, flags);
return;
}

@@ -3303,9 +3236,7 @@ void mem_cgroup_replace_page_cache(struct page *oldpage,
{
struct mem_cgroup *memcg;
struct page_cgroup *pc;
- struct zone *zone;
enum charge_type type = MEM_CGROUP_CHARGE_TYPE_CACHE;
- unsigned long flags;

pc = lookup_page_cgroup(oldpage);
/* fix accounting on old pages */
@@ -3318,20 +3249,12 @@ void mem_cgroup_replace_page_cache(struct page *oldpage,
if (PageSwapBacked(oldpage))
type = MEM_CGROUP_CHARGE_TYPE_SHMEM;

- zone = page_zone(newpage);
- pc = lookup_page_cgroup(newpage);
/*
* Even if newpage->mapping was NULL before starting replacement,
* the newpage may be on LRU(or pagevec for LRU) already. We lock
* LRU while we overwrite pc->mem_cgroup.
*/
- spin_lock_irqsave(&zone->lru_lock, flags);
- if (PageLRU(newpage))
- del_page_from_lru_list(zone, newpage, page_lru(newpage));
- __mem_cgroup_commit_charge(memcg, newpage, 1, pc, type);
- if (PageLRU(newpage))
- add_page_to_lru_list(zone, newpage, page_lru(newpage));
- spin_unlock_irqrestore(&zone->lru_lock, flags);
+ __mem_cgroup_commit_charge_lrucare(newpage, memcg, type);
}

#ifdef CONFIG_DEBUG_VM
--
1.7.4.1



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