[RFC][PATCH] fix swap entries is not reclaimed in proper way formemg v3.

[KAMEZAWA Hiroyuki]

This is new one. (using new logic.) Maybe enough light-weight and caches all cases.

Thanks,
-Kame
==
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>

Because free_swap_and_cache() function is called under spinlocks,
it can't sleep and use trylock_page() instead of lock_page().
By this, swp_entry which is not used after zap_xx can exists as
SwapCache, which will be never used.
This kind of SwapCache is reclaimed by global LRU when it's found
at LRU rotation.

When memory cgroup is used,  the global LRU will not be kicked and
stale Swap Caches will not be reclaimed. This is problematic because
memcg's swap entry accounting is leaked and memcg can't know it.
To catch this stale SwapCache, we have to chase it and check the
swap is alive or not again.

This patch adds a function to chase stale swap cache and reclaim it
in modelate way. When zap_xxx fails to remove swap ent, it will be
recoreded into buffer and memcg's "work" will reclaim it later.
No sleep, no memory allocation under free_swap_and_cache().

This patch also adds stale-swap-cache-congestion logic and try to avoid having
too much stale swap caches at the same time.

Implementation is naive but maybe the cost meets trade-off.

How to test:
  1. set limit of memory to very small (1-2M?). 
  2. run some amount of program and run page reclaim/swap-in.
  3. kill programs by SIGKILL etc....then, Stale Swap Cache will
     be increased. After this patch, stale swap caches are reclaimed
     and mem+swap controller will not go to OOM.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
 include/linux/swap.h |   16 ++++++
 mm/memcontrol.c      |  128 +++++++++++++++++++++++++++++++++++++++++++++++++++
 mm/swap_state.c      |   11 +++-
 mm/swapfile.c        |   11 ++++
 mm/vmscan.c          |    3 +
 5 files changed, 168 insertions(+), 1 deletion(-)

Index: mmotm-2.6.30-Apr21/include/linux/swap.h
===================================================================
--- mmotm-2.6.30-Apr21.orig/include/linux/swap.h
+++ mmotm-2.6.30-Apr21/include/linux/swap.h
@@ -336,11 +336,27 @@ static inline void disable_swap_token(vo
 
 #ifdef CONFIG_CGROUP_MEM_RES_CTLR
 extern void mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent);
+extern void memcg_mark_swapent_stale(swp_entry_t ent);
+extern void memcg_sanity_check_swapin(struct page *page, swp_entry_t ent);
+extern int memcg_stale_swap_congestion(void);
 #else
 static inline void
 mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent)
 {
 }
+
+static void memcg_mark_swapent_stale(swp_entry_t ent)
+{
+}
+
+static void memcg_sanity_check_swapin(struct page *page, swp_entry_t ent)
+{
+}
+
+static inline int memcg_stale_swap_congestion(void)
+{
+	return 0;
+}
 #endif
 #ifdef CONFIG_CGROUP_MEM_RES_CTLR_SWAP
 extern void mem_cgroup_uncharge_swap(swp_entry_t ent);
Index: mmotm-2.6.30-Apr21/mm/memcontrol.c
===================================================================
--- mmotm-2.6.30-Apr21.orig/mm/memcontrol.c
+++ mmotm-2.6.30-Apr21/mm/memcontrol.c
@@ -1702,6 +1702,131 @@ int mem_cgroup_shmem_charge_fallback(str
 	return ret;
 }
 
+#ifdef CONFIG_SWAP
+/*
+ * Stale Swap Cache Handler.
+ * Stale Swap Cache is a Swap Cache which will never be used. In general,
+ * Swap Cache is zapped by free_swap_and_cache() (via zap_pte_range() etc.).
+ * But in racy case, free_swap_and_cache() doesn't free swap entries and
+ * it's expected that Swap Cache will be freed by global LRU rotation.
+ *
+ * But if memory cgroup is used, global lru rotation may not happen and
+ * Stale Swap Cache (and unused swap entry) will never be reclaimed. In bad
+ * case, this can cause OOM (under memcg) and other problems.
+ *
+ * Following is GC code for stale swap caches.
+ */
+
+#define STALE_ENTS (512)
+#define STALE_ENTS_MAP (STALE_ENTS/BITS_PER_LONG)
+
+static struct stale_swap_control {
+	spinlock_t lock;
+	int num;
+	int congestion;
+	unsigned long usemap[STALE_ENTS_MAP];
+	swp_entry_t ents[STALE_ENTS];
+	struct delayed_work gc_work;
+} ssc;
+
+static void memcg_fixup_stale_swapcache(struct work_struct *work)
+{
+	int pos = 0;
+	swp_entry_t entry;
+	struct page *page;
+	int forget, ret;
+
+	while (ssc.num) {
+		spin_lock(&ssc.lock);
+		pos = find_next_bit(ssc.usemap, STALE_ENTS, pos);
+		spin_unlock(&ssc.lock);
+
+		if (pos >= STALE_ENTS)
+			break;
+
+		entry = ssc.ents[pos];
+
+		forget = 1;
+		page = lookup_swap_cache(entry);
+		if (page) {
+			lock_page(page);
+			ret = try_to_free_swap(page);
+			/* If it's still under I/O, don't forget it */
+			if (!ret && PageWriteback(page))
+				forget = 0;
+			unlock_page(page);
+		}
+		if (forget) {
+			spin_lock(&ssc.lock);
+			clear_bit(pos, ssc.usemap);
+			ssc.num--;
+			if (ssc.num < STALE_ENTS/2)
+				ssc.congestion = 0;
+			spin_unlock(&ssc.lock);
+		}
+		pos++;
+	}
+	if (ssc.num) /* schedule me again */
+		schedule_delayed_work(&ssc.gc_work, HZ/10);
+	return;
+}
+
+static void schedule_ssc_gc(void)
+{
+	/* 10ms margin to wait for a page unlocked */
+	schedule_delayed_work(&ssc.gc_work, HZ/10);
+}
+
+/* We found lock_page() contention at zap_page. then revisit this later */
+void memcg_mark_swapent_stale(swp_entry_t ent)
+{
+	int pos;
+
+	spin_lock(&ssc.lock);
+	WARN_ON(ssc.num >= STALE_ENTS);
+	if (ssc.num < STALE_ENTS) {
+		pos = find_first_zero_bit(ssc.usemap, STALE_ENTS);
+		ssc.ents[pos] = ent;
+		set_bit(pos, ssc.usemap);
+		ssc.num++;
+		if (ssc.num > STALE_ENTS/2)
+			ssc.congestion = 1;
+	}
+	spin_unlock(&ssc.lock);
+	schedule_ssc_gc();
+}
+
+/* If too many stale swap caches, avoid too much swap I/O */
+int memcg_stale_swap_congestion(void)
+{
+	smp_mb();
+	if (ssc.congestion) {
+		schedule_ssc_gc();
+		return 1;
+	}
+	return 0;
+}
+
+static void setup_stale_swapcache_control(void)
+{
+	memset(&ssc, 0, sizeof(ssc));
+	spin_lock_init(&ssc.lock);
+	INIT_DELAYED_WORK(&ssc.gc_work, memcg_fixup_stale_swapcache);
+}
+
+#else
+
+int memcg_stale_swap_congestion(void)
+{
+	return 0;
+}
+
+static void setup_stale_swapcache_control(void)
+{
+}
+
+#endif /* CONFIG_SWAP */
+
 static DEFINE_MUTEX(set_limit_mutex);
 
 static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
@@ -2464,6 +2589,7 @@ static struct mem_cgroup *parent_mem_cgr
 	return mem_cgroup_from_res_counter(mem->res.parent, res);
 }
 
+
 #ifdef CONFIG_CGROUP_MEM_RES_CTLR_SWAP
 static void __init enable_swap_cgroup(void)
 {
@@ -2493,6 +2619,7 @@ mem_cgroup_create(struct cgroup_subsys *
 	/* root ? */
 	if (cont->parent == NULL) {
 		enable_swap_cgroup();
+		setup_stale_swapcache_control();
 		parent = NULL;
 	} else {
 		parent = mem_cgroup_from_cont(cont->parent);
@@ -2588,3 +2715,4 @@ static int __init disable_swap_account(c
 }
 __setup("noswapaccount", disable_swap_account);
 #endif
+
Index: mmotm-2.6.30-Apr21/mm/swap_state.c
===================================================================
--- mmotm-2.6.30-Apr21.orig/mm/swap_state.c
+++ mmotm-2.6.30-Apr21/mm/swap_state.c
@@ -313,6 +313,7 @@ struct page *read_swap_cache_async(swp_e
 			/*
 			 * Initiate read into locked page and return.
 			 */
+			memcg_sanity_check_swapin(new_page, entry);
 			lru_cache_add_anon(new_page);
 			swap_readpage(NULL, new_page);
 			return new_page;
@@ -360,8 +361,16 @@ struct page *swapin_readahead(swp_entry_
 	 * No, it's very unlikely that swap layout would follow vma layout,
 	 * more likely that neighbouring swap pages came from the same node:
 	 * so use the same "addr" to choose the same node for each swap read.
+	 *
+	 * If memory cgroup is used, Stale Swap Cache congestion check is
+	 * done and no readahed if there are too much stale swap caches.
 	 */
-	nr_pages = valid_swaphandles(entry, &offset);
+	if (memcg_stale_swap_congestion()) {
+		offset = swp_offset(entry);
+		nr_pages = 1;
+	} else
+		nr_pages = valid_swaphandles(entry, &offset);
+
 	for (end_offset = offset + nr_pages; offset < end_offset; offset++) {
 		/* Ok, do the async read-ahead now */
 		page = read_swap_cache_async(swp_entry(swp_type(entry), offset),
Index: mmotm-2.6.30-Apr21/mm/swapfile.c
===================================================================
--- mmotm-2.6.30-Apr21.orig/mm/swapfile.c
+++ mmotm-2.6.30-Apr21/mm/swapfile.c
@@ -570,6 +570,16 @@ int try_to_free_swap(struct page *page)
 	return 1;
 }
 
+#ifdef CONFIG_CGROUP_MEM_RES_CTLR
+void memcg_sanity_check_swapin(struct page *page, swp_entry_t entry)
+{
+	VM_BUG_ON(!PageSwapCache(page));
+	VM_BUG_ON(!PageLocked(page));
+	/* This page is Locked */
+	if (!page_swapcount(page))
+		memcg_mark_swapent_stale(entry);
+}
+#endif
 /*
  * Free the swap entry like above, but also try to
  * free the page cache entry if it is the last user.
@@ -589,6 +599,7 @@ int free_swap_and_cache(swp_entry_t entr
 			if (page && !trylock_page(page)) {
 				page_cache_release(page);
 				page = NULL;
+				memcg_mark_swapent_stale(entry);
 			}
 		}
 		spin_unlock(&swap_lock);
Index: mmotm-2.6.30-Apr21/mm/vmscan.c
===================================================================
--- mmotm-2.6.30-Apr21.orig/mm/vmscan.c
+++ mmotm-2.6.30-Apr21/mm/vmscan.c
@@ -661,6 +661,9 @@ static unsigned long shrink_page_list(st
 		if (PageAnon(page) && !PageSwapCache(page)) {
 			if (!(sc->gfp_mask & __GFP_IO))
 				goto keep_locked;
+			/* avoid making more stale swap caches */
+			if (memcg_stale_swap_congestion())
+				goto keep_locked;
 			if (!add_to_swap(page))
 				goto activate_locked;
 			may_enter_fs = 1;

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