[PATCH] temporarily disable swap token on memory pressure

Some users (hi Zwane) have seen a problem when running a workload that
eats nearly all of physical memory - th system does an OOM kill, even
when there is still a lot of swap free.

The problem appears to be a very big task that is holding the swap
token, and the VM has a very hard time finding any other page in the
system that is swappable.

Instead of ignoring the swap token when sc->priority reaches 0, we could
simply take the swap token away from the memory hog and make sure we
don't give it back to the memory hog for a few seconds.

This patch resolves the problem Zwane ran into.

Signed-off-by: Rik van Riel <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/rmap.h

···
 /*
  * Called from mm/vmscan.c to handle paging out
  */
-int page_referenced(struct page *, int is_locked, int ignore_token);
+int page_referenced(struct page *, int is_locked);
 int try_to_unmap(struct page *);
 
 /*
···
 #define anon_vma_prepare(vma)	(0)
 #define anon_vma_link(vma)	do {} while (0)
 
-#define page_referenced(page,l,i) TestClearPageReferenced(page)
+#define page_referenced(page,l) TestClearPageReferenced(page)
 #define try_to_unmap(page)	SWAP_FAIL
 
 #endif	/* CONFIG_MMU */

include/linux/swap.h

···
 		__put_swap_token(mm);
 }
 
+static inline void disable_swap_token(void)
+{
+	put_swap_token(swap_token_mm);
+}
+
 #else /* CONFIG_SWAP */
 
 #define total_swap_pages			0
···
 #define put_swap_token(x) do { } while(0)
 #define grab_swap_token()  do { } while(0)
 #define has_swap_token(x) 0
+#define disable_swap_token() do { } while(0)
 
 #endif /* CONFIG_SWAP */
 #endif /* __KERNEL__*/

mm/rmap.c

···
  * repeatedly from either page_referenced_anon or page_referenced_file.
  */
 static int page_referenced_one(struct page *page,
-	struct vm_area_struct *vma, unsigned int *mapcount, int ignore_token)
+	struct vm_area_struct *vma, unsigned int *mapcount)
 {
 	struct mm_struct *mm = vma->vm_mm;
 	unsigned long address;
···
 
 	/* Pretend the page is referenced if the task has the
 	   swap token and is in the middle of a page fault. */
-	if (mm != current->mm && !ignore_token && has_swap_token(mm) &&
+	if (mm != current->mm && has_swap_token(mm) &&
 			rwsem_is_locked(&mm->mmap_sem))
 		referenced++;
 
···
 	return referenced;
 }
 
-static int page_referenced_anon(struct page *page, int ignore_token)
+static int page_referenced_anon(struct page *page)
 {
 	unsigned int mapcount;
 	struct anon_vma *anon_vma;
···
 
 	mapcount = page_mapcount(page);
 	list_for_each_entry(vma, &anon_vma->head, anon_vma_node) {
-		referenced += page_referenced_one(page, vma, &mapcount,
-							ignore_token);
+		referenced += page_referenced_one(page, vma, &mapcount);
 		if (!mapcount)
 			break;
 	}
···
  *
  * This function is only called from page_referenced for object-based pages.
  */
-static int page_referenced_file(struct page *page, int ignore_token)
+static int page_referenced_file(struct page *page)
 {
 	unsigned int mapcount;
 	struct address_space *mapping = page->mapping;
···
 			referenced++;
 			break;
 		}
-		referenced += page_referenced_one(page, vma, &mapcount,
-							ignore_token);
+		referenced += page_referenced_one(page, vma, &mapcount);
 		if (!mapcount)
 			break;
 	}
···
  * Quick test_and_clear_referenced for all mappings to a page,
  * returns the number of ptes which referenced the page.
  */
-int page_referenced(struct page *page, int is_locked, int ignore_token)
+int page_referenced(struct page *page, int is_locked)
 {
 	int referenced = 0;
-
-	if (!swap_token_default_timeout)
-		ignore_token = 1;
 
 	if (page_test_and_clear_young(page))
 		referenced++;
···
 
 	if (page_mapped(page) && page->mapping) {
 		if (PageAnon(page))
-			referenced += page_referenced_anon(page, ignore_token);
+			referenced += page_referenced_anon(page);
 		else if (is_locked)
-			referenced += page_referenced_file(page, ignore_token);
+			referenced += page_referenced_file(page);
 		else if (TestSetPageLocked(page))
 			referenced++;
 		else {
 			if (page->mapping)
-				referenced += page_referenced_file(page,
-								ignore_token);
+				referenced += page_referenced_file(page);
 			unlock_page(page);
 		}
 	}

mm/thrash.c

···
 	/* We have the token. Let others know we still need it. */
 	if (has_swap_token(current->mm)) {
 		current->mm->recent_pagein = 1;
+		if (unlikely(!swap_token_default_timeout))
+			disable_swap_token();
 		return;
 	}
 
 	if (time_after(jiffies, swap_token_check)) {
 
-		/* Can't get swapout protection if we exceed our RSS limit. */
-		// if (current->mm->rss > current->mm->rlimit_rss)
-		//	return;
+		if (!swap_token_default_timeout) {
+			swap_token_check = jiffies + SWAP_TOKEN_CHECK_INTERVAL;
+			return;
+		}
 
 		/* ... or if we recently held the token. */
 		if (time_before(jiffies, current->mm->swap_token_time))
···
 {
 	spin_lock(&swap_token_lock);
 	if (likely(mm == swap_token_mm)) {
+		mm->swap_token_time = jiffies + SWAP_TOKEN_CHECK_INTERVAL;
 		swap_token_mm = &init_mm;
 		swap_token_check = jiffies;
 	}

mm/vmscan.c

···
 		if (PageWriteback(page))
 			goto keep_locked;
 
-		referenced = page_referenced(page, 1, sc->priority <= 0);
+		referenced = page_referenced(page, 1);
 		/* In active use or really unfreeable?  Activate it. */
 		if (referenced && page_mapping_inuse(page))
 			goto activate_locked;
···
 		if (page_mapped(page)) {
 			if (!reclaim_mapped ||
 			    (total_swap_pages == 0 && PageAnon(page)) ||
-			    page_referenced(page, 0, sc->priority <= 0)) {
+			    page_referenced(page, 0)) {
 				list_add(&page->lru, &l_active);
 				continue;
 			}
···
 		sc.nr_reclaimed = 0;
 		sc.priority = priority;
 		sc.swap_cluster_max = SWAP_CLUSTER_MAX;
+		if (!priority)
+			disable_swap_token();
 		shrink_caches(zones, &sc);
 		shrink_slab(sc.nr_scanned, gfp_mask, lru_pages);
 		if (reclaim_state) {
···
 	for (priority = DEF_PRIORITY; priority >= 0; priority--) {
 		int end_zone = 0;	/* Inclusive.  0 = ZONE_DMA */
 		unsigned long lru_pages = 0;
+
+		/* The swap token gets in the way of swapout... */
+		if (!priority)
+			disable_swap_token();
 
 		all_zones_ok = 1;
 
···
 	sc.nr_reclaimed = 0;
 	/* scan at the highest priority */
 	sc.priority = 0;
+	disable_swap_token();
 
 	if (nr_pages > SWAP_CLUSTER_MAX)
 		sc.swap_cluster_max = nr_pages;