staging/zcache: Fix/improve zcache writeback code, tie to a config option

It was observed by Andrea Arcangeli in 2011 that zcache can get "full"
and there must be some way for compressed swap pages to be (uncompressed
and then) sent through to the backing swap disk.  A prototype of this
functionality, called "unuse", was added in 2012 as part of a major update
to zcache (aka "zcache2"), but was left unfinished due to the unfortunate
temporary fork of zcache.

This earlier version of the code had an unresolved memory leak
and was anyway dependent on not-yet-upstream frontswap and mm changes.
The code was meanwhile adapted by Seth Jennings for similar
functionality in zswap (which he calls "flush").  Seth also made some
clever simplifications which are herein ported back to zcache.  As a
result of those simplifications, the frontswap changes are no longer
necessary, but a slightly different (and simpler) set of mm changes are
still required [1].  The memory leak is also fixed.

Due to feedback from akpm in a zswap thread, this functionality in zcache
has now been renamed from "unuse" to "writeback".

Although this zcache writeback code now works, there are open questions
as how best to handle the policy that drives it.  As a result, this
patch also ties writeback to a new config option.  And, since the
code still depends on not-yet-upstreamed mm patches, to avoid build
problems, the config option added by this patch temporarily depends
on "BROKEN"; this config dependency can be removed in trees that
contain the necessary mm patches.

[1] https://lkml.org/lkml/2013/1/29/540/ https://lkml.org/lkml/2013/1/29/539/

Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/zcache/Kconfig

@@ -24,3 +24,20 @@ config RAMSTER
 	  while minimizing total RAM across the cluster.  RAMster, like
 	  zcache2, compresses swap pages into local RAM, but then remotifies
 	  the compressed pages to another node in the RAMster cluster.
+
+# Depends on not-yet-upstreamed mm patches to export end_swap_bio_write and
+# __add_to_swap_cache, and implement __swap_writepage (which is swap_writepage
+# without the frontswap call. When these are in-tree, the dependency on
+# BROKEN can be removed
+config ZCACHE_WRITEBACK
+	bool "Allow compressed swap pages to be writtenback to swap disk"
+	depends on ZCACHE=y && BROKEN
+	default n
+	help
+	  Zcache caches compressed swap pages (and other data) in RAM which
+	  often improves performance by avoiding I/O's due to swapping.
+	  In some workloads with very long-lived large processes, it can
+	  instead reduce performance.  Writeback decompresses zcache-compressed
+	  pages (in LRU order) when under memory pressure and writes them to
+	  the backing swap disk to ameliorate this problem.  Policy driving
+	  writeback is still under development.

drivers/staging/zcache/zcache-main.c

@@ -22,6 +22,10 @@
 #include <linux/atomic.h>
 #include <linux/math64.h>
 #include <linux/crypto.h>
+#include <linux/swap.h>
+#include <linux/swapops.h>
+#include <linux/pagemap.h>
+#include <linux/writeback.h>
 
 #include <linux/cleancache.h>
 #include <linux/frontswap.h>
@@ -55,6 +59,9 @@ static inline void frontswap_tmem_exclusive_gets(bool b)
 }
 #endif
 
+/* enable (or fix code) when Seth's patches are accepted upstream */
+#define zcache_writeback_enabled 0
+
 static int zcache_enabled __read_mostly;
 static int disable_cleancache __read_mostly;
 static int disable_frontswap __read_mostly;
@@ -181,6 +188,8 @@ static unsigned long zcache_last_active_anon_pageframes;
 static unsigned long zcache_last_inactive_anon_pageframes;
 static unsigned long zcache_eph_nonactive_puts_ignored;
 static unsigned long zcache_pers_nonactive_puts_ignored;
+static unsigned long zcache_writtenback_pages;
+static long zcache_outstanding_writeback_pages;
 
 #ifdef CONFIG_DEBUG_FS
 #include <linux/debugfs.h>
@@ -239,6 +248,9 @@ static int zcache_debugfs_init(void)
 	zdfs64("eph_zbytes_max", S_IRUGO, root, &zcache_eph_zbytes_max);
 	zdfs64("pers_zbytes", S_IRUGO, root, &zcache_pers_zbytes);
 	zdfs64("pers_zbytes_max", S_IRUGO, root, &zcache_pers_zbytes_max);
+	zdfs("outstanding_writeback_pages", S_IRUGO, root,
+				&zcache_outstanding_writeback_pages);
+	zdfs("writtenback_pages", S_IRUGO, root, &zcache_writtenback_pages);
 	return 0;
 }
 #undef	zdebugfs
@@ -285,6 +297,18 @@ void zcache_dump(void)
 	pr_info("zcache: eph_zpages_max=%lu\n", zcache_eph_zpages_max);
 	pr_info("zcache: pers_zpages=%lu\n", zcache_pers_zpages);
 	pr_info("zcache: pers_zpages_max=%lu\n", zcache_pers_zpages_max);
+	pr_info("zcache: last_active_file_pageframes=%lu\n",
+				zcache_last_active_file_pageframes);
+	pr_info("zcache: last_inactive_file_pageframes=%lu\n",
+				zcache_last_inactive_file_pageframes);
+	pr_info("zcache: last_active_anon_pageframes=%lu\n",
+				zcache_last_active_anon_pageframes);
+	pr_info("zcache: last_inactive_anon_pageframes=%lu\n",
+				zcache_last_inactive_anon_pageframes);
+	pr_info("zcache: eph_nonactive_puts_ignored=%lu\n",
+				zcache_eph_nonactive_puts_ignored);
+	pr_info("zcache: pers_nonactive_puts_ignored=%lu\n",
+				zcache_pers_nonactive_puts_ignored);
 	pr_info("zcache: eph_zbytes=%llu\n",
 				(unsigned long long)zcache_eph_zbytes);
 	pr_info("zcache: eph_zbytes_max=%llu\n",
@@ -292,7 +316,10 @@ void zcache_dump(void)
 	pr_info("zcache: pers_zbytes=%llu\n",
 				(unsigned long long)zcache_pers_zbytes);
 	pr_info("zcache: pers_zbytes_max=%llu\n",
-			(unsigned long long)zcache_pers_zbytes_max);
+				(unsigned long long)zcache_pers_zbytes_max);
+	pr_info("zcache: outstanding_writeback_pages=%lu\n",
+				zcache_outstanding_writeback_pages);
+	pr_info("zcache: writtenback_pages=%lu\n", zcache_writtenback_pages);
 }
 #endif
 
@@ -449,14 +476,6 @@ static struct page *zcache_alloc_page(void)
 	return page;
 }
 
-#ifdef FRONTSWAP_HAS_UNUSE
-static void zcache_unacct_page(void)
-{
-	zcache_pageframes_freed =
-		atomic_inc_return(&zcache_pageframes_freed_atomic);
-}
-#endif
-
 static void zcache_free_page(struct page *page)
 {
 	long curr_pageframes;
@@ -959,7 +978,7 @@ static struct page *zcache_evict_eph_pageframe(void)
 					&zcache_eph_zbytes_atomic);
 	zcache_eph_zpages = atomic_sub_return(zpages,
 					&zcache_eph_zpages_atomic);
-	zcache_evicted_eph_zpages++;
+	zcache_evicted_eph_zpages += zpages;
 	zcache_eph_pageframes =
 		atomic_dec_return(&zcache_eph_pageframes_atomic);
 	zcache_evicted_eph_pageframes++;
@@ -967,77 +986,253 @@ out:
 	return page;
 }
 
-#ifdef FRONTSWAP_HAS_UNUSE
+#ifdef CONFIG_ZCACHE_WRITEBACK
+
+static atomic_t zcache_outstanding_writeback_pages_atomic = ATOMIC_INIT(0);
+
 static void unswiz(struct tmem_oid oid, u32 index,
 				unsigned *type, pgoff_t *offset);
 
 /*
- *  Choose an LRU persistent pageframe and attempt to "unuse" it by
- *  calling frontswap_unuse on both zpages.
+ *  Choose an LRU persistent pageframe and attempt to write it back to
+ *  the backing swap disk by calling frontswap_writeback on both zpages.
  *
  *  This is work-in-progress.
  */
 
-static int zcache_frontswap_unuse(void)
+static void zcache_end_swap_write(struct bio *bio, int err)
+{
+	end_swap_bio_write(bio, err);
+	zcache_outstanding_writeback_pages =
+	  atomic_dec_return(&zcache_outstanding_writeback_pages_atomic);
+	zcache_writtenback_pages++;
+}
+
+/*
+ * zcache_get_swap_cache_page
+ *
+ * This is an adaption of read_swap_cache_async()
+ *
+ * If success, page is returned in retpage
+ * Returns 0 if page was already in the swap cache, page is not locked
+ * Returns 1 if the new page needs to be populated, page is locked
+ */
+static int zcache_get_swap_cache_page(int type, pgoff_t offset,
+				struct page *new_page)
+{
+	struct page *found_page;
+	swp_entry_t entry = swp_entry(type, offset);
+	int err;
+
+	BUG_ON(new_page == NULL);
+	do {
+		/*
+		 * First check the swap cache.  Since this is normally
+		 * called after lookup_swap_cache() failed, re-calling
+		 * that would confuse statistics.
+		 */
+		found_page = find_get_page(&swapper_space, entry.val);
+		if (found_page)
+			return 0;
+
+		/*
+		 * call radix_tree_preload() while we can wait.
+		 */
+		err = radix_tree_preload(GFP_KERNEL);
+		if (err)
+			break;
+
+		/*
+		 * Swap entry may have been freed since our caller observed it.
+		 */
+		err = swapcache_prepare(entry);
+		if (err == -EEXIST) { /* seems racy */
+			radix_tree_preload_end();
+			continue;
+		}
+		if (err) { /* swp entry is obsolete ? */
+			radix_tree_preload_end();
+			break;
+		}
+
+		/* May fail (-ENOMEM) if radix-tree node allocation failed. */
+		__set_page_locked(new_page);
+		SetPageSwapBacked(new_page);
+		err = __add_to_swap_cache(new_page, entry);
+		if (likely(!err)) {
+			radix_tree_preload_end();
+			lru_cache_add_anon(new_page);
+			return 1;
+		}
+		radix_tree_preload_end();
+		ClearPageSwapBacked(new_page);
+		__clear_page_locked(new_page);
+		/*
+		 * add_to_swap_cache() doesn't return -EEXIST, so we can safely
+		 * clear SWAP_HAS_CACHE flag.
+		 */
+		swapcache_free(entry, NULL);
+		/* FIXME: is it possible to get here without err==-ENOMEM?
+		 * If not, we can dispense with the do loop, use goto retry */
+	} while (err != -ENOMEM);
+
+	return -ENOMEM;
+}
+
+/*
+ * Given a frontswap zpage in zcache (identified by type/offset) and
+ * an empty page, put the page into the swap cache, use frontswap
+ * to get the page from zcache into the empty page, then give it
+ * to the swap subsystem to send to disk (carefully avoiding the
+ * possibility that frontswap might snatch it back).
+ * Returns < 0 if error, 0 if successful, and 1 if successful but
+ * the newpage passed in not needed and should be freed.
+ */
+static int zcache_frontswap_writeback_zpage(int type, pgoff_t offset,
+					struct page *newpage)
+{
+	struct page *page = newpage;
+	int ret;
+	struct writeback_control wbc = {
+		.sync_mode = WB_SYNC_NONE,
+	};
+
+	ret = zcache_get_swap_cache_page(type, offset, page);
+	if (ret < 0)
+		return ret;
+	else if (ret == 0) {
+		/* more uptodate page is already in swapcache */
+		__frontswap_invalidate_page(type, offset);
+		return 1;
+	}
+
+	BUG_ON(!frontswap_has_exclusive_gets); /* load must also invalidate */
+	/* FIXME: how is it possible to get here when page is unlocked? */
+	__frontswap_load(page);
+	SetPageUptodate(page);  /* above does SetPageDirty, is that enough? */
+
+	/* start writeback */
+	SetPageReclaim(page);
+	/*
+	 * Return value is ignored here because it doesn't change anything
+	 * for us.  Page is returned unlocked.
+	 */
+	(void)__swap_writepage(page, &wbc, zcache_end_swap_write);
+	page_cache_release(page);
+	zcache_outstanding_writeback_pages =
+	    atomic_inc_return(&zcache_outstanding_writeback_pages_atomic);
+
+	return 0;
+}
+
+/*
+ * The following is still a magic number... we want to allow forward progress
+ * for writeback because it clears out needed RAM when under pressure, but
+ * we don't want to allow writeback to absorb and queue too many GFP_KERNEL
+ * pages if the swap device is very slow.
+ */
+#define ZCACHE_MAX_OUTSTANDING_WRITEBACK_PAGES 6400
+
+/*
+ * Try to allocate two free pages, first using a non-aggressive alloc,
+ * then by evicting zcache ephemeral (clean pagecache) pages, and last
+ * by aggressive GFP_KERNEL alloc.  We allow zbud to choose a pageframe
+ * consisting of 1-2 zbuds/zpages, then call the writeback_zpage helper
+ * function above for each.
+ */
+static int zcache_frontswap_writeback(void)
 {
 	struct tmem_handle th[2];
-	int ret = -ENOMEM;
-	int nzbuds, unuse_ret;
+	int ret = 0;
+	int nzbuds, writeback_ret;
 	unsigned type;
-	struct page *newpage1 = NULL, *newpage2 = NULL;
+	struct page *znewpage1 = NULL, *znewpage2 = NULL;
 	struct page *evictpage1 = NULL, *evictpage2 = NULL;
+	struct page *newpage1 = NULL, *newpage2 = NULL;
+	struct page *page1 = NULL, *page2 = NULL;
 	pgoff_t offset;
 
-	newpage1 = alloc_page(ZCACHE_GFP_MASK);
-	newpage2 = alloc_page(ZCACHE_GFP_MASK);
-	if (newpage1 == NULL)
+	znewpage1 = alloc_page(ZCACHE_GFP_MASK);
+	znewpage2 = alloc_page(ZCACHE_GFP_MASK);
+	if (znewpage1 == NULL)
 		evictpage1 = zcache_evict_eph_pageframe();
-	if (newpage2 == NULL)
+	if (znewpage2 == NULL)
 		evictpage2 = zcache_evict_eph_pageframe();
-	if (evictpage1 == NULL || evictpage2 == NULL)
+
+	if ((evictpage1 == NULL || evictpage2 == NULL) &&
+	    atomic_read(&zcache_outstanding_writeback_pages_atomic) >
+				ZCACHE_MAX_OUTSTANDING_WRITEBACK_PAGES) {
 		goto free_and_out;
-	/* ok, we have two pages pre-allocated */
+	}
+	if (znewpage1 == NULL && evictpage1 == NULL)
+		newpage1 = alloc_page(GFP_KERNEL);
+	if (znewpage2 == NULL && evictpage2 == NULL)
+		newpage2 = alloc_page(GFP_KERNEL);
+	if (newpage1 == NULL || newpage2 == NULL)
+			goto free_and_out;
+
+	/* ok, we have two pageframes pre-allocated, get a pair of zbuds */
 	nzbuds = zbud_make_zombie_lru(&th[0], NULL, NULL, false);
 	if (nzbuds == 0) {
 		ret = -ENOENT;
 		goto free_and_out;
 	}
+
+	/* process the first zbud */
 	unswiz(th[0].oid, th[0].index, &type, &offset);
-	unuse_ret = frontswap_unuse(type, offset,
-				newpage1 != NULL ? newpage1 : evictpage1,
-				ZCACHE_GFP_MASK);
-	if (unuse_ret != 0)
+	page1 = (znewpage1 != NULL) ? znewpage1 :
+			((newpage1 != NULL) ? newpage1 : evictpage1);
+	writeback_ret = zcache_frontswap_writeback_zpage(type, offset, page1);
+	if (writeback_ret < 0) {
+		ret = -ENOMEM;
 		goto free_and_out;
-	else if (evictpage1 != NULL)
-		zcache_unacct_page();
-	newpage1 = NULL;
-	evictpage1 = NULL;
-	if (nzbuds == 2) {
-		unswiz(th[1].oid, th[1].index, &type, &offset);
-		unuse_ret = frontswap_unuse(type, offset,
-				newpage2 != NULL ? newpage2 : evictpage2,
-				ZCACHE_GFP_MASK);
-		if (unuse_ret != 0)
-			goto free_and_out;
-		else if (evictpage2 != NULL)
-			zcache_unacct_page();
 	}
-	ret = 0;
-	goto out;
+	if (evictpage1 != NULL)
+		zcache_pageframes_freed =
+			atomic_inc_return(&zcache_pageframes_freed_atomic);
+	if (writeback_ret == 0) {
+		/* zcache_get_swap_cache_page will free, don't double free */
+		znewpage1 = NULL;
+		newpage1 = NULL;
+		evictpage1 = NULL;
+	}
+	if (nzbuds < 2)
+		goto free_and_out;
+
+	/* if there is a second zbud, process it */
+	unswiz(th[1].oid, th[1].index, &type, &offset);
+	page2 = (znewpage2 != NULL) ? znewpage2 :
+			((newpage2 != NULL) ? newpage2 : evictpage2);
+	writeback_ret = zcache_frontswap_writeback_zpage(type, offset, page2);
+	if (writeback_ret < 0) {
+		ret = -ENOMEM;
+		goto free_and_out;
+	}
+	if (evictpage2 != NULL)
+		zcache_pageframes_freed =
+			atomic_inc_return(&zcache_pageframes_freed_atomic);
+	if (writeback_ret == 0) {
+		znewpage2 = NULL;
+		newpage2 = NULL;
+		evictpage2 = NULL;
+	}
 
 free_and_out:
+	if (znewpage1 != NULL)
+		page_cache_release(znewpage1);
+	if (znewpage2 != NULL)
+		page_cache_release(znewpage2);
 	if (newpage1 != NULL)
-		__free_page(newpage1);
+		page_cache_release(newpage1);
 	if (newpage2 != NULL)
-		__free_page(newpage2);
+		page_cache_release(newpage2);
 	if (evictpage1 != NULL)
 		zcache_free_page(evictpage1);
 	if (evictpage2 != NULL)
 		zcache_free_page(evictpage2);
-out:
 	return ret;
 }
-#endif
+#endif /* CONFIG_ZCACHE_WRITEBACK */
 
 /*
  * When zcache is disabled ("frozen"), pools can be created and destroyed,
@@ -1051,7 +1246,10 @@ static bool zcache_freeze;
 /*
  * This zcache shrinker interface reduces the number of ephemeral pageframes
  * used by zcache to approximately the same as the total number of LRU_FILE
- * pageframes in use.
+ * pageframes in use, and now also reduces the number of persistent pageframes
+ * used by zcache to approximately the same as the total number of LRU_ANON
+ * pageframes in use.  FIXME POLICY: Probably the writeback should only occur
+ * if the eviction doesn't free enough pages.
  */
 static int shrink_zcache_memory(struct shrinker *shrink,
 				struct shrink_control *sc)
@@ -1060,11 +1258,9 @@ static int shrink_zcache_memory(struct shrinker *shrink,
 	int ret = -1;
 	int nr = sc->nr_to_scan;
 	int nr_evict = 0;
-	int nr_unuse = 0;
+	int nr_writeback = 0;
 	struct page *page;
-#ifdef FRONTSWAP_HAS_UNUSE
-	int unuse_ret;
-#endif
+	int  file_pageframes_inuse, anon_pageframes_inuse;
 
 	if (nr <= 0)
 		goto skip_evict;
@@ -1080,8 +1276,12 @@ static int shrink_zcache_memory(struct shrinker *shrink,
 		global_page_state(NR_LRU_BASE + LRU_ACTIVE_FILE);
 	zcache_last_inactive_file_pageframes =
 		global_page_state(NR_LRU_BASE + LRU_INACTIVE_FILE);
-	nr_evict = zcache_eph_pageframes - zcache_last_active_file_pageframes +
-		zcache_last_inactive_file_pageframes;
+	file_pageframes_inuse = zcache_last_active_file_pageframes +
+				zcache_last_inactive_file_pageframes;
+	if (zcache_eph_pageframes > file_pageframes_inuse)
+		nr_evict = zcache_eph_pageframes - file_pageframes_inuse;
+	else
+		nr_evict = 0;
 	while (nr_evict-- > 0) {
 		page = zcache_evict_eph_pageframe();
 		if (page == NULL)
@@ -1093,18 +1293,20 @@ static int shrink_zcache_memory(struct shrinker *shrink,
 		global_page_state(NR_LRU_BASE + LRU_ACTIVE_ANON);
 	zcache_last_inactive_anon_pageframes =
 		global_page_state(NR_LRU_BASE + LRU_INACTIVE_ANON);
-	nr_unuse = zcache_pers_pageframes - zcache_last_active_anon_pageframes +
-		zcache_last_inactive_anon_pageframes;
-#ifdef FRONTSWAP_HAS_UNUSE
-	/* rate limit for testing */
-	if (nr_unuse > 32)
-		nr_unuse = 32;
-	while (nr_unuse-- > 0) {
-		unuse_ret = zcache_frontswap_unuse();
-		if (unuse_ret == -ENOMEM)
+	anon_pageframes_inuse = zcache_last_active_anon_pageframes +
+				zcache_last_inactive_anon_pageframes;
+	if (zcache_pers_pageframes > anon_pageframes_inuse)
+		nr_writeback = zcache_pers_pageframes - anon_pageframes_inuse;
+	else
+		nr_writeback = 0;
+	while (nr_writeback-- > 0) {
+#ifdef CONFIG_ZCACHE_WRITEBACK
+		int writeback_ret;
+		writeback_ret = zcache_frontswap_writeback();
+		if (writeback_ret == -ENOMEM)
+#endif
 			break;
 	}
-#endif
 	in_progress = false;
 
 skip_evict:
@@ -1343,7 +1545,7 @@ static int zcache_local_new_pool(uint32_t flags)
 int zcache_autocreate_pool(unsigned int cli_id, unsigned int pool_id, bool eph)
 {
 	struct tmem_pool *pool;
-	struct zcache_client *cli = NULL;
+	struct zcache_client *cli;
 	uint32_t flags = eph ? 0 : TMEM_POOL_PERSIST;
 	int ret = -1;
 
@@ -1520,7 +1722,7 @@ static inline struct tmem_oid oswiz(unsigned type, u32 ind)
 	return oid;
 }
 
-#ifdef FRONTSWAP_HAS_UNUSE
+#ifdef CONFIG_ZCACHE_WRITEBACK
 static void unswiz(struct tmem_oid oid, u32 index,
 				unsigned *type, pgoff_t *offset)
 {