[RFC/PATCH 0/3] swap/frontswap: allow movement of zcache pages to swap

[RFC/PATCH 0/3] swap/frontswap: allow movement of zcache pages to swap

[Dan Magenheimer]

INTRODUCTION

This is an initial patchset attempting to address Andrea Arcangeli's concern
expressed in https://lwn.net/Articles/516538/ "Moving zcache towards the
mainline".  It works, but not well, so feedback/help/expertise is definitely
needed!  Note that the zcache code that invokes this already exists in
zcache2, currently housed in Linus' 3.7-rc0 tree in drivers/staging/ramster,
so is not included in this patchset.  All relevant code in zcache is
conveniently marked with "#ifdef FRONTSWAP_UNUSE" which this patchset enables.

This patchset currently applies cleanly in Linus' tree following commit
33c2a174120b2c1baec9d1dac513f9d4b761b26a

[PROMOTION NOTE: Due to the great zcache compromise, the author won't take a
position on whether this "unuse" capability is a new zcache feature nor
whether this capability is necessary for production use of zcache.  This post
merely documents the motivation, mechanism, and policy of a possible solution
and provides a work-to-date patch, and requests discussion and feedback from
mm experts to ensure that, at some point in zcache's future, this important
capability can be completed and made available to zcache users.]

MOTIVATION

Several mm developers have noted that one of the key limiters to zcache
is that zcache may fill up with pages from frontswap and, once full,
there is no mechanism (or policy) to "evict" pages to make room for
newer pages.  This is problematic for some workloads, especially those
with a mix of long-lived-rarely-running threads (type A) such as system
services that run at boot and shutdown, and short-lived-always-running
threads (type B).  In a non-zcache system under memory pressure, type A
pages will be swapped to ensure that type B pages remain in RAM.  With
zcache, if even with compression there is insufficient RAM for both,
type A pages will fill zcache and type B pages will be swapped, resulting
in noticably reduced performance for type B threads.  Not good.

So it would be good if zcache had some mechanism for saying "oops, this
frontswap page saved in zcache should go out to the swap device after all."
We will call this "eviction".  (Note that zcache already supports eviction
of cleancache pages because these need only be "reclaimed"... the data
contained in cleancache pages can be discarded.  For the remainder of this
discussion, "eviction" implies a frontswap page.)

This begs the question: What page should zcache choose to evict?  While it
is impossible to predict what pages will be used when, the memory management
subsystem normally employs some form of LRU queue to minimize the probability
that an actively used page will be evicted.  Note, however, that evicting
one zcache page ("zpage") may or may not free up a pageframe as two (or,
in the case of the zsmalloc, more than two) zpages may reside in the same
pageframe.  Since freeing up a part of a pageframe has little value and,
indeed, even swapping a page to a swap device requires a full pageframe,
the zcache eviction policy must evict all zpages in a pageframe.  We will
call this "unuse".  For the remainder of this discussion, we will assume
a zcache pageframe contains exactly two zpages (though this may change in
the future).

MECHANISM

Should zcache write entire pageframes to the swap device, with multiple
zpages contained intact?  This might be possible, and could be explored,
but raises some interesting metadata challenges;  a fair amount of
additional RAM would be necessary to track these sets of zpages.  It might
also be easier if each swap disk had a "shadow": one swap device for
uncompressed pages, and a shadow device for compressed pages, else we
must take great care to avoid overwriting one with the other, which would
likely require some fairly invasive changes to the swap subsystem, which
already has a number of interesting coherency problems solved.  A shadow
swap disk would also require additional devices and/or disk space, which
may not be available, plus userland/sysadmin changes, which would be
difficult to mainstream.

So, we'd like an implementation that has a low requirement for in-RAM
metadata and has no requirement for additional swap device/space or
additional userland changes.

To achieve this, we will rely heavily on the existing swapcache.  When
zcache wishes to unuse a zcache pageframe, it first allocates two pageframes,
one for each zpage, and decompresses each zpage into a pageframe.  It then
frontswap to mark a new bit (in a one-bit-per-swap-page array) called a
"denial" bit.  Next it puts the uncompressed pageframe back into the swap
cache, at the least-recently-used end of the anonymous-inactive queue which,
presumably, makes it a candidate for immediate swapwrite.  At this point,
zcache is now free to release the pageframe that contained the two zpages.
Soon, memory pressure causes kswapd to select some pages to write
to swap.  As with all swap attempts, frontswap will first be called,
but since the denial bit is set, frontswap will reject the page
and the swap subsystem will write the page to the true swapdisk.

There are a number of housekeeping details, but that's the proposed
mechanism, implemented in this patchset, in a nutshell.

POLICY

There are some policy questions: How do we maintain an LRU queue
of zpages?  How do we know when zcache is "full"?  If zcache is
full, how do we ensure we can allocate two fresh pageframes for
decompression, and won't this potentially cause an OOM?  The
new zcache implementation (aka zcache2) attempts a trial policy
for each of these, but without a known working mechanism, the
policy may be irrelevant or wrong.  So let's focus first on
getting a working mechanism, OK?

CALL FOR INPUT/HELP

The patchset proposed does work and, in limited testing, does not OOM.
However, performance is much slower than expected and, with extensive
debug output, it appears that "immediate swapwrite" is not very immediate.
This may be related to the performance degradation.  Or there may be
a stupid bug lingering.  Or... the patchset may be completely braindead!
Any help or input in getting this working (or perhaps justifying why a
completely different mechanism might work better) would be appreciated!

Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>

---
Diffstat:

 include/linux/frontswap.h |   57 ++++++++++++++++++++++++++++++++
 include/linux/swap.h      |   12 +++++++
 mm/frontswap.c            |   29 ++++++++++++++++
 mm/swap.c                 |   16 +++++++++
 mm/swap_state.c           |   80 +++++++++++++++++++++++++++++++++++++++++++++
 mm/swapfile.c             |   18 +++++++---
 6 files changed, 207 insertions(+), 5 deletions(-)

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[RFC/PATCH 1/3] swap: allow adding of pages to tail of anonymous inactive queue

[Dan Magenheimer]

When moving a page of anonymous data out of zcache and back
into swap cache, such pages are VERY inactive, and we want
them to be swapped to disk ASAP.  So we need to add them
at the tail of the proper queue.

Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>
---
 include/linux/swap.h |   10 ++++++++++
 mm/swap.c            |   16 ++++++++++++++++
 2 files changed, 26 insertions(+), 0 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 388e706..d3c7281 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -225,6 +225,7 @@ extern unsigned int nr_free_pagecache_pages(void);
 
 /* linux/mm/swap.c */
 extern void __lru_cache_add(struct page *, enum lru_list lru);
+extern void __lru_cache_add_tail(struct page *, enum lru_list lru);
 extern void lru_cache_add_lru(struct page *, enum lru_list lru);
 extern void lru_add_page_tail(struct page *page, struct page *page_tail,
 			      struct lruvec *lruvec);
@@ -247,6 +248,15 @@ static inline void lru_cache_add_anon(struct page *page)
 {
 	__lru_cache_add(page, LRU_INACTIVE_ANON);
 }
+ 
+/**
+ * lru_cache_add_tail: add a page to the tail of the page lists
+ * @page: the page to add
+ */
+static inline void lru_cache_add_anon_tail(struct page *page)
+{
+	__lru_cache_add_tail(page, LRU_INACTIVE_ANON);
+}
 
 static inline void lru_cache_add_file(struct page *page)
 {
diff --git a/mm/swap.c b/mm/swap.c
index 7782588..67216d8 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -456,6 +456,22 @@ void __lru_cache_add(struct page *page, enum lru_list lru)
 	put_cpu_var(lru_add_pvecs);
 }
 EXPORT_SYMBOL(__lru_cache_add);
+ 
+void __lru_cache_add_tail(struct page *page, enum lru_list lru)
+{
+	struct pagevec *pvec = &get_cpu_var(lru_add_pvecs)[lru];
+	unsigned long flags;
+
+	page_cache_get(page);
+	if (!pagevec_add(pvec, page)) {
+		__pagevec_lru_add(pvec, lru);
+		local_irq_save(flags);
+		pagevec_move_tail(pvec);
+		local_irq_restore(flags);
+	}
+	put_cpu_var(lru_add_pvecs);
+}
+EXPORT_SYMBOL(__lru_cache_add_tail);
 
 /**
  * lru_cache_add_lru - add a page to a page list
-- 
1.7.1

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[RFC/PATCH 2/3] swap: add read_frontswap_async to move a page from frontswap to swapcache

[Dan Magenheimer]

We would like to move a "swap page" identified by swaptype/offset
out of frontswap and into swap cache.  Add read_frontswap_async
that, given an unused new page and a gfp_mask (for necessary radix
tree work), attempts to do that and communicates success, failure,
or the fact that a (possibly dirty) copy already exists in swap cache.
This new routine will be called from zcache (via frontswap) to
allow pages to be swapped to a true swap device when zcache gets "full".

Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>
---
 include/linux/swap.h |    1 +
 mm/swap_state.c      |   80 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 81 insertions(+), 0 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index d3c7281..8a59ddb 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -352,6 +352,7 @@ extern void free_pages_and_swap_cache(struct page **, int);
 extern struct page *lookup_swap_cache(swp_entry_t);
 extern struct page *read_swap_cache_async(swp_entry_t, gfp_t,
 			struct vm_area_struct *vma, unsigned long addr);
+extern int read_frontswap_async(int, pgoff_t, struct page *, gfp_t);
 extern struct page *swapin_readahead(swp_entry_t, gfp_t,
 			struct vm_area_struct *vma, unsigned long addr);
 
diff --git a/mm/swap_state.c b/mm/swap_state.c
index 0cb36fb..ad790bf 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -18,6 +18,7 @@
 #include <linux/pagevec.h>
 #include <linux/migrate.h>
 #include <linux/page_cgroup.h>
+#include <linux/frontswap.h>
 
 #include <asm/pgtable.h>
 
@@ -351,6 +352,85 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 	return found_page;
 }
 
+/*
+ * Similar to read_swap_cache_async except we know the page is in frontswap
+ * and we are trying to place it in swapcache so we can remove it from
+ * frontswap. Success means the data in frontswap can be thrown away,
+ * ENOMEM means it cannot, and -EEXIST means a (possibly dirty) copy
+ * already exists in the swapcache.
+ */
+int read_frontswap_async(int type, pgoff_t offset, struct page *new_page,
+				gfp_t gfp_mask)
+{
+	struct page *found_page;
+	swp_entry_t entry;
+	int ret = 0;
+
+	entry = swp_entry(type, offset);
+	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) {
+			/* its already in the swap cache */
+			ret = -EEXIST;
+			break;
+		}
+
+
+		/*
+		 * call radix_tree_preload() while we can wait.
+		 */
+		ret = radix_tree_preload(gfp_mask);
+		if (ret)
+			break;
+
+		/*
+		 * Swap entry may have been freed since our caller observed it.
+		 */
+		ret = swapcache_prepare(entry);
+		if (ret == -EEXIST) {	/* seems racy */
+			radix_tree_preload_end();
+			continue;
+		}
+		if (ret) {		/* 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);
+		ret = __add_to_swap_cache(new_page, entry);
+		if (likely(!ret)) {
+			radix_tree_preload_end();
+			/* FIXME: how do I add this at tail of lru? */
+			SetPageDirty(new_page);
+			lru_cache_add_anon_tail(new_page);
+			/* Get page (from frontswap) and return */
+			if (frontswap_load(new_page) == 0)
+				SetPageUptodate(new_page);
+			unlock_page(new_page);
+			ret = 0;
+			goto out;
+		}
+		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);
+	} while (ret != -ENOMEM);
+
+out:
+	return ret;
+}
+
 /**
  * swapin_readahead - swap in pages in hope we need them soon
  * @entry: swap entry of this memory
-- 
1.7.1

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[RFC/PATCH 3/3] frontswap/swap: allow frontswap "unuse" and add metadata for tracking it

[Dan Magenheimer]

We wish for transcendent memory backends to be able to push
frontswap pages back into the swap cache and need to ensure
that such a page, once pushed back, doesn't get immediately
recaptured by frontswap.  We add frontswap_unuse to do the
pushing via the recently added read_frontswap_async.  We
also add metadata to track when a page has been pushed and
code to manage (and count with debugfs) this metadata.

The initialization/destruction code for the metadata (aka
frontswap_denial_map) is a bit clunky in swapfile.c but
cleanup can be addressed when all the unuse code is working.

Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>
---
 include/linux/frontswap.h |   57 +++++++++++++++++++++++++++++++++++++++++++++
 include/linux/swap.h      |    1 +
 mm/frontswap.c            |   29 +++++++++++++++++++++++
 mm/swapfile.c             |   18 ++++++++++----
 4 files changed, 100 insertions(+), 5 deletions(-)

diff --git a/include/linux/frontswap.h b/include/linux/frontswap.h
index 3044254..f48bb34 100644
--- a/include/linux/frontswap.h
+++ b/include/linux/frontswap.h
@@ -21,6 +21,8 @@ extern unsigned long frontswap_curr_pages(void);
 extern void frontswap_writethrough(bool);
 #define FRONTSWAP_HAS_EXCLUSIVE_GETS
 extern void frontswap_tmem_exclusive_gets(bool);
+#define FRONTSWAP_HAS_UNUSE
+extern int frontswap_unuse(int, pgoff_t, struct page *, gfp_t);
 
 extern void __frontswap_init(unsigned type);
 extern int __frontswap_store(struct page *page);
@@ -61,6 +63,38 @@ static inline unsigned long *frontswap_map_get(struct swap_info_struct *p)
 {
 	return p->frontswap_map;
 }
+
+static inline int frontswap_test_denial(struct swap_info_struct *sis, pgoff_t offset)
+{
+	int ret = 0;
+
+	if (frontswap_enabled && sis->frontswap_denial_map)
+		ret = test_bit(offset, sis->frontswap_denial_map);
+	return ret;
+}
+
+static inline void frontswap_set_denial(struct swap_info_struct *sis, pgoff_t offset)
+{
+	if (frontswap_enabled && sis->frontswap_denial_map)
+		set_bit(offset, sis->frontswap_denial_map);
+}
+
+static inline void frontswap_clear_denial(struct swap_info_struct *sis, pgoff_t offset)
+{
+	if (frontswap_enabled && sis->frontswap_denial_map)
+		clear_bit(offset, sis->frontswap_denial_map);
+}
+
+static inline void frontswap_denial_map_set(struct swap_info_struct *p,
+				     unsigned long *map)
+{
+	p->frontswap_denial_map = map;
+}
+
+static inline unsigned long *frontswap_denial_map_get(struct swap_info_struct *p)
+{
+	return p->frontswap_denial_map;
+}
 #else
 /* all inline routines become no-ops and all externs are ignored */
 
@@ -88,6 +122,29 @@ static inline unsigned long *frontswap_map_get(struct swap_info_struct *p)
 {
 	return NULL;
 }
+
+static inline int frontswap_test_denial(struct swap_info_struct *sis, pgoff_t offset)
+{
+	return 0;
+}
+
+static inline void frontswap_set_denial(struct swap_info_struct *sis, pgoff_t offset)
+{
+}
+
+static inline void frontswap_clear_denial(struct swap_info_struct *sis, pgoff_t offset)
+{
+}
+
+static inline void frontswap_map_set_denial(struct swap_info_struct *p,
+				     unsigned long *map)
+{
+}
+
+static inline unsigned long *frontswap_map_get_denial(struct swap_info_struct *p)
+{
+	return NULL;
+}
 #endif
 
 static inline int frontswap_store(struct page *page)
diff --git a/include/linux/swap.h b/include/linux/swap.h
index 8a59ddb..aef02bc 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -200,6 +200,7 @@ struct swap_info_struct {
 	unsigned int old_block_size;	/* seldom referenced */
 #ifdef CONFIG_FRONTSWAP
 	unsigned long *frontswap_map;	/* frontswap in-use, one bit per page */
+	unsigned long *frontswap_denial_map;	/* deny frontswap, 1bit/page */
 	atomic_t frontswap_pages;	/* frontswap pages in-use counter */
 #endif
 };
diff --git a/mm/frontswap.c b/mm/frontswap.c
index 2890e67..1af07d1 100644
--- a/mm/frontswap.c
+++ b/mm/frontswap.c
@@ -61,6 +61,8 @@ static u64 frontswap_loads;
 static u64 frontswap_succ_stores;
 static u64 frontswap_failed_stores;
 static u64 frontswap_invalidates;
+static u64 frontswap_unuses;
+static u64 frontswap_denials;
 
 static inline void inc_frontswap_loads(void) {
 	frontswap_loads++;
@@ -151,6 +153,11 @@ int __frontswap_store(struct page *page)
 	BUG_ON(sis == NULL);
 	if (frontswap_test(sis, offset))
 		dup = 1;
+	if (frontswap_test_denial(sis, offset) && (dup == 0)) {
+		frontswap_clear_denial(sis, offset);
+		frontswap_denials++;
+		goto out;
+	}
 	ret = frontswap_ops.store(type, offset, page);
 	if (ret == 0) {
 		frontswap_set(sis, offset);
@@ -169,6 +176,7 @@ int __frontswap_store(struct page *page)
 	if (frontswap_writethrough_enabled)
 		/* report failure so swap also writes to swap device */
 		ret = -1;
+out:
 	return ret;
 }
 EXPORT_SYMBOL(__frontswap_store);
@@ -213,6 +221,7 @@ void __frontswap_invalidate_page(unsigned type, pgoff_t offset)
 	if (frontswap_test(sis, offset)) {
 		frontswap_ops.invalidate_page(type, offset);
 		__frontswap_clear(sis, offset);
+		frontswap_clear_denial(sis, offset);
 		inc_frontswap_invalidates();
 	}
 }
@@ -351,6 +360,24 @@ unsigned long frontswap_curr_pages(void)
 }
 EXPORT_SYMBOL(frontswap_curr_pages);
 
+int frontswap_unuse(int type, pgoff_t offset,
+			struct page *newpage, gfp_t gfp_mask)
+{
+	struct swap_info_struct *sis = swap_info[type];
+	int ret = 0;
+
+	frontswap_set_denial(sis, offset);
+	ret = read_frontswap_async(type, offset, newpage, gfp_mask);
+	if (ret == 0 || ret == -EEXIST) {
+		(*frontswap_ops.invalidate_page)(type, offset);
+		atomic_dec(&sis->frontswap_pages);
+		frontswap_clear(sis, offset);
+		frontswap_unuses++;
+	}
+	return ret;
+}
+EXPORT_SYMBOL(frontswap_unuse);
+
 static int __init init_frontswap(void)
 {
 #ifdef CONFIG_DEBUG_FS
@@ -363,6 +390,8 @@ static int __init init_frontswap(void)
 				&frontswap_failed_stores);
 	debugfs_create_u64("invalidates", S_IRUGO,
 				root, &frontswap_invalidates);
+	debugfs_create_u64("unuses", S_IRUGO, root, &frontswap_unuses);
+	debugfs_create_u64("denials", S_IRUGO, root, &frontswap_denials);
 #endif
 	return 0;
 }
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 14e254c..b3d6266 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1445,7 +1445,8 @@ static int setup_swap_extents(struct swap_info_struct *sis, sector_t *span)
 
 static void enable_swap_info(struct swap_info_struct *p, int prio,
 				unsigned char *swap_map,
-				unsigned long *frontswap_map)
+				unsigned long *frontswap_map,
+				unsigned long *frontswap_denial_map)
 {
 	int i, prev;
 
@@ -1456,6 +1457,7 @@ static void enable_swap_info(struct swap_info_struct *p, int prio,
 		p->prio = --least_priority;
 	p->swap_map = swap_map;
 	frontswap_map_set(p, frontswap_map);
+	frontswap_denial_map_set(p, frontswap_denial_map);
 	p->flags |= SWP_WRITEOK;
 	nr_swap_pages += p->pages;
 	total_swap_pages += p->pages;
@@ -1557,7 +1559,8 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 		 * sys_swapoff for this swap_info_struct at this point.
 		 */
 		/* re-insert swap space back into swap_list */
-		enable_swap_info(p, p->prio, p->swap_map, frontswap_map_get(p));
+		enable_swap_info(p, p->prio, p->swap_map,
+			frontswap_map_get(p), frontswap_denial_map_get(p));
 		goto out_dput;
 	}
 
@@ -1588,6 +1591,7 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 	mutex_unlock(&swapon_mutex);
 	vfree(swap_map);
 	vfree(frontswap_map_get(p));
+	vfree(frontswap_denial_map_get(p));
 	/* Destroy swap account informatin */
 	swap_cgroup_swapoff(type);
 
@@ -1948,6 +1952,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	unsigned long maxpages;
 	unsigned char *swap_map = NULL;
 	unsigned long *frontswap_map = NULL;
+	unsigned long *frontswap_denial_map = NULL;
 	struct page *page = NULL;
 	struct inode *inode = NULL;
 
@@ -2032,8 +2037,10 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 		goto bad_swap;
 	}
 	/* frontswap enabled? set up bit-per-page map for frontswap */
-	if (frontswap_enabled)
-		frontswap_map = vzalloc(maxpages / sizeof(long));
+	if (frontswap_enabled) {
+ 		frontswap_map = vzalloc(maxpages / sizeof(long));
+		frontswap_denial_map = vzalloc(maxpages / sizeof(long));
+	}
 
 	if (p->bdev) {
 		if (blk_queue_nonrot(bdev_get_queue(p->bdev))) {
@@ -2049,7 +2056,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	if (swap_flags & SWAP_FLAG_PREFER)
 		prio =
 		  (swap_flags & SWAP_FLAG_PRIO_MASK) >> SWAP_FLAG_PRIO_SHIFT;
-	enable_swap_info(p, prio, swap_map, frontswap_map);
+	enable_swap_info(p, prio, swap_map,
+				frontswap_map, frontswap_denial_map);
 
 	printk(KERN_INFO "Adding %uk swap on %s.  "
 			"Priority:%d extents:%d across:%lluk %s%s%s\n",
-- 
1.7.1

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