[PATCH 4/5] [RFC][HACK] tmpfs: Purge volatile ranges on writepage instead of using shrinker

[John Stultz <john.stultz@linaro.org>]

In order to avoid using a shrinker, purge volatile ranges on writepage.
This requires deactivating/activating the page ranges together to
ensure relative LRU behavior in purging the volatile ranges.

This no longer requires the volatile range lru so remove that code.
Also add volatile range infrastructure to find a range that contains
a given page.

One concern with this approach is that it adds the overhead of having
to activate or deactivate each page in the range when we mark or unmark
a range as volatile. Since users of this interface are volunteering
memory for the kernel to possibly take, users may not feel so generous
if the cost of marking and umarking ranges is high.

Cost of calling MARK_VOLATILE then UNMARK_VOLATILE on a 1meg chunk:
Before this patch:	~25us
With this patch:	~495us

So the 20x cost increase makes this approach less favorable. But
hopefully someone can suggest an idea to improve things?

NOTE: On systems without swap, the VM won't shrink anonymous memory,
so writepage is never called and volatile ranges won't be purged.
This issue will be addressed in a following patch.

I've only been able to do minimal testing, so there's probably much
still wrong with this patch. But hopefully it will provide a concrete
approach for discussion.

v2:
 * Rework page activation/deactivation using pagevec to batch
   operations.  This improves performance 3x from prior, but
   performance still is ~20x slower then the lru shrinker method.

CC: Andrew Morton <akpm@linux-foundation.org>
CC: Android Kernel Team <kernel-team@android.com>
CC: Robert Love <rlove@google.com>
CC: Mel Gorman <mel@csn.ul.ie>
CC: Hugh Dickins <hughd@google.com>
CC: Dave Hansen <dave@linux.vnet.ibm.com>
CC: Rik van Riel <riel@redhat.com>
CC: Dmitry Adamushko <dmitry.adamushko@gmail.com>
CC: Dave Chinner <david@fromorbit.com>
CC: Neil Brown <neilb@suse.de>
CC: Andrea Righi <andrea@betterlinux.com>
CC: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
CC: Taras Glek <tgek@mozilla.com>
CC: Mike Hommey <mh@glandium.org>
CC: Jan Kara <jack@suse.cz>
CC: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
CC: Michel Lespinasse <walken@google.com>
CC: Minchan Kim <minchan@kernel.org>
CC: linux-mm@kvack.org <linux-mm@kvack.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
---
 include/linux/volatile.h |   11 ++---
 mm/shmem.c               |  108 +++++++++++++++++++++++++---------------------
 mm/volatile.c            |   88 ++++++++++---------------------------
 3 files changed, 84 insertions(+), 123 deletions(-)

diff --git a/include/linux/volatile.h b/include/linux/volatile.h
index 6f41b98..fcae047 100644
--- a/include/linux/volatile.h
+++ b/include/linux/volatile.h
@@ -5,15 +5,11 @@
 
 struct volatile_fs_head {
 	struct mutex lock;
-	struct list_head lru_head;
-	s64 unpurged_page_count;
 };
 
 
 #define DEFINE_VOLATILE_FS_HEAD(name) struct volatile_fs_head name = {	\
 	.lock = __MUTEX_INITIALIZER(name.lock),				\
-	.lru_head = LIST_HEAD_INIT(name.lru_head),			\
-	.unpurged_page_count = 0,					\
 }
 
 
@@ -34,12 +30,11 @@ extern long volatile_range_remove(struct volatile_fs_head *head,
 				struct address_space *mapping,
 				pgoff_t start_index, pgoff_t end_index);
 
-extern s64 volatile_range_lru_size(struct volatile_fs_head *head);
-
 extern void volatile_range_clear(struct volatile_fs_head *head,
 					struct address_space *mapping);
 
-extern s64 volatile_ranges_pluck_lru(struct volatile_fs_head *head,
-				struct address_space **mapping,
+int volatile_page_in_range(struct volatile_fs_head *head,
+				struct page *page,
 				pgoff_t *start, pgoff_t *end);
+
 #endif /* _LINUX_VOLATILE_H */
diff --git a/mm/shmem.c b/mm/shmem.c
index d85d237..9c6b2cd 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -627,6 +627,41 @@ static int shmem_setattr(struct dentry *dentry, struct iattr *attr)
 
 static DEFINE_VOLATILE_FS_HEAD(shmem_volatile_head);
 
+
+void modify_range(struct address_space *mapping,
+			pgoff_t start, pgoff_t end,
+			void(*activate_func)(struct page*))
+{
+	struct pagevec pvec;
+	pgoff_t index = start;
+	int i;
+
+
+	pagevec_init(&pvec, 0);
+	while (index <= end && pagevec_lookup(&pvec, mapping, index,
+			min(end - index, (pgoff_t)PAGEVEC_SIZE - 1) + 1)) {
+		mem_cgroup_uncharge_start();
+		for (i = 0; i < pagevec_count(&pvec); i++) {
+			struct page *page = pvec.pages[i];
+
+			/* We rely upon deletion not changing page->index */
+			index = page->index;
+			if (index > end)
+				break;
+
+			activate_func(page);
+		}
+		pagevec_release(&pvec);
+		mem_cgroup_uncharge_end();
+		cond_resched();
+		index++;
+	}
+
+}
+
+
+
+
 static int shmem_mark_volatile(struct inode *inode, loff_t offset, loff_t len)
 {
 	pgoff_t start, end;
@@ -643,7 +678,11 @@ static int shmem_mark_volatile(struct inode *inode, loff_t offset, loff_t len)
 				((loff_t) start << PAGE_CACHE_SHIFT),
 				((loff_t) end << PAGE_CACHE_SHIFT)-1);
 		ret = 0;
+
 	}
+
+	modify_range(&inode->i_data, start, end, &deactivate_page);
+
 	volatile_range_unlock(&shmem_volatile_head);
 
 	return ret;
@@ -660,6 +699,9 @@ static int shmem_unmark_volatile(struct inode *inode, loff_t offset, loff_t len)
 	volatile_range_lock(&shmem_volatile_head);
 	ret = volatile_range_remove(&shmem_volatile_head, &inode->i_data,
 								start, end);
+
+	modify_range(&inode->i_data, start, end, &activate_page);
+
 	volatile_range_unlock(&shmem_volatile_head);
 
 	return ret;
@@ -672,55 +714,6 @@ static void shmem_clear_volatile(struct inode *inode)
 	volatile_range_unlock(&shmem_volatile_head);
 }
 
-static
-int shmem_volatile_shrink(struct shrinker *ignored, struct shrink_control *sc)
-{
-	s64 nr_to_scan = sc->nr_to_scan;
-	const gfp_t gfp_mask = sc->gfp_mask;
-	struct address_space *mapping;
-	pgoff_t start, end;
-	int ret;
-	s64 page_count;
-
-	if (nr_to_scan && !(gfp_mask & __GFP_FS))
-		return -1;
-
-	volatile_range_lock(&shmem_volatile_head);
-	page_count = volatile_range_lru_size(&shmem_volatile_head);
-	if (!nr_to_scan)
-		goto out;
-
-	do {
-		ret = volatile_ranges_pluck_lru(&shmem_volatile_head,
-							&mapping, &start, &end);
-		if (ret) {
-			shmem_truncate_range(mapping->host,
-				((loff_t) start << PAGE_CACHE_SHIFT),
-				((loff_t) end << PAGE_CACHE_SHIFT)-1);
-
-			nr_to_scan -= end-start;
-			page_count -= end-start;
-		};
-	} while (ret && (nr_to_scan > 0));
-
-out:
-	volatile_range_unlock(&shmem_volatile_head);
-
-	return page_count;
-}
-
-static struct shrinker shmem_volatile_shrinker = {
-	.shrink = shmem_volatile_shrink,
-	.seeks = DEFAULT_SEEKS,
-};
-
-static int __init shmem_shrinker_init(void)
-{
-	register_shrinker(&shmem_volatile_shrinker);
-	return 0;
-}
-arch_initcall(shmem_shrinker_init);
-
 
 static void shmem_evict_inode(struct inode *inode)
 {
@@ -884,15 +877,30 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
 	struct address_space *mapping;
 	struct inode *inode;
 	swp_entry_t swap;
-	pgoff_t index;
+	pgoff_t index, start, end;
 
 	BUG_ON(!PageLocked(page));
 	mapping = page->mapping;
 	index = page->index;
 	inode = mapping->host;
 	info = SHMEM_I(inode);
+
 	if (info->flags & VM_LOCKED)
 		goto redirty;
+
+	volatile_range_lock(&shmem_volatile_head);
+	if (volatile_page_in_range(&shmem_volatile_head, page, &start, &end)) {
+		unlock_page(page);
+		volatile_range_unlock(&shmem_volatile_head);
+		shmem_truncate_range(inode,
+				((loff_t) start << PAGE_CACHE_SHIFT),
+				((loff_t) end << PAGE_CACHE_SHIFT)-1);
+
+		return 0;
+	}
+	volatile_range_unlock(&shmem_volatile_head);
+
+
 	if (!total_swap_pages)
 		goto redirty;
 
diff --git a/mm/volatile.c b/mm/volatile.c
index d05a767..862f2ae 100644
--- a/mm/volatile.c
+++ b/mm/volatile.c
@@ -53,7 +53,6 @@
 
 
 struct volatile_range {
-	struct list_head		lru;
 	struct prio_tree_node		node;
 	unsigned int			purged;
 	struct address_space		*mapping;
@@ -159,15 +158,8 @@ static inline void vrange_resize(struct volatile_fs_head *head,
 				struct volatile_range *vrange,
 				pgoff_t start_index, pgoff_t end_index)
 {
-	pgoff_t old_size, new_size;
-
-	old_size = vrange->node.last - vrange->node.start;
-	new_size = end_index-start_index;
-
-	if (!vrange->purged)
-		head->unpurged_page_count += new_size - old_size;
-
 	prio_tree_remove(root, &vrange->node);
+	INIT_PRIO_TREE_NODE(&vrange->node);
 	vrange->node.start = start_index;
 	vrange->node.last = end_index;
 	prio_tree_insert(root, &vrange->node);
@@ -189,15 +181,7 @@ static void vrange_add(struct volatile_fs_head *head,
 				struct prio_tree_root *root,
 				struct volatile_range *vrange)
 {
-
 	prio_tree_insert(root, &vrange->node);
-
-	/* Only add unpurged ranges to LRU */
-	if (!vrange->purged) {
-		head->unpurged_page_count += vrange->node.last - vrange->node.start;
-		list_add_tail(&vrange->lru, &head->lru_head);
-	}
-
 }
 
 
@@ -206,10 +190,6 @@ static void vrange_del(struct volatile_fs_head *head,
 				struct prio_tree_root *root,
 				struct volatile_range *vrange)
 {
-	if (!vrange->purged) {
-		head->unpurged_page_count -= vrange->node.last - vrange->node.start;
-		list_del(&vrange->lru);
-	}
 	prio_tree_remove(root, &vrange->node);
 	kfree(vrange);
 }
@@ -416,62 +396,40 @@ out:
 	return ret;
 }
 
-/**
- * volatile_range_lru_size: Returns the number of unpurged pages on the lru
- * @head: per-fs volatile head
- *
- * Returns the number of unpurged pages on the LRU
- *
- * Must lock the volatile_fs_head before calling!
- *
- */
-s64 volatile_range_lru_size(struct volatile_fs_head *head)
-{
-	WARN_ON(!mutex_is_locked(&head->lock));
-	return head->unpurged_page_count;
-}
-
 
-/**
- * volatile_ranges_pluck_lru: Returns mapping and size of lru unpurged range
- * @head: per-fs volatile head
- * @mapping: dbl pointer to mapping who's range is being purged
- * @start: Pointer to starting address of range being purged
- * @end: Pointer to ending address of range being purged
- *
- * Returns the mapping, start and end values of the least recently used
- * range. Marks the range as purged and removes it from the LRU.
- *
- * Must lock the volatile_fs_head before calling!
- *
- * Returns 1 on success if a range was returned
- * Return 0 if no ranges were found.
- */
-s64 volatile_ranges_pluck_lru(struct volatile_fs_head *head,
-				struct address_space **mapping,
+int volatile_page_in_range(struct volatile_fs_head *head,
+				struct page *page,
 				pgoff_t *start, pgoff_t *end)
 {
-	struct volatile_range *range;
+	struct prio_tree_root *root;
+	struct prio_tree_node *node;
+	struct prio_tree_iter iter;
+	struct volatile_range *vrange;
+	int ret	= 0;
 
 	WARN_ON(!mutex_is_locked(&head->lock));
 
-	if (list_empty(&head->lru_head))
+	root = mapping_to_root(page->mapping);
+	if (!root)
 		return 0;
 
-	range = list_first_entry(&head->lru_head, struct volatile_range, lru);
-
-	*start = range->node.start;
-	*end = range->node.last;
-	*mapping = range->mapping;
-
-	head->unpurged_page_count -= *end - *start;
-	list_del(&range->lru);
-	range->purged = 1;
+	prio_tree_iter_init(&iter, root, page->index, page->index);
+	node = prio_tree_next(&iter);
+	if (node) {
+		vrange = container_of(node, struct volatile_range, node);
 
-	return 1;
+		if (!vrange->purged) {
+			*start = vrange->node.start;
+			*end = vrange->node.last;
+			vrange->purged = 1;
+			ret = 1;
+		}
+	}
+	return ret;
 }
 
 
+
 /*
  * Cleans up any volatile ranges.
  */
-- 
1.7.9.5

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>