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Subject[PATCH 2/2] mm/zswap: refoctor the get/put routines
Date
The refcount routine was not fit the kernel get/put semantic exactly,
There were too many judgement statements on refcount and it could be minus.

This patch does the following:

- move refcount judgement to zswap_entry_put() to hide resource free function.

- add a new function zswap_entry_find_get(), so that callers can use easily
in the following pattern:

zswap_entry_find_get
.../* do something */
zswap_entry_put

- to eliminate compile error, move some functions declaration

This patch is based on Minchan Kim <minchan@kernel.org> 's idea and suggestion.

Signed-off-by: Weijie Yang <weijie.yang@samsung.com>
---
mm/zswap.c | 176 +++++++++++++++++++++++++++++-------------------------------
1 file changed, 85 insertions(+), 91 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index f25394a..986563c 100755
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -217,6 +217,7 @@ static struct zswap_entry *zswap_entry_cache_alloc(gfp_t gfp)
if (!entry)
return NULL;
entry->refcount = 1;
+ RB_CLEAR_NODE(&entry->rbnode);
return entry;
}

@@ -225,19 +226,6 @@ static void zswap_entry_cache_free(struct zswap_entry *entry)
kmem_cache_free(zswap_entry_cache, entry);
}

-/* caller must hold the tree lock */
-static void zswap_entry_get(struct zswap_entry *entry)
-{
- entry->refcount++;
-}
-
-/* caller must hold the tree lock */
-static int zswap_entry_put(struct zswap_entry *entry)
-{
- entry->refcount--;
- return entry->refcount;
-}
-
/*********************************
* rbtree functions
**********************************/
@@ -285,6 +273,59 @@ static int zswap_rb_insert(struct rb_root *root, struct zswap_entry *entry,
return 0;
}

+static void zswap_rb_erase(struct rb_root *root, struct zswap_entry *entry)
+{
+ if (!RB_EMPTY_NODE(&entry->rbnode)) {
+ rb_erase(&entry->rbnode, root);
+ RB_CLEAR_NODE(&entry->rbnode);
+ }
+}
+
+/*
+ * Carries out the common pattern of freeing and entry's zsmalloc allocation,
+ * freeing the entry itself, and decrementing the number of stored pages.
+ */
+static void zswap_free_entry(struct zswap_tree *tree, struct zswap_entry *entry)
+{
+ zbud_free(tree->pool, entry->handle);
+ zswap_entry_cache_free(entry);
+ atomic_dec(&zswap_stored_pages);
+ zswap_pool_pages = zbud_get_pool_size(tree->pool);
+}
+
+/* caller must hold the tree lock */
+static void zswap_entry_get(struct zswap_entry *entry)
+{
+ entry->refcount++;
+}
+
+/* caller must hold the tree lock
+* remove from the tree and free it, if nobody reference the entry
+*/
+static void zswap_entry_put(struct zswap_tree *tree, struct zswap_entry *entry)
+{
+ int refcount = --entry->refcount;
+
+ BUG_ON(refcount < 0);
+ if (refcount == 0) {
+ zswap_rb_erase(&tree->rbroot, entry);
+ zswap_free_entry(tree, entry);
+ }
+}
+
+/* caller must hold the tree lock */
+static struct zswap_entry *zswap_entry_find_get(struct rb_root *root,
+ pgoff_t offset)
+{
+ struct zswap_entry *entry = NULL;
+
+ entry = zswap_rb_search(root, offset);
+ if (entry)
+ zswap_entry_get(entry);
+
+ return entry;
+}
+
/*********************************
* per-cpu code
**********************************/
@@ -368,18 +409,6 @@ static bool zswap_is_full(void)
zswap_pool_pages);
}

-/*
- * Carries out the common pattern of freeing and entry's zsmalloc allocation,
- * freeing the entry itself, and decrementing the number of stored pages.
- */
-static void zswap_free_entry(struct zswap_tree *tree, struct zswap_entry *entry)
-{
- zbud_free(tree->pool, entry->handle);
- zswap_entry_cache_free(entry);
- atomic_dec(&zswap_stored_pages);
- zswap_pool_pages = zbud_get_pool_size(tree->pool);
-}
-
/*********************************
* writeback code
**********************************/
@@ -503,7 +532,7 @@ static int zswap_writeback_entry(struct zbud_pool *pool, unsigned long handle)
struct page *page;
u8 *src, *dst;
unsigned int dlen;
- int ret, refcount;
+ int ret;
struct writeback_control wbc = {
.sync_mode = WB_SYNC_NONE,
};
@@ -518,13 +547,12 @@ static int zswap_writeback_entry(struct zbud_pool *pool, unsigned long handle)

/* find and ref zswap entry */
spin_lock(&tree->lock);
- entry = zswap_rb_search(&tree->rbroot, offset);
+ entry = zswap_entry_find_get(&tree->rbroot, offset);
if (!entry) {
/* entry was invalidated */
spin_unlock(&tree->lock);
return 0;
}
- zswap_entry_get(entry);
spin_unlock(&tree->lock);
BUG_ON(offset != entry->offset);

@@ -563,42 +591,34 @@ static int zswap_writeback_entry(struct zbud_pool *pool, unsigned long handle)
zswap_written_back_pages++;

spin_lock(&tree->lock);
-
/* drop local reference */
- zswap_entry_put(entry);
- /* drop the initial reference from entry creation */
- refcount = zswap_entry_put(entry);
-
+ zswap_entry_put(tree, entry);
/*
- * There are three possible values for refcount here:
- * (1) refcount is 1, load is in progress, unlink from rbtree,
- * load will free
- * (2) refcount is 0, (normal case) entry is valid,
- * remove from rbtree and free entry
- * (3) refcount is -1, invalidate happened during writeback,
- * free entry
- */
- if (refcount >= 0) {
- /* no invalidate yet, remove from rbtree */
- rb_erase(&entry->rbnode, &tree->rbroot);
- }
+ * There are two possible situations for entry here:
+ * (1) refcount is 1(normal case), entry is valid and on the tree
+ * (2) refcount is 0, entry is freed and not on the tree
+ * because invalidate happened during writeback
+ * to handle (1), search the tree and free the entry
+ */
+ if (entry == zswap_rb_search(&tree->rbroot, offset))
+ zswap_entry_put(tree, entry);
spin_unlock(&tree->lock);
- if (refcount <= 0) {
- /* free the entry */
- zswap_free_entry(tree, entry);
- return 0;
- }
- return -EAGAIN;

+ goto end;
+
+ /*
+ * if we get here due to ZSWAP_SWAPCACHE_EXIST
+ * a load may happening concurrently
+ * it is safe and okay to not free the entry
+ * if we free the entry in the following put
+ * it it either okay to return !0
+ */
fail:
spin_lock(&tree->lock);
- refcount = zswap_entry_put(entry);
- if (refcount <= 0) {
- /* invalidate happened, consider writeback as success */
- zswap_free_entry(tree, entry);
- ret = 0;
- }
+ zswap_entry_put(tree, entry);
spin_unlock(&tree->lock);
+
+end:
return ret;
}

@@ -682,11 +702,8 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
if (ret == -EEXIST) {
zswap_duplicate_entry++;
/* remove from rbtree */
- rb_erase(&dupentry->rbnode, &tree->rbroot);
- if (!zswap_entry_put(dupentry)) {
- /* free */
- zswap_free_entry(tree, dupentry);
- }
+ zswap_rb_erase(&tree->rbroot, dupentry);
+ zswap_entry_put(tree, dupentry);
}
} while (ret == -EEXIST);
spin_unlock(&tree->lock);
@@ -715,17 +732,16 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
struct zswap_entry *entry;
u8 *src, *dst;
unsigned int dlen;
- int refcount, ret;
+ int ret;

/* find */
spin_lock(&tree->lock);
- entry = zswap_rb_search(&tree->rbroot, offset);
+ entry = zswap_entry_find_get(&tree->rbroot, offset);
if (!entry) {
/* entry was written back */
spin_unlock(&tree->lock);
return -1;
}
- zswap_entry_get(entry);
spin_unlock(&tree->lock);

/* decompress */
@@ -740,22 +756,9 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
BUG_ON(ret);

spin_lock(&tree->lock);
- refcount = zswap_entry_put(entry);
- if (likely(refcount)) {
- spin_unlock(&tree->lock);
- return 0;
- }
+ zswap_entry_put(tree, entry);
spin_unlock(&tree->lock);

- /*
- * We don't have to unlink from the rbtree because
- * zswap_writeback_entry() or zswap_frontswap_invalidate page()
- * has already done this for us if we are the last reference.
- */
- /* free */
-
- zswap_free_entry(tree, entry);
-
return 0;
}

@@ -764,7 +767,6 @@ static void zswap_frontswap_invalidate_page(unsigned type, pgoff_t offset)
{
struct zswap_tree *tree = zswap_trees[type];
struct zswap_entry *entry;
- int refcount;

/* find */
spin_lock(&tree->lock);
@@ -776,20 +778,12 @@ static void zswap_frontswap_invalidate_page(unsigned type, pgoff_t offset)
}

/* remove from rbtree */
- rb_erase(&entry->rbnode, &tree->rbroot);
+ zswap_rb_erase(&tree->rbroot, entry);

/* drop the initial reference from entry creation */
- refcount = zswap_entry_put(entry);
+ zswap_entry_put(tree, entry);

spin_unlock(&tree->lock);
-
- if (refcount) {
- /* writeback in progress, writeback will free */
- return;
- }
-
- /* free */
- zswap_free_entry(tree, entry);
}

/* frees all zswap entries for the given swap type */
@@ -815,7 +809,7 @@ static void zswap_frontswap_invalidate_area(unsigned type)
*/
while ((node = rb_first(&tree->rbroot))) {
entry = rb_entry(node, struct zswap_entry, rbnode);
- rb_erase(&entry->rbnode, &tree->rbroot);
+ zswap_rb_erase(&tree->rbroot, entry);
zbud_free(tree->pool, entry->handle);
zswap_entry_cache_free(entry);
atomic_dec(&zswap_stored_pages);
--
1.7.10.4


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