On Tue, Sep 24, 2013 at 9:03 AM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> On Mon, Sep 23, 2013 at 04:21:49PM +0800, Weijie Yang wrote:
> >
> > Modify:
> > - check the refcount in fail path, free memory if it is not referenced.
>
> Hmm, I don't like this because zswap refcount routine is already mess for me.
> I'm not sure why it was designed from the beginning. I hope we should fix it first.
>
> 1. zswap_rb_serach could include zswap_entry_get semantic if it founds a entry from
> the tree. Of course, we should ranme it as find_get_zswap_entry like find_get_page.
> 2. zswap_entry_put could hide resource free function like zswap_free_entry so that
> all of caller can use it easily following pattern.
>
> find_get_zswap_entry
> ...
> ...
> zswap_entry_put
>
> Of course, zswap_entry_put have to check the entry is in the tree or not
> so if someone already removes it from the tree, it should avoid double remove.
>
> One of the concern I can think is that approach extends critical section
> but I think it would be no problem because more bottleneck would be [de]compress
> functions. If it were really problem, we can mitigate a problem with moving
> unnecessary functions out of zswap_free_entry because it seem to be rather
> over-enginnering.
I refactor the zswap refcount routine according to Minchan's idea.
Here is the new patch, Any suggestion is welcomed.
To Seth and Bob, would you please review it again?
mm/zswap.c | 116
++++++++++++++++++++++++++++++++++++++++++++++++++++----------------------------------------------------------------
1 file changed, 52 insertions(+), 64 deletions(-)
diff --git a/mm/zswap.c b/mm/zswap.c
old mode 100644
new mode 100755
index deda2b6..bd04910
--- 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;
}
@@ -232,10 +233,20 @@ static void zswap_entry_get(struct zswap_entry *entry)
}
/* caller must hold the tree lock */
-static int zswap_entry_put(struct zswap_entry *entry)
+static int zswap_entry_put(struct zswap_tree *tree, struct zswap_entry *entry)
{
- entry->refcount--;
- return entry->refcount;
+ int refcount = --entry->refcount;
+
+ if (refcount <= 0) {
+ if (!RB_EMPTY_NODE(&entry->rbnode)) {
+ rb_erase(&entry->rbnode, &tree->rbroot);
+ RB_CLEAR_NODE(&entry->rbnode);
+ }
+
+ zswap_free_entry(tree, entry);
+ }
+
+ return refcount;
}
/*********************************
@@ -258,6 +269,17 @@ static struct zswap_entry *zswap_rb_search(struct rb_root *root, pgoff_t offset)
return NULL;
}
+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;
+}
+
/*
* In the case that a entry with the same offset is found, a pointer to
* the existing entry is stored in dupentry and the function returns -EEXIST
@@ -387,7 +409,7 @@ static void zswap_free_entry(struct zswap_tree *tree, struct zswap_entry *entry)
enum zswap_get_swap_ret {
ZSWAP_SWAPCACHE_NEW,
ZSWAP_SWAPCACHE_EXIST,
- ZSWAP_SWAPCACHE_NOMEM
+ ZSWAP_SWAPCACHE_FAIL,
};
/*
@@ -401,9 +423,9 @@ enum zswap_get_swap_ret {
* added to the swap cache, and returned in retpage.
*
* If success, the swap cache 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
- * Returns <0 on error
+ * Returns ZSWAP_SWAPCACHE_EXIST if page was already in the swap cache
+ * Returns ZSWAP_SWAPCACHE_NEW if the new page needs to be populated, page is locked
+ * Returns ZSWAP_SWAPCACHE_FAIL on error
*/
static int zswap_get_swap_cache_page(swp_entry_t entry,
struct page **retpage)
@@ -475,7 +497,7 @@ static int zswap_get_swap_cache_page(swp_entry_t entry,
if (new_page)
page_cache_release(new_page);
if (!found_page)
- return ZSWAP_SWAPCACHE_NOMEM;
+ return ZSWAP_SWAPCACHE_FAIL;
*retpage = found_page;
return ZSWAP_SWAPCACHE_EXIST;
}
@@ -517,23 +539,22 @@ 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);
/* try to allocate swap cache page */
switch (zswap_get_swap_cache_page(swpentry, &page)) {
- case ZSWAP_SWAPCACHE_NOMEM: /* no memory */
+ case ZSWAP_SWAPCACHE_FAIL: /* no memory or invalidate happened */
ret = -ENOMEM;
goto fail;
- case ZSWAP_SWAPCACHE_EXIST: /* page is unlocked */
+ case ZSWAP_SWAPCACHE_EXIST:
/* page is already in the swap cache, ignore for now */
page_cache_release(page);
ret = -EEXIST;
@@ -562,38 +583,28 @@ 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);
+ refcount = zswap_entry_put(tree, entry);
/* drop the initial reference from entry creation */
- refcount = zswap_entry_put(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 */
+ if (refcount > 0) {
rb_erase(&entry->rbnode, &tree->rbroot);
+ RB_CLEAR_NODE(&entry->rbnode);
+ refcount = 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;
fail:
spin_lock(&tree->lock);
- zswap_entry_put(entry);
+ refcount = zswap_entry_put(tree, entry);
spin_unlock(&tree->lock);
- return ret;
+
+end:
+ if (refcount <= 0)
+ return 0;
+ else
+ return -EAGAIN;
}
/*********************************
@@ -677,10 +688,8 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
zswap_duplicate_entry++;
/* remove from rbtree */
rb_erase(&dupentry->rbnode, &tree->rbroot);
- if (!zswap_entry_put(dupentry)) {
- /* free */
- zswap_free_entry(tree, dupentry);
- }
+ RB_CLEAR_NODE(&dupentry->rbnode);
+ zswap_entry_put(tree, dupentry);
}
} while (ret == -EEXIST);
spin_unlock(&tree->lock);
@@ -713,13 +722,12 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
/* 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 */
@@ -734,22 +742,9 @@ 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;
}
@@ -771,19 +766,12 @@ static void zswap_frontswap_invalidate_page(unsigned type, pgoff_t offset)
/* remove from rbtree */
rb_erase(&entry->rbnode, &tree->rbroot);
+ RB_CLEAR_NODE(&entry->rbnode);
/* 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 */
--
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