Date:	Mon, 14 Oct 2013 09:31:24 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Weijie Yang <weijie.yang@...sung.com>
Cc:	akpm@...ux-foundation.org, sjenning@...ux.vnet.ibm.com,
	bob.liu@...cle.com, weijie.yang.kh@...il.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v3 2/3] mm/zswap: bugfix: memory leak when invalidate and
 reclaim occur concurrently

Hello,

On Sat, Oct 12, 2013 at 05:37:55PM +0800, Weijie Yang wrote:
> Hi, all
> 
> I thought out a new way to resolve this problem: use CAS instead of refcount.
> 
> It not only resolve this problem, but also fix another issue,
> can be expanded to support frontswap get_and_free mode easily.
> And I use it in an zswap variant which writeback zbud page directly.
> 
> If it is accepted, I will resent it instead of the previous refactor patch

You're adding atomic operation and making code more complicated to me rather
than simple get/put scheme. Please, write a good decsription why it's better,
not vague words like "fix another issue, get_and_free and zswap variant".

Please convince us.

> 
> please see below, Request For Comments, Thanks.
> 
> On Fri, Oct 11, 2013 at 3:13 PM, Minchan Kim <minchan@...nel.org> wrote:
> > On Thu, Sep 26, 2013 at 11:42:17AM +0800, Weijie Yang wrote:
> > > On Tue, Sep 24, 2013 at 9:03 AM, Minchan Kim <minchan@...nel.org> 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?
> > 
> > Yeah, Seth, Bob. You guys are right persons to review this because this
> > scheme was suggested by me who is biased so it couldn't be a fair. ;-)
> > But anyway, I will review code itself.
> 
> Consider the following scenario:
> thread 0: reclaim entry x(get refcount, but not call zswap_get_swap_cache_page)
> thread 1: call zswap_frontswap_invalidate_page to invalidate entry x.
> 	finished, entry x and its zbud is not freed as its refcount != 0
> 	now, the swap_map[x] = 0
> thread 0: now call zswap_get_swap_cache_page
> 	swapcache_prepare return -ENOENT because entry x is not used any more
> 	zswap_get_swap_cache_page return ZSWAP_SWAPCACHE_NOMEM
> 	zswap_writeback_entry do nothing except put refcount
> Now, the memory of zswap_entry x and its zpage leak.
> 
> Consider the another scenario:
> zswap entry x with offset A is already stored in zswap backend.
> thread 0: reclaim entry x(get refcount, but not call zswap_get_swap_cache_page)
> thread 1: store new page with the same offset A, alloc a new zswap entry y.
> 	This is a duplicate store and finished.
> 	shrink_page_list() finish, now the new page is not in swap_cache
> thread 0: zswap_get_swap_cache_page get called.
> 	old page data is added to swap_cache
> Now, swap cache has old data rather than new data for offset A.
> Error will happen If do_swap_page() get page from swap_cache.
> 
> Modify:
>  - re-design the zswap_entry concurrent protection by using a CAS-access flag
>    instead of the refcount get/put semantic
> 
>  - use ZSWAP_SWAPCACHE_FAIL instead of ZSWAP_SWAPCACHE_NOMEM as the fail path
>    can be not only caused by nomem but also by invalidate.
> 
> Signed-off-by: Weijie Yang <weijie.yang@...sung.com>
> ---
>  mm/zswap.c |  177 +++++++++++++++++++++++++++---------------------------------
>  1 file changed, 80 insertions(+), 97 deletions(-)
>  mode change 100644 => 100755 mm/zswap.c
> 
> diff --git a/mm/zswap.c b/mm/zswap.c
> old mode 100644
> new mode 100755
> index cbd9578..fcaaecb
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -158,21 +158,23 @@ static void zswap_comp_exit(void)
>   * page within zswap.
>   *
>   * rbnode - links the entry into red-black tree for the appropriate swap type
> - * refcount - the number of outstanding reference to the entry. This is needed
> - *            to protect against premature freeing of the entry by code
> - *            concurent calls to load, invalidate, and writeback.  The lock
> - *            for the zswap_tree structure that contains the entry must
> - *            be held while changing the refcount.  Since the lock must
> - *            be held, there is no reason to also make refcount atomic.
>   * offset - the swap offset for the entry.  Index into the red-black tree.
> + * pos - the position or status of this entry. see below macro definition
> + *    change it by CAS, we hold this entry on success or retry if fail
> + *    protect against concurrent access by load, invalidate or writeback
> + *    even though the probability of concurrent access is very small
>   * handle - zsmalloc allocation handle that stores the compressed page data
>   * length - the length in bytes of the compressed page data.  Needed during
>   *           decompression
>   */
> +#define POS_POOL      1  /* stay in pool still */
> +#define POS_LOAD      2  /* is loading */
> +#define POS_RECLAIM   3  /* is reclaiming */
> +#define POS_FLUSH     4  /* is invalidating */
>  struct zswap_entry {
>  	struct rb_node rbnode;
>  	pgoff_t offset;
> -	int refcount;
> +	atomic_t pos;
>  	unsigned int length;
>  	unsigned long handle;
>  };
> @@ -216,7 +218,6 @@ static struct zswap_entry *zswap_entry_cache_alloc(gfp_t gfp)
>  	entry = kmem_cache_alloc(zswap_entry_cache, gfp);
>  	if (!entry)
>  		return NULL;
> -	entry->refcount = 1;
>  	return entry;
>  }
>  
> @@ -225,18 +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
> @@ -380,6 +369,23 @@ static void zswap_free_entry(struct zswap_tree *tree, struct zswap_entry *entry)
>  	zswap_pool_pages = zbud_get_pool_size(tree->pool);
>  }
>  
> +/* caller must hold the tree lock, entry is on the tree
> +* seldom called fortunately
> +* return 0: free this entry successfully
> +* return < 0: fail
> +*/
> +static int zswap_invalidate_entry(struct zswap_tree *tree, struct zswap_entry *entry)
> +{
> +	if (atomic_cmpxchg(&entry->pos, POS_POOL, POS_FLUSH) == POS_POOL) {
> +		rb_erase(&entry->rbnode, &tree->rbroot);
> +		zswap_free_entry(tree, entry);
> +		return 0;
> +	}
> +
> +	/* encounter reclaim called by another thread */
> +	return -1;
> +}
> +
>  /*********************************
>  * writeback code
>  **********************************/
> @@ -387,7 +393,7 @@ 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 +407,10 @@ 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,
> + * 	the new page is added to swap cache and locked
> + * Returns ZSWAP_SWAPCACHE_FAIL on error
>   */
>  static int zswap_get_swap_cache_page(swp_entry_t entry,
>  				struct page **retpage)
> @@ -475,7 +482,7 @@ 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;
>  }
> @@ -502,7 +509,7 @@ 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,
>  	};
> @@ -523,17 +530,23 @@ static int zswap_writeback_entry(struct zbud_pool *pool, unsigned long handle)
>  		spin_unlock(&tree->lock);
>  		return 0;
>  	}
> -	zswap_entry_get(entry);
> +	/* maybe encounter load or invalidate called by another thread
> +	* hold or give up this entry
> +	*/
> +	if (atomic_cmpxchg(&entry->pos, POS_POOL, POS_RECLAIM) != POS_POOL) {
> +		spin_unlock(&tree->lock);
> +		return -EAGAIN;
> +	}
>  	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:
>  		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;
> @@ -561,38 +574,23 @@ static int zswap_writeback_entry(struct zbud_pool *pool, unsigned long handle)
>  	page_cache_release(page);
>  	zswap_written_back_pages++;
>  
> +	/* once we hold this entry and get here, we can free entry safely
> +	* during the period which we hold this entry:
> +	* 1. if a thread call do_swap_page concurrently
> +	* we will get ZSWAP_SWAPCACHE_EXIST returned or
> +	* do_swap_page will hit page we added in the swap cache
> +	* 2. if a thread call invalidate concurrently
> +	* invalidate thread should wait until this function end
> +	*/
>  	spin_lock(&tree->lock);
> -
> -	/* drop local reference */
> -	zswap_entry_put(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 */
> -		rb_erase(&entry->rbnode, &tree->rbroot);
> -	}
> +	rb_erase(&entry->rbnode, &tree->rbroot);
>  	spin_unlock(&tree->lock);
> -	if (refcount <= 0) {
> -		/* free the entry */
> -		zswap_free_entry(tree, entry);
> -		return 0;
> -	}
> -	return -EAGAIN;
> +	zswap_free_entry(tree, entry);
> +
> +	return 0;
>  
>  fail:
> -	spin_lock(&tree->lock);
> -	zswap_entry_put(entry);
> -	spin_unlock(&tree->lock);
> +	BUG_ON(atomic_cmpxchg(&entry->pos, POS_RECLAIM, POS_POOL) != POS_RECLAIM);
>  	return ret;
>  }
>  
> @@ -668,19 +666,15 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
>  	entry->offset = offset;
>  	entry->handle = handle;
>  	entry->length = dlen;
> +	atomic_set(&entry->pos, POS_POOL);
>  
>  	/* map */
>  	spin_lock(&tree->lock);
>  	do {
>  		ret = zswap_rb_insert(&tree->rbroot, entry, &dupentry);
> -		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);
> -			}
> +		if (unlikely(ret == -EEXIST)) {
> +			if (zswap_invalidate_entry(tree, dupentry) == 0)
> +				zswap_duplicate_entry++;
>  		}
>  	} while (ret == -EEXIST);
>  	spin_unlock(&tree->lock);
> @@ -709,9 +703,10 @@ 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 */
> +find:
>  	spin_lock(&tree->lock);
>  	entry = zswap_rb_search(&tree->rbroot, offset);
>  	if (!entry) {
> @@ -719,7 +714,14 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
>  		spin_unlock(&tree->lock);
>  		return -1;
>  	}
> -	zswap_entry_get(entry);
> +	/* encounter entry reclaimed by another thread
> +	* just retry until that thread get ZSWAP_SWAPCACHE_EXIST returned
> +	* as we hold the entry, we can do anything: get or get_and_free
> +	*/
> +	if (atomic_cmpxchg(&entry->pos, POS_POOL, POS_LOAD) != POS_POOL) {
> +		spin_unlock(&tree->lock);
> +		goto find;
> +	}
>  	spin_unlock(&tree->lock);
>  
>  	/* decompress */
> @@ -733,22 +735,7 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
>  	zbud_unmap(tree->pool, entry->handle);
>  	BUG_ON(ret);
>  
> -	spin_lock(&tree->lock);
> -	refcount = zswap_entry_put(entry);
> -	if (likely(refcount)) {
> -		spin_unlock(&tree->lock);
> -		return 0;
> -	}
> -	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);
> +	BUG_ON(atomic_cmpxchg(&entry->pos, POS_LOAD, POS_POOL) != POS_LOAD);
>  
>  	return 0;
>  }
> @@ -758,9 +745,9 @@ 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 */
> +find:
>  	spin_lock(&tree->lock);
>  	entry = zswap_rb_search(&tree->rbroot, offset);
>  	if (!entry) {
> @@ -769,19 +756,18 @@ static void zswap_frontswap_invalidate_page(unsigned type, pgoff_t offset)
>  		return;
>  	}
>  
> +	/* encounter entry reclaimed by another thread
> +	* just retry until that reclaim end.
> +	*/
> +	if (atomic_cmpxchg(&entry->pos, POS_POOL, POS_FLUSH) != POS_POOL) {
> +		spin_unlock(&tree->lock);
> +		goto find;
> +	}
> +
>  	/* remove from rbtree */
>  	rb_erase(&entry->rbnode, &tree->rbroot);
> -
> -	/* drop the initial reference from entry creation */
> -	refcount = zswap_entry_put(entry);
> -
>  	spin_unlock(&tree->lock);
>  
> -	if (refcount) {
> -		/* writeback in progress, writeback will free */
> -		return;
> -	}
> -
>  	/* free */
>  	zswap_free_entry(tree, entry);
>  }
> @@ -809,10 +795,7 @@ 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);
> -		zbud_free(tree->pool, entry->handle);
> -		zswap_entry_cache_free(entry);
> -		atomic_dec(&zswap_stored_pages);
> +		zswap_invalidate_entry(tree, entry);
>  	}
>  	tree->rbroot = RB_ROOT;
>  	spin_unlock(&tree->lock);
> -- 
> 1.7.10.4
> 
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-- 
Kind regards,
Minchan Kim
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