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@xxxxxxxxxx> 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@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?
> >
> > 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@xxxxxxxxxxx>
> ---
> 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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