Hi,
In order to make things clear and simple,
I will re-send this v3 2/3 patch. split it to two patches:
1. just fix the bug, as same as the first v3 2/3 patch of this email thread
2. refactor the get/put routine based on Minchan's suggestion.
As for the last CAS patch, I will add more decsription and resend it
as a RFC patch.
Thanks,
On Mon, Oct 14, 2013 at 8:31 AM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> 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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>> see: http://www.linux-mm.org/ .
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>
> --
> Kind regards,
> Minchan Kim
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