Re: [PATCH v2] zswap: replace RB tree with xarray

[Chengming Zhou]

On 2024/3/1 02:58, Chris Li wrote:
> Hi Chengming,
> 
> Thanks for the review and feedback.
> 
> On Thu, Feb 29, 2024 at 1:44 AM Chengming Zhou
> <zhouchengming@xxxxxxxxxxxxx> wrote:
>>
>> Hi Chris,
>>
>> On 2024/2/29 16:46, Chris Li wrote:
>>> Very deep RB tree requires rebalance at times. That
>>> contributes to the zswap fault latencies. Xarray does not
>>> need to perform tree rebalance. Replacing RB tree to xarray
>>> can have some small performance gain.
>>>
>>> One small difference is that xarray insert might fail with
>>> ENOMEM, while RB tree insert does not allocate additional
>>> memory.
>>>
>>> The zswap_entry size will reduce a bit due to removing the
>>> RB node, which has two pointers and a color field. Xarray
>>> store the pointer in the xarray tree rather than the
>>> zswap_entry. Every entry has one pointer from the xarray
>>> tree. Overall, switching to xarray should save some memory,
>>> if the swap entries are densely packed.
>>>
>>> Notice the zswap_rb_search and zswap_rb_insert always
>>> followed by zswap_rb_erase. Fold the entry erase into
>>> zswap_xa_search_and_erase and zswap_xa_insert. That saves
>>> one tree lookup as well.
>>>
>>> Remove zswap_invalidate_entry due to no need to call
>>> zswap_rb_erase any more. Use zswap_free_entry instead.
>>>
>>> The "struct zswap_tree" has been replaced by "struct xarray".
>>> The tree spin lock has transferred to the xarray lock.
>>>
>>> Thanks to Chengming for providing the kernel build test number.
>>>
>>> Run the kernel build testing 5 times for each version, averages:
>>> (memory.max=2GB, zswap shrinker and writeback enabled, one 50GB swapfile.)
>>>
>>>                               mm-266f922c0b5e       zswap-xarray-test
>>> real                          63.43                 63.12
>>> user                          1063.78               1062.59
>>> sys                           272.49                265.66
>>>
>>> The sys time is about 2.5% faster.
>>>
>>> Tested-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
>>> ---
>>>
>>>
>>> Signed-off-by: Chris Li <chrisl@xxxxxxxxxx>
>>> ---
>>> Changes in v2:
>>> - Replace struct zswap_tree with struct xarray.
>>> - Remove zswap_tree spinlock, use xarray lock instead.
>>> - Fold zswap_rb_erase() into zswap_xa_search_and_delete() and zswap_xa_insert().
>>> - Delete zswap_invalidate_entry(), use zswap_free_entry() instead.
>>> - Link to v1: https://lore.kernel.org/r/20240117-zswap-xarray-v1-0-6daa86c08fae@xxxxxxxxxx
>>> ---
>>>  mm/zswap.c | 173 +++++++++++++++++++++++--------------------------------------
>>>  1 file changed, 64 insertions(+), 109 deletions(-)
>>>
>>> diff --git a/mm/zswap.c b/mm/zswap.c
>>> index 011e068eb355..ac9ef14d88be 100644
>>> --- a/mm/zswap.c
>>> +++ b/mm/zswap.c
>>> @@ -20,7 +20,6 @@
>>>  #include <linux/spinlock.h>
>>>  #include <linux/types.h>
>>>  #include <linux/atomic.h>
>>> -#include <linux/rbtree.h>
>>>  #include <linux/swap.h>
>>>  #include <linux/crypto.h>
>>>  #include <linux/scatterlist.h>
>>> @@ -71,6 +70,8 @@ static u64 zswap_reject_compress_poor;
>>>  static u64 zswap_reject_alloc_fail;
>>>  /* Store failed because the entry metadata could not be allocated (rare) */
>>>  static u64 zswap_reject_kmemcache_fail;
>>> +/* Store failed because xarray can't insert the entry*/
>>> +static u64 zswap_reject_xarray_fail;
>>>
>>>  /* Shrinker work queue */
>>>  static struct workqueue_struct *shrink_wq;
>>> @@ -196,7 +197,6 @@ static struct {
>>>   * This structure contains the metadata for tracking a single compressed
>>>   * page within zswap.
>>>   *
>>> - * rbnode - links the entry into red-black tree for the appropriate swap type
>>>   * swpentry - associated swap entry, the offset indexes into the red-black tree
>>>   * length - the length in bytes of the compressed page data.  Needed during
>>>   *          decompression. For a same value filled page length is 0, and both
>>> @@ -208,7 +208,6 @@ static struct {
>>>   * lru - handle to the pool's lru used to evict pages.
>>>   */
>>>  struct zswap_entry {
>>> -     struct rb_node rbnode;
>>>       swp_entry_t swpentry;
>>>       unsigned int length;
>>>       struct zswap_pool *pool;
>>> @@ -220,12 +219,7 @@ struct zswap_entry {
>>>       struct list_head lru;
>>>  };
>>>
>>> -struct zswap_tree {
>>> -     struct rb_root rbroot;
>>> -     spinlock_t lock;
>>> -};
>>> -
>>> -static struct zswap_tree *zswap_trees[MAX_SWAPFILES];
>>> +static struct xarray *zswap_trees[MAX_SWAPFILES];
>>>  static unsigned int nr_zswap_trees[MAX_SWAPFILES];
>>>
>>>  /* RCU-protected iteration */
>>> @@ -253,10 +247,10 @@ static bool zswap_has_pool;
>>>  * helpers and fwd declarations
>>>  **********************************/
>>>
>>> -static inline struct zswap_tree *swap_zswap_tree(swp_entry_t swp)
>>> +static inline struct xarray *swap_zswap_tree(swp_entry_t swp)
>>>  {
>>> -     return &zswap_trees[swp_type(swp)][swp_offset(swp)
>>> -             >> SWAP_ADDRESS_SPACE_SHIFT];
>>> +     return zswap_trees[swp_type(swp)] + (swp_offset(swp)
>>> +             >> SWAP_ADDRESS_SPACE_SHIFT);
>>>  }
>>>
>>>  #define zswap_pool_debug(msg, p)                             \
>>> @@ -805,60 +799,38 @@ void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg)
>>>  }
>>>
>>>  /*********************************
>>> -* rbtree functions
>>> +* xarray functions
>>>  **********************************/
>>> -static struct zswap_entry *zswap_rb_search(struct rb_root *root, pgoff_t offset)
>>> +static struct zswap_entry *zswap_xa_search_and_erase(struct xarray *tree, pgoff_t offset)
>>>  {
>>> -     struct rb_node *node = root->rb_node;
>>> -     struct zswap_entry *entry;
>>> -     pgoff_t entry_offset;
>>> -
>>> -     while (node) {
>>> -             entry = rb_entry(node, struct zswap_entry, rbnode);
>>> -             entry_offset = swp_offset(entry->swpentry);
>>> -             if (entry_offset > offset)
>>> -                     node = node->rb_left;
>>> -             else if (entry_offset < offset)
>>> -                     node = node->rb_right;
>>> -             else
>>> -                     return entry;
>>> -     }
>>> -     return NULL;
>>> +     return xa_erase(tree, offset);
>>>  }
>>>
>>>  /*
>>> + * Expects xa_lock to be held on 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
>>> + * the existing entry is stored in old and erased from the tree.
>>> + * Function return error on insert.
>>>   */
>>> -static int zswap_rb_insert(struct rb_root *root, struct zswap_entry *entry,
>>> -                     struct zswap_entry **dupentry)
>>> +static int zswap_xa_insert(struct xarray *tree, struct zswap_entry *entry,
>>> +                        struct zswap_entry **old)
>>>  {
>>> -     struct rb_node **link = &root->rb_node, *parent = NULL;
>>> -     struct zswap_entry *myentry;
>>> -     pgoff_t myentry_offset, entry_offset = swp_offset(entry->swpentry);
>>> -
>>> -     while (*link) {
>>> -             parent = *link;
>>> -             myentry = rb_entry(parent, struct zswap_entry, rbnode);
>>> -             myentry_offset = swp_offset(myentry->swpentry);
>>> -             if (myentry_offset > entry_offset)
>>> -                     link = &(*link)->rb_left;
>>> -             else if (myentry_offset < entry_offset)
>>> -                     link = &(*link)->rb_right;
>>> -             else {
>>> -                     *dupentry = myentry;
>>> -                     return -EEXIST;
>>> -             }
>>> -     }
>>> -     rb_link_node(&entry->rbnode, parent, link);
>>> -     rb_insert_color(&entry->rbnode, root);
>>> -     return 0;
>>> -}
>>> +     int err;
>>> +     struct zswap_entry *e;
>>> +     pgoff_t offset = swp_offset(entry->swpentry);
>>>
>>> -static void zswap_rb_erase(struct rb_root *root, struct zswap_entry *entry)
>>> -{
>>> -     rb_erase(&entry->rbnode, root);
>>> -     RB_CLEAR_NODE(&entry->rbnode);
>>> +     e = __xa_store(tree, offset, entry, GFP_KERNEL);
>>> +     err = xa_err(e);
>>> +
>>> +     if (err) {
>>> +             e = __xa_erase(tree, offset);
> 
> zswap_xa_insert will always erase the old entry, even when __xa_store fails.
> 
>>> +             if (err == -ENOMEM)
>>> +                     zswap_reject_alloc_fail++;
>>> +             else
>>> +                     zswap_reject_xarray_fail++;
>>> +     }
>>> +     *old = e;
> 
> Old pointer is set regardless of the error.

Ok, I get it. The "old" pointer is always set on return.

> 
>>> +     return err;
>>>  }
>>>
>>>  /*********************************
>>> @@ -872,7 +844,6 @@ static struct zswap_entry *zswap_entry_cache_alloc(gfp_t gfp, int nid)
>>>       entry = kmem_cache_alloc_node(zswap_entry_cache, gfp, nid);
>>>       if (!entry)
>>>               return NULL;
>>> -     RB_CLEAR_NODE(&entry->rbnode);
>>>       return entry;
>>>  }
>>>
>>> @@ -914,17 +885,6 @@ static void zswap_entry_free(struct zswap_entry *entry)
>>>       zswap_update_total_size();
>>>  }
>>>
>>> -/*
>>> - * The caller hold the tree lock and search the entry from the tree,
>>> - * so it must be on the tree, remove it from the tree and free it.
>>> - */
>>> -static void zswap_invalidate_entry(struct zswap_tree *tree,
>>> -                                struct zswap_entry *entry)
>>> -{
>>> -     zswap_rb_erase(&tree->rbroot, entry);
>>> -     zswap_entry_free(entry);
>>> -}
>>> -
>>>  /*********************************
>>>  * compressed storage functions
>>>  **********************************/
>>> @@ -1113,7 +1073,9 @@ static void zswap_decompress(struct zswap_entry *entry, struct page *page)
>>>  static int zswap_writeback_entry(struct zswap_entry *entry,
>>>                                swp_entry_t swpentry)
>>>  {
>>> -     struct zswap_tree *tree;
>>> +     struct xarray *tree;
>>> +     pgoff_t offset = swp_offset(swpentry);
>>> +     struct zswap_entry *e;
>>>       struct folio *folio;
>>>       struct mempolicy *mpol;
>>>       bool folio_was_allocated;
>>> @@ -1150,19 +1112,14 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
>>>        * be dereferenced.
>>>        */
>>>       tree = swap_zswap_tree(swpentry);
>>> -     spin_lock(&tree->lock);
>>> -     if (zswap_rb_search(&tree->rbroot, swp_offset(swpentry)) != entry) {
>>> -             spin_unlock(&tree->lock);
>>> +     e = zswap_xa_search_and_erase(tree, offset);
>>> +     if (e != entry) {
>>
>> IIUC, here we should use xa_cmpxchg() instead of erasing it unconditionally.
> 
> Good catch, I agree with your suggestion. I will spin a V3 to correct that.
> 
>>
>>>               delete_from_swap_cache(folio);
>>>               folio_unlock(folio);
>>>               folio_put(folio);
>>>               return -ENOMEM;
>>>       }
>>>
>>> -     /* Safe to deref entry after the entry is verified above. */
>>> -     zswap_rb_erase(&tree->rbroot, entry);
>>> -     spin_unlock(&tree->lock);
>>> -
>>>       zswap_decompress(entry, &folio->page);
>>>
>>>       count_vm_event(ZSWPWB);
>>> @@ -1471,10 +1428,11 @@ bool zswap_store(struct folio *folio)
>>>  {
>>>       swp_entry_t swp = folio->swap;
>>>       pgoff_t offset = swp_offset(swp);
>>> -     struct zswap_tree *tree = swap_zswap_tree(swp);
>>> -     struct zswap_entry *entry, *dupentry;
>>> +     struct xarray *tree = swap_zswap_tree(swp);
>>> +     struct zswap_entry *entry, *old;
>>>       struct obj_cgroup *objcg = NULL;
>>>       struct mem_cgroup *memcg = NULL;
>>> +     int err;
>>>
>>>       VM_WARN_ON_ONCE(!folio_test_locked(folio));
>>>       VM_WARN_ON_ONCE(!folio_test_swapcache(folio));
>>> @@ -1562,21 +1520,25 @@ bool zswap_store(struct folio *folio)
>>>       }
>>>
>>>       /* map */
>>> -     spin_lock(&tree->lock);
>>> +     xa_lock(tree);
>>>       /*
>>>        * The folio may have been dirtied again, invalidate the
>>>        * possibly stale entry before inserting the new entry.
>>>        */
>>> -     if (zswap_rb_insert(&tree->rbroot, entry, &dupentry) == -EEXIST) {
>>> -             zswap_invalidate_entry(tree, dupentry);
>>> -             WARN_ON(zswap_rb_insert(&tree->rbroot, entry, &dupentry));
>>> +     err = zswap_xa_insert(tree, entry, &old);
>>> +     if (old)
>>> +             zswap_entry_free(old);
>>
>> Maybe it's safer to check old after !err, since "old" variable is not initialized
>> to NULL, and zswap_xa_insert() maybe won't overwrite "old" to NULL when err return?
> 
> That is the intended behavior.
> 
> See the above in zswap_xa_insert(). It will always erase and return
> "old" even when the __xa_store() has an error.
> That is because by the time zswap needs to store a new entry at this
> swap entry. The old data is already outdated. We should just remove
> the old data. If __xa_store failed due to out of memory. That is the
> same as allocating an entry out of memory. It is fine to fail
> swap_store. Then the folio will just stay in the swap cache for the
> next time.
> 
> Do you see any ill effects can be caused by deleting the old entry on
> xa_insert error?

No, you're right, we should always delete/free old zswap entry no matter
store success or fail.

> 
>>> +     if (err) {
>>> +             xa_unlock(tree);
>>> +             goto free_zpool;
>>>       }
>>> +
>>>       if (entry->length) {
>>>               INIT_LIST_HEAD(&entry->lru);
>>>               zswap_lru_add(&zswap.list_lru, entry);
>>>               atomic_inc(&zswap.nr_stored);
>>>       }
>>
>> It seems that we can put this part out of the xarray lock section, then it's enough to
>> just use xa_insert().

I wanted to mean xa_store() here.

> 
> It is not enough protection. Consider this race:
> 
>  CPU1                         CPU2
> 
> xa_insert()
>                                    entry = swap_xa_search_and_erase()
>                                    zswap_free_entry(entry)
> 
> if (entry->length)
>      ...
> CPU1 is using entry after free.

Hmm, right, but I don't know how could this race happen? Since the folio we store is
the owner of swap entry, which couldn't be deleted meanwhile, right?

Another problem I just notice is that if xa_store() failed, zswap_same_filled_pages
won't be correct. (Maybe we should move zswap_same_filled_pages inc)

Thanks.