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

[Chris Li <chrisl@kernel.org>]

Hi Chengming,

On Fri, Mar 1, 2024 at 1:38 AM Chris Li <chrisl@kernel.org> wrote:
>
> On Thu, Feb 29, 2024 at 11:24 PM Chengming Zhou
> <zhouchengming@bytedance.com> wrote:
> >
> > 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@bytedance.com> 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 The "struct zswap_tree" has been replaced by "struct xarray".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@bytedance.com>
> > >>> ---
> > >>>
> > >>>
> > >>> Signed-off-by: Chris Li <chrisl@kernel.org>
> > >>> ---
> > >>> 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@kernel.org
> > >>> ---
> > >>>  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?
>
> I will need to think about it more. Agree the current folio can't
> delete itself. It is possible the folio lock was enough to prevent the
> race.

After sleeping on it a bit, I think it should be safe to reduce the
unlock range and use xa_store directly.

Does anyone have any other objects? If not, I will use xa_store directly for V3.

Chris