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 The "struct zswap_tree" has been replaced by "struct xarray".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. > > + 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? > > + 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(). 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. Chris
