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 The "struct zswap_tree" has been replaced by "struct xarray".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.
