Hi Chengming, On Fri, Mar 1, 2024 at 1:38 AM Chris Li <chrisl@xxxxxxxxxx> wrote: > > On Thu, Feb 29, 2024 at 11:24 PM Chengming Zhou > <zhouchengming@xxxxxxxxxxxxx> 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@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 The "struct zswap_tree" has been replaced by "struct xarray".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? > > 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
