From f7942430e40f14c6d2ca48a1875add509938c07d Mon Sep 17 00:00:00 2001 From: Johannes Weiner Date: Mon, 12 Dec 2016 16:43:41 -0800 Subject: [PATCH] lib: radix-tree: native accounting of exceptional entries The way the page cache is sneaking shadow entries of evicted pages into the radix tree past the node entry accounting and tracking them manually in the upper bits of node->count is fraught with problems. These shadow entries are marked in the tree as exceptional entries, which are a native concept to the radix tree. Maintain an explicit counter of exceptional entries in the radix tree node. Subsequent patches will switch shadow entry tracking over to that counter. DAX and shmem are the other users of exceptional entries. Since slot replacements that change the entry type from regular to exceptional must now be accounted, introduce a __radix_tree_replace() function that does replacement and accounting, and switch DAX and shmem over. The increase in radix tree node size is temporary. A followup patch switches the shadow tracking to this new scheme and we'll no longer need the upper bits in node->count and shrink that back to one byte. Link: http://lkml.kernel.org/r/20161117192945.GA23430@cmpxchg.org Signed-off-by: Johannes Weiner Reviewed-by: Jan Kara Cc: Kirill A. Shutemov Cc: Hugh Dickins Cc: Matthew Wilcox Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- fs/dax.c | 5 +++-- include/linux/radix-tree.h | 10 ++++++--- lib/radix-tree.c | 46 +++++++++++++++++++++++++++++++++++--- mm/shmem.c | 8 +++---- 4 files changed, 57 insertions(+), 12 deletions(-) diff --git a/fs/dax.c b/fs/dax.c index 014defd2e744..db78bae0dc0f 100644 --- a/fs/dax.c +++ b/fs/dax.c @@ -643,12 +643,13 @@ static void *dax_insert_mapping_entry(struct address_space *mapping, } mapping->nrexceptional++; } else { + struct radix_tree_node *node; void **slot; void *ret; - ret = __radix_tree_lookup(page_tree, index, NULL, &slot); + ret = __radix_tree_lookup(page_tree, index, &node, &slot); WARN_ON_ONCE(ret != entry); - radix_tree_replace_slot(slot, new_entry); + __radix_tree_replace(page_tree, node, slot, new_entry); } if (vmf->flags & FAULT_FLAG_WRITE) radix_tree_tag_set(page_tree, index, PAGECACHE_TAG_DIRTY); diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h index af3581b8a451..7ced8a70cc8b 100644 --- a/include/linux/radix-tree.h +++ b/include/linux/radix-tree.h @@ -85,9 +85,10 @@ static inline bool radix_tree_is_internal_node(void *ptr) #define RADIX_TREE_COUNT_MASK ((1UL << RADIX_TREE_COUNT_SHIFT) - 1) struct radix_tree_node { - unsigned char shift; /* Bits remaining in each slot */ - unsigned char offset; /* Slot offset in parent */ - unsigned int count; + unsigned char shift; /* Bits remaining in each slot */ + unsigned char offset; /* Slot offset in parent */ + unsigned int count; /* Total entry count */ + unsigned char exceptional; /* Exceptional entry count */ union { struct { /* Used when ascending tree */ @@ -276,6 +277,9 @@ void *__radix_tree_lookup(struct radix_tree_root *root, unsigned long index, struct radix_tree_node **nodep, void ***slotp); void *radix_tree_lookup(struct radix_tree_root *, unsigned long); void **radix_tree_lookup_slot(struct radix_tree_root *, unsigned long); +void __radix_tree_replace(struct radix_tree_root *root, + struct radix_tree_node *node, + void **slot, void *item); bool __radix_tree_delete_node(struct radix_tree_root *root, struct radix_tree_node *node); void *radix_tree_delete_item(struct radix_tree_root *, unsigned long, void *); diff --git a/lib/radix-tree.c b/lib/radix-tree.c index 8e6d552c40dd..7885796d35ae 100644 --- a/lib/radix-tree.c +++ b/lib/radix-tree.c @@ -220,10 +220,10 @@ static void dump_node(struct radix_tree_node *node, unsigned long index) { unsigned long i; - pr_debug("radix node: %p offset %d tags %lx %lx %lx shift %d count %d parent %p\n", + pr_debug("radix node: %p offset %d tags %lx %lx %lx shift %d count %d exceptional %d parent %p\n", node, node->offset, node->tags[0][0], node->tags[1][0], node->tags[2][0], - node->shift, node->count, node->parent); + node->shift, node->count, node->exceptional, node->parent); for (i = 0; i < RADIX_TREE_MAP_SIZE; i++) { unsigned long first = index | (i << node->shift); @@ -522,8 +522,13 @@ static int radix_tree_extend(struct radix_tree_root *root, node->offset = 0; node->count = 1; node->parent = NULL; - if (radix_tree_is_internal_node(slot)) + if (radix_tree_is_internal_node(slot)) { entry_to_node(slot)->parent = node; + } else { + /* Moving an exceptional root->rnode to a node */ + if (radix_tree_exceptional_entry(slot)) + node->exceptional = 1; + } node->slots[0] = slot; slot = node_to_entry(node); rcu_assign_pointer(root->rnode, slot); @@ -649,6 +654,8 @@ int __radix_tree_insert(struct radix_tree_root *root, unsigned long index, if (node) { unsigned offset = get_slot_offset(node, slot); node->count++; + if (radix_tree_exceptional_entry(item)) + node->exceptional++; BUG_ON(tag_get(node, 0, offset)); BUG_ON(tag_get(node, 1, offset)); BUG_ON(tag_get(node, 2, offset)); @@ -746,6 +753,37 @@ void *radix_tree_lookup(struct radix_tree_root *root, unsigned long index) } EXPORT_SYMBOL(radix_tree_lookup); +/** + * __radix_tree_replace - replace item in a slot + * @root: radix tree root + * @node: pointer to tree node + * @slot: pointer to slot in @node + * @item: new item to store in the slot. + * + * For use with __radix_tree_lookup(). Caller must hold tree write locked + * across slot lookup and replacement. + */ +void __radix_tree_replace(struct radix_tree_root *root, + struct radix_tree_node *node, + void **slot, void *item) +{ + void *old = rcu_dereference_raw(*slot); + int exceptional; + + WARN_ON_ONCE(radix_tree_is_internal_node(item)); + WARN_ON_ONCE(!!item - !!old); + + exceptional = !!radix_tree_exceptional_entry(item) - + !!radix_tree_exceptional_entry(old); + + WARN_ON_ONCE(exceptional && !node && slot != (void **)&root->rnode); + + if (node) + node->exceptional += exceptional; + + rcu_assign_pointer(*slot, item); +} + /** * radix_tree_tag_set - set a tag on a radix tree node * @root: radix tree root @@ -1561,6 +1599,8 @@ void *radix_tree_delete_item(struct radix_tree_root *root, delete_sibling_entries(node, node_to_entry(slot), offset); node->slots[offset] = NULL; node->count--; + if (radix_tree_exceptional_entry(entry)) + node->exceptional--; __radix_tree_delete_node(root, node); diff --git a/mm/shmem.c b/mm/shmem.c index ec7aa562343e..3149ddee8f55 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -300,18 +300,18 @@ void shmem_uncharge(struct inode *inode, long pages) static int shmem_radix_tree_replace(struct address_space *mapping, pgoff_t index, void *expected, void *replacement) { + struct radix_tree_node *node; void **pslot; void *item; VM_BUG_ON(!expected); VM_BUG_ON(!replacement); - pslot = radix_tree_lookup_slot(&mapping->page_tree, index); - if (!pslot) + item = __radix_tree_lookup(&mapping->page_tree, index, &node, &pslot); + if (!item) return -ENOENT; - item = radix_tree_deref_slot_protected(pslot, &mapping->tree_lock); if (item != expected) return -ENOENT; - radix_tree_replace_slot(pslot, replacement); + __radix_tree_replace(&mapping->page_tree, node, pslot, replacement); return 0; }