[RFC PATCH V2 3/6] ext4: add operations on delayed extent tree

[Yongqiang Yang <xiaoqiangnk@gmail.com>]

This patch adds operations on a delayed extent tree.

Signed-off-by; Yongqiang Yang <xiaoqiangnk@gmail.com>
---
 fs/ext4/Makefile          |    2 +-
 fs/ext4/delayed_extents.c |  412 +++++++++++++++++++++++++++++++++++++++++++++
 fs/ext4/delayed_extents.h |   18 ++
 3 files changed, 431 insertions(+), 1 deletions(-)
 create mode 100644 fs/ext4/delayed_extents.c

diff --git a/fs/ext4/Makefile b/fs/ext4/Makefile
index 56fd8f86..ee16ad3 100644
--- a/fs/ext4/Makefile
+++ b/fs/ext4/Makefile
@@ -7,7 +7,7 @@ obj-$(CONFIG_EXT4_FS) += ext4.o
 ext4-y	:= balloc.o bitmap.o dir.o file.o fsync.o ialloc.o inode.o page-io.o \
 		ioctl.o namei.o super.o symlink.o hash.o resize.o extents.o \
 		ext4_jbd2.o migrate.o mballoc.o block_validity.o move_extent.o \
-		mmp.o indirect.o
+		mmp.o indirect.o delayed_extents.o
 
 ext4-$(CONFIG_EXT4_FS_XATTR)		+= xattr.o xattr_user.o xattr_trusted.o
 ext4-$(CONFIG_EXT4_FS_POSIX_ACL)	+= acl.o
diff --git a/fs/ext4/delayed_extents.c b/fs/ext4/delayed_extents.c
new file mode 100644
index 0000000..8da7b78
--- /dev/null
+++ b/fs/ext4/delayed_extents.c
@@ -0,0 +1,412 @@
+/* 
+ *  fs/ext4/delayed_extents.c
+ *
+ * Written by Yongqiang Yang <xiaoqiangnk@gmail.com>
+ *
+ * Ext4 delayed extents core functions.
+ */
+#include <linux/rbtree.h>
+#include "ext4.h"
+#include "delayed_extents.h"
+#include "ext4_extents.h"
+
+/*
+ * delayed extent implementation for ext4.
+ *
+ *
+ * ==========================================================================
+ * 1. Why delayed extent implementation ?
+ *
+ * Without delayed extent, ext4 identifies a delayed extent by looking up
+ * page cache, this has several deficiencies - complicated, buggy, and
+ * inefficient code.
+ *
+ * FIEMAP, SEEK_HOLE/DATA, bigalloc, punch hole and writeout all need to know if
+ * a block or a range of blocks are belonged to a delayed extent.
+ *
+ * Let us have a look at how they do without delayed extents implementation.
+ *   --	FIEMAP
+ *	FIEMAP looks up page cache to identify delayed allocations from holes.
+ *
+ *   --	SEEK_HOLE/DATA
+ *	SEEK_HOLE/DATA has the same problem as FIEMAP.
+ *
+ *   --	bigalloc
+ *	bigalloc looks up page cache to figure out if a block is already
+ *	under delayed allocation or not to determine whether quota reserving
+ *	is needed for the cluster.
+ *
+ *   -- punch hole
+ *	punch hole looks up page cache to identify a delayed extent.
+ *
+ *   --	writeout
+ *	Writeout looks up whole page cache to see if a buffer is mapped, If
+ *	there are not very many delayed buffers, then it is time comsuming.
+ *
+ * With delayed extents implementation, FIEMAP, SEEK_HOLE/DATA, bigalloc and
+ * writeout can figure out if a block or a range of blocks is under delayed
+ * allocation(belonged to a delayed extent) or not by searching the delayed
+ * extent tree.
+ *
+ *
+ * ==========================================================================
+ * 2. ext4 delayed extents impelmentation
+ *
+ *   --	delayed extent
+ *	A delayed extent is a range of blocks which are contiguous logically and
+ *	under delayed allocation.  Unlike extent in ext4, delayed extent in ext4
+ *	is a in-memory struct, there is no corresponding on-disk data.  There is
+ *	no limit on length of delayed extent, so a delayed extent can contain as
+ *	many blocks as they are contiguous logically.
+ *
+ *   --	delayed extent tree
+ *	Every inode has a delayed extent tree and all under delayed allocation
+ *	blocks are added to the tree as dealyed extents.  Delayed extents in
+ *	the tree are ordered by logical block no.
+ *
+ *   --	operations on a delayed extent tree
+ *	There are three operations on a delayed extent tree: find next delayed
+ *	extent, adding a space(a range of blocks) and removing a space.
+ *
+ *   --	race on a delayed extent tree
+ *	Delayed extent tree is protected inode->i_data_sem like extent tree.
+ *
+ *
+ * ==========================================================================
+ * 3. performance analysis
+ *   --	overhead
+ *	1. Apart from operations on a delayed extent tree, we need to
+ *	down_write(inode->i_data_sem) in delayed write path to maintain delayed
+ *	extent tree, this can have impact on parallel read-write and write-write
+ *
+ *	2. There is a cache extent for write access, so if writes are not very
+ *	random, adding space operaions are in O(1) time.
+ *
+ *   -- gain
+ *	3. Code is much simpler, more readable, more maintainable and
+ *      more efficient.
+ *
+ * 4. TODO
+ *   -- cache_de in de_find_extent
+ *	de_find_extent is used heavily by bgalloc, so de_find_extent should use
+ *	cache_de.
+ *
+ */
+static struct kmem_cache *ext4_de_cachep;
+
+int __init ext4_init_de(void)
+{
+	ext4_de_cachep = KMEM_CACHE(delayed_extent, SLAB_RECLAIM_ACCOUNT);
+	if (ext4_de_cachep == NULL)
+		return -ENOMEM;
+	return 0;
+}
+
+void ext4_exit_de(void)
+{
+	if (ext4_de_cachep)
+		kmem_cache_destroy(ext4_de_cachep);
+}
+
+void ext4_de_init_tree(struct ext4_de_tree *tree)
+{
+	tree->root = RB_ROOT;
+	tree->cache_de = NULL;
+}
+
+#ifdef DE_DEBUG
+static void ext4_de_print_tree(struct inode *inode)
+{
+	struct ext4_de_tree *tree;
+	struct rb_node *node;
+
+	printk(KERN_DEBUG "delayed extents for inode %lu:", inode->i_ino);
+	tree = &EXT4_I(inode)->i_de_tree;
+	node = rb_first(&tree->root);
+	while(node) {
+		struct delayed_extent *de; 
+		de = rb_entry(node, struct delayed_extent, rb_node);
+		printk(KERN_DEBUG " [%u/%u)", de->start, de->len);
+		node = rb_next(node);
+	}
+	printk(KERN_DEBUG "\n");
+}
+#else
+#define ext4_de_print_tree(inode)
+#endif
+
+static inline ext4_lblk_t delayed_extent_end(struct delayed_extent *de)
+{
+	if (de->start + de->len < de->start)
+		return (ext4_lblk_t)-1;
+	return de->start + de->len;
+}
+
+/*
+ * search through the tree for an delayed_extent with a given offset.  If
+ * it can't be found, try to find next extent.
+ */
+static struct delayed_extent * __de_tree_search(struct rb_root *root,
+						ext4_lblk_t offset)
+{
+	struct rb_node *node = root->rb_node;
+	struct delayed_extent *de = NULL;
+ 
+	while (node) {
+		de = rb_entry(node, struct delayed_extent, rb_node);
+		if (offset < de->start)
+			node = node->rb_left;
+		else if (offset >= delayed_extent_end(de))
+			node = node->rb_right;
+		else
+			return de;
+	}
+
+	if (de && offset < de->start)
+		return de;
+
+	if (de && offset >= delayed_extent_end(de))
+		return rb_entry(rb_next(&de->rb_node),
+				struct delayed_extent, rb_node);
+
+	return NULL;
+}
+
+/*
+ * ext4_de_first_extent_since: find the 1st delayed extent covering @start
+ * if it exists, otherwise, the next extent after @start.
+ *
+ * @inode: the inode which owns delayed extents
+ * @offset: logic block
+ *
+ * Returns next block beyond the found extent.
+ * Delayed extent is returned via @de. 
+ */
+ext4_lblk_t ext4_de_find_extent(struct inode *inode, struct delayed_extent *de)
+{
+	struct ext4_de_tree *tree;
+	struct delayed_extent *de1;
+	struct rb_node *node;
+
+	de->len = 0;
+	tree = &EXT4_I(inode)->i_de_tree;
+	de1 = __de_tree_search(&tree->root, de->start);
+	if (de1) {
+		de->start = de1->start;
+		de->len = de1->len;
+		node = rb_next(&de1->rb_node);
+		if (node) {
+			de1 = rb_entry(node, struct delayed_extent, rb_node);
+			return de1->start;
+		}
+	}
+
+	return EXT_MAX_BLOCKS;
+}
+
+static struct delayed_extent *
+ext4_de_alloc_extent(ext4_lblk_t start, ext4_lblk_t len)
+{
+	struct delayed_extent *de;
+	de = kmem_cache_alloc(ext4_de_cachep, GFP_NOFS);
+	if (de == NULL)
+		return NULL;
+	de->start = start;
+	de->len = len;
+	return de;
+}
+
+static void ext4_de_free_extent(struct delayed_extent *de)
+{
+	kmem_cache_free(ext4_de_cachep, de);
+}
+
+static void ext4_de_try_to_merge_left(struct ext4_de_tree *tree,
+				      struct delayed_extent *de)
+{
+	struct delayed_extent *de1;
+	struct rb_node *node;
+
+	node = rb_prev(&de->rb_node);
+	if (!node)
+		return;
+
+	de1 = rb_entry(node, struct delayed_extent, rb_node);
+	if (delayed_extent_end(de1) == de->start) {
+		de1->len += de->len;
+		rb_erase(&de->rb_node, &tree->root);
+		if (de == tree->cache_de)
+			tree->cache_de = de1;
+		ext4_de_free_extent(de);
+	}
+}
+
+static void ext4_de_try_to_merge_right(struct ext4_de_tree *tree,
+				       struct delayed_extent *de)
+{
+	struct delayed_extent *de1;
+	struct rb_node *node;
+
+	node = rb_next(&de->rb_node);
+	if (!node)
+		return;
+
+	de1 = rb_entry(node, struct delayed_extent, rb_node);
+	if (de1->start == delayed_extent_end(de)) {
+		de->len += de1->len;
+		rb_erase(node, &tree->root);
+		if (de1 == tree->cache_de)
+			tree->cache_de = de;
+		ext4_de_free_extent(de1);
+	}
+}
+
+/*
+ * ext4_de_add_space adds a space to a delayed extent tree.
+ * Caller holds inode->i_data_sem.
+ *
+ * ext4_de_add_space is callyed by ext4_dealyed_write_begin and
+ * ext4_de_remove_space.
+ *
+ * Return 0 on success, error code on failure.
+ */
+int ext4_de_add_space(struct inode *inode, ext4_lblk_t offset, ext4_lblk_t len)
+{
+	struct ext4_de_tree *tree = &EXT4_I(inode)->i_de_tree;
+	struct rb_node **p = &tree->root.rb_node;
+	struct rb_node *parent = NULL;
+	struct delayed_extent *de;
+	ext4_lblk_t end = offset + len;
+
+	BUG_ON(end <= offset);
+
+	de = tree->cache_de;
+	de_debug("add [%u/%u) to delayed extent list of inode %lu\n",
+		 offset, len, inode->i_ino);
+
+	if (de && delayed_extent_end(de) == offset) {
+		de->len += len;
+		ext4_de_try_to_merge_right(tree, de);
+		goto out;
+	} else if (de && de->start == end) {
+		de->start = offset;
+		de->len += len;
+		ext4_de_try_to_merge_left(tree, de);
+		goto out;
+	} else if (de && de->start <= offset &&
+		   delayed_extent_end(de) >= end)
+		goto out;
+
+	while (*p) {
+		parent = *p;
+		de = rb_entry(parent, struct delayed_extent, rb_node);
+
+		if (offset < de->start) {
+			if (end == de->start) {
+				de->len += len;
+				de->start = offset;
+				goto out;
+			}
+			p = &(*p)->rb_left;
+		} else if (offset > delayed_extent_end(de)) {
+			if (delayed_extent_end(de) == offset) {
+				de->len += len;
+				goto out;
+			}
+			p = &(*p)->rb_right;
+		} else
+			goto out;
+	}
+
+	de = ext4_de_alloc_extent(offset, len);
+	if (!de)
+		return -ENOMEM;
+	rb_link_node(&de->rb_node, parent, p);
+	rb_insert_color(&de->rb_node, &tree->root);
+
+out:
+	tree->cache_de = de;
+	ext4_de_print_tree(inode);
+
+	return 0;
+}
+
+/*
+ * ext4_de_remove_space() removes a space from a delayed extent tree.
+ * Caller holds inode->i_data_sem.
+ *
+ * Return 0 on success, error code on failure.
+ */
+int ext4_de_remove_space(struct inode *inode, ext4_lblk_t offset,
+			 ext4_lblk_t len)
+{
+	struct rb_node *node;
+	struct ext4_de_tree *tree;
+	struct delayed_extent *de;
+	struct delayed_extent orig_de;
+	ext4_lblk_t len1, len2, end;
+
+	de_debug("remove [%u/%u) from delayed extent list of inode %lu\n",
+		 offset, len, inode->i_ino);
+
+	end = offset + len;
+	BUG_ON(end <= offset);
+	tree = &EXT4_I(inode)->i_de_tree;
+	de = __de_tree_search(&tree->root, offset);
+	if (!de)
+		goto out;
+
+	orig_de.start = de->start;
+	orig_de.len = de->len;
+	len1 = offset > de->start ? offset - de->start : 0;
+	len2 = delayed_extent_end(de) > end ?
+	       delayed_extent_end(de) - end : 0;
+	if (len1 > 0)
+		de->len = len1;
+	if (len2 > 0) {
+		if (len1 > 0) {
+			int err;
+			err = ext4_de_add_space(inode, end, len2);
+			if (err) {
+				de->start = orig_de.start;
+				de->len = orig_de.len;
+				return err;
+			}
+		} else {
+			de->start = end;
+			de->len = len2;
+		}
+		goto out;
+	}
+
+	if (len1 > 0) {
+		node = rb_next(&de->rb_node);
+		if (!node)
+			de = rb_entry(node, struct delayed_extent, rb_node);
+		else
+			de = NULL;
+	}
+
+	while (de && delayed_extent_end(de) <= end) {
+		node = rb_next(&de->rb_node);
+		rb_erase(&de->rb_node, &tree->root);
+		if (de == tree->cache_de)
+			tree->cache_de = NULL;
+		ext4_de_free_extent(de);
+		if (!node) {
+			de = NULL;
+			break;
+		}
+		de = rb_entry(node, struct delayed_extent, rb_node);
+	}
+
+	if (de && de->start < end) {
+		len1 = delayed_extent_end(de) - end;
+		de->start = end;
+		de->len = len1;
+	}
+
+out:
+	ext4_de_print_tree(inode);
+	return 0;
+}
diff --git a/fs/ext4/delayed_extents.h b/fs/ext4/delayed_extents.h
index 66d6390..3f649d9 100644
--- a/fs/ext4/delayed_extents.h
+++ b/fs/ext4/delayed_extents.h
@@ -8,6 +8,13 @@
 #ifndef _EXT4_DELAYED_EXTENTS_H
 #define _EXT4_DELAYED_EXTENTS_H
 
+#define DE_DEBUG
+#ifdef DE_DEBUG
+#define de_debug(a...)		printk(a)
+#else
+#define de_debug(a...)
+#endif
+
 struct delayed_extent {
 	struct rb_node rb_node;
 	ext4_lblk_t start;	/* first block extent covers */
@@ -19,4 +26,15 @@ struct ext4_de_tree {
 	struct delayed_extent *cache_de;	/* recently accessed extent */
 };
 
+extern int __init ext4_init_de(void);
+extern void ext4_exit_de(void);
+extern void ext4_de_init_tree(struct ext4_de_tree *tree);
+
+extern int ext4_de_add_space(struct inode *inode, ext4_lblk_t start,
+				ext4_lblk_t len);
+extern int ext4_de_remove_space(struct inode *inode, ext4_lblk_t start,
+				ext4_lblk_t len);
+extern ext4_lblk_t ext4_de_find_extent(struct inode *inode,
+				struct delayed_extent *de);
+
 #endif /* _EXT4_DELAYED_EXTENTS_H */
-- 
1.7.5.1