Date:	Thu, 29 Sep 2011 16:05:57 +0800
From:	Tao Ma <tm@....ma>
To:	Yongqiang Yang <xiaoqiangnk@...il.com>
CC:	linux-ext4@...r.kernel.org, jack@...e.cz, jeff.liu@...cle.com,
	achender@...ux.vnet.ibm.com, adityakali@...gle.com
Subject: Re: [RFC PATCH V2 3/6] ext4: add operations on delayed extent tree

Hi yongqiang,
On 09/29/2011 01:08 PM, Yongqiang Yang wrote:
> This patch adds operations on a delayed extent tree.
> 
> Signed-off-by; Yongqiang Yang <xiaoqiangnk@...il.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
<snip>
> +/*
> + * 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);
what if the de is the most right one? rb_next should return NULL in this
case I guess? What is more, you will return a non-NULL value and use it
later in the caller. The kernel will panic. Or do I miss something here?
> +
> +	return NULL;
> +}
> +
> +/*
> + * ext4_de_first_extent_since: find the 1st delayed extent covering @start
> + * if it exists, otherwise, the next extent after @start.
This comment seems to be unrelated to the below function.
> + *
> + * @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);
Sorry, but I don't understand what you are going to do here. In
__de_tree_search, you have already use rb_next to get the next de if
start < del->start. why we still need a rb_next here?
> +		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;
> +	}
is the above what  __de_tree_search try to do?
btw, we'd better have a BUG_ON when we meet with an intersection since
you only check the offset in your rb_tree search. So what if offset <
de->start while offset + len > de->start? It would cause your algorithm
not work I guess.
> +
> +	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);
I don't find any error in your ext4_de_add_space.
> +			if (err) {
> +				de->start = orig_de.start;
> +				de->len = orig_de.len;
> +				return err;
> +			}
> +		} else {
> +			de->start = end;
> +			de->len = len2;
> +		}
> +		goto out;
> +	}

Thanks
Tao
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