Re: [PATCH 1/1 version2] Extent overlap bugfix in ext4

[Mingming Cao <cmm@us.ibm.com>]

Hi, Amit,

Have you looked at ext4_ext_walk_space()? It calculate the right extent 
length to allocate to avoid overlap before calling block allocation 
callback function is called.

Amit K. Arora wrote:
>  /*
> + * ext4_ext_check_overlap:
> + * check if a portion of the "newext" extent overlaps with an
> + * existing extent.
> + *
> + * If there is an overlap discovered, it returns the (logical) block
> + * number of the first block in the next extent (the existing extent
> + * which covers few of the new requested blocks)
> + * If there is no overlap found, it returns 0.
> + */

What if the start logical block of the exisitng extent is 0 and there is 
overlap? I think that is possible. For example, the exisitng extent is 
(0,100) and you want to insert new extent (0,500), this will certainly 
fail to report the overlap.


> +unsigned int ext4_ext_check_overlap(struct inode *inode,

We shall be consistant with other data type used for logical block, 
right now is unsigned long. Probably replace that with ext4_fsblk_t type 
when that cleanup is introduced.

> +					struct ext4_extent *newext,
> +					struct ext4_ext_path *path)
> +{
> +	unsigned int depth, b1, len1, b2;
> +
unsigned long type for b1 and b2.

> +	b1 = le32_to_cpu(newext->ee_block);
> +	len1 = le16_to_cpu(newext->ee_len);
> +	depth = ext_depth(inode);
> +	if (!path[depth].p_ext)
> +		goto out;
> +	b2 = le32_to_cpu(path[depth].p_ext->ee_block);
> +
> +	/* get the next allocated block if the extent in the path
> +	 * is before the requested block(s) */
> +	if (b2 < b1) {
> +		b2 = ext4_ext_next_allocated_block(path);
> +		if (b2 == EXT_MAX_BLOCK)
> +			goto out;
> +	}
> +
> +	if (b1 + len1 > b2)
> +		return b2;
> +out:
> +	return 0;
> +}
> +

Since this overlap check function is called inside 
ext4_ext_insert_extent(), I think this function should check for all 
kinds of overlaps. Here you only check if the new extent is overlap with 
the next extent. Looking at ext4_ext_walk_space(), there are total three 
kinds of overlaps:
1) righ port of new extent overlap with path->p_ext,
2) left port of new extent overlap with path->p_ext
2) right port of new extent overlap with next extent

I think we are almost repeating the same logic in ext4_ext_walk_space() 
here.

> +/*
>   * ext4_ext_insert_extent:
>   * tries to merge requsted extent into the existing extent or
>   * inserts requested extent as new one into the tree,
> @@ -1133,12 +1170,25 @@ int ext4_ext_insert_extent(handle_t *han
>  	struct ext4_extent *nearex; /* nearest extent */
>  	struct ext4_ext_path *npath = NULL;
>  	int depth, len, err, next;
> +	unsigned int oblock;
>
unsigned long type for oblock

>  	BUG_ON(newext->ee_len == 0);
>  	depth = ext_depth(inode);
>  	ex = path[depth].p_ext;
>  	BUG_ON(path[depth].p_hdr == NULL);
> 
> +	/* check for overlap */
> +	oblock = ext4_ext_check_overlap(inode, newext, path);
> +	if (oblock) {
> +		printk(KERN_ERR "ERROR: newext=%u/%u overlaps with an "
> +				"existing extent, which starts with %u\n",
> +				le32_to_cpu(newext->ee_block),
> +				le16_to_cpu(newext->ee_len),
> +				oblock);
> +		ext4_ext_show_leaf(inode, path);
> +		BUG();
> +	}

How about return true or false from ext4_ext_check_overlap()? Inside 
that function put the correct new extent logical block number and extent 
length that safe to insert? Afterall the returning oblock is used in 
ext4_ext_get_blocks() to calculate the safe extent to allocate.

> +
>  	/* try to insert block into found extent and return */
>  	if (ex && ext4_can_extents_be_merged(inode, ex, newext)) {
>  		ext_debug("append %d block to %d:%d (from %llu)\n",
> @@ -1984,6 +2034,10 @@ int ext4_ext_get_blocks(handle_t *handle
>  		 */
>  		if (ee_len > EXT_MAX_LEN)
>  			goto out2;
> +
> +		if (iblock < ee_block && iblock + max_blocks >= ee_block)
> +			allocated = ee_block - iblock;
> +
>  		/* if found extent covers block, simply return it */
>  	        if (iblock >= ee_block && iblock < ee_block + ee_len) {
>  			newblock = iblock - ee_block + ee_start;

Here I realize that the way that ext4_ext_get_blocks() handles the 
requested extent has hole on the right side is: simply returns the left 
port of the extent which already has blocks allocated. This is actually 
what non_extent get_blocks does also. caller of get_blocks() including 
preallocation code in ioctl will continue calling get_blocks to allocate 
blocks for the hole.

Probably we shall make this clear in the comment.
> @@ -2016,7 +2070,17 @@ int ext4_ext_get_blocks(handle_t *handle
> 
>  	/* allocate new block */
>  	goal = ext4_ext_find_goal(inode, path, iblock);
> -	allocated = max_blocks;
> +
> +	/* Check if we can really insert (iblock)::(iblock+max_blocks) extent */
> +	newex.ee_block = cpu_to_le32(iblock);
> +	if (!allocated) {
> +		newex.ee_len = cpu_to_le16(max_blocks);
> +		allocated = ext4_ext_check_overlap(inode, &newex, path);
> +		if (allocated)
> +			allocated = allocated - iblock;
> +		else
> +			allocated = max_blocks;
> +	}
>  	newblock = ext4_new_blocks(handle, inode, goal, &allocated, &err);
>  	if (!newblock)
>  		goto out2;
> @@ -2024,7 +2088,6 @@ int ext4_ext_get_blocks(handle_t *handle
>  			goal, newblock, allocated);
> 
>  	/* try to insert new extent into found leaf and return */
> -	newex.ee_block = cpu_to_le32(iblock);
>  	ext4_ext_store_pblock(&newex, newblock);
>  	newex.ee_len = cpu_to_le16(allocated);
>  	err = ext4_ext_insert_extent(handle, inode, path, &newex);
> Index: linux-2.6.19.prealloc/include/linux/ext4_fs_extents.h
> ===================================================================
> --- linux-2.6.19.prealloc.orig/include/linux/ext4_fs_extents.h
> +++ linux-2.6.19.prealloc/include/linux/ext4_fs_extents.h
> @@ -190,6 +190,7 @@ ext4_ext_invalidate_cache(struct inode *
> 
>  extern int ext4_extent_tree_init(handle_t *, struct inode *);
>  extern int ext4_ext_calc_credits_for_insert(struct inode *, struct ext4_ext_path *);
> +extern unsigned int ext4_ext_check_overlap(struct inode *, struct ext4_extent *, struct ext4_ext_path *);
>  extern int ext4_ext_insert_extent(handle_t *, struct inode *, struct ext4_ext_path *, struct ext4_extent *);
>  extern int ext4_ext_walk_space(struct inode *, unsigned long, unsigned long, ext_prepare_callback, void *);
>  extern struct ext4_ext_path * ext4_ext_find_extent(struct inode *, int, struct ext4_ext_path *);
>