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 *);
-
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