Note: This patch is being resubmitted as part of the recall of all the
patches for ext4. It uses 2.6.20-rc5 version as the base.
Problem Description:
-------------------
The ext4_ext_get_blocks() and ext4_ext_insert_extent() routines do not
have a complete check for extent overlap, when a new extent needs to be
inserted in an inode. With the current implementation, an overlap is
possible when the new extent being inserted has ee_block that is not
part of any of the existing extents, but the tail/center portion of this
new extent _is_. This is possible only when we are writing/preallocating
blocks across a hole.
Though this problem was discovered while stress testing persistent
preallocation patches (using modified fsx-linux); this essentially is an
independent problem and should be fixed by a separate patch. Hence this
fix.
The Fix:
-------
The suggested patch fixes this by having a check in get_blocks() for
finding if the new extent overlaps with an existing one. If it does, the
length of the new extent is modified such that the overlap does not
happen at all.
Other option discussed:
----------------------
The other option discussed was to not to use ext4_ext_get_blocks() for
persistent preallocation, and use ext4_ext_walk_space() with some helper
function instead. This was considered because walk_space() already does
a complete check for overlap and hence we can avoid duplication of this
part of the logic in get_blocks(). But, again, there will be a
duplication of code in the new helper function that may be required for
this (like, calling ext4_new_blocks() and ext4_ext_insert_extent()).
Updates from the original (first) version:
-----------------------------------------
This patch takes care of following review comments from Mingming, Alex
and Suparna:
(a) Not to use ext4_ext_find_extent() in check_overlap(), since it is an
expensive operation.
(b) Use "unsigned long" for (logical) block numbers everywhere.
(c) Return true/false by check_overlap(), rather than extent pointer or
the block number.
(d) Update the length of the new extent in check_overlap(), if there is
an overlap detected.
(e) No need to have a check in insert_extent() (i.e. no BUG_ON required)
Here is the patch:
Signed-off-by: Amit Arora <aarora@xxxxxxxxxx>
---
fs/ext4/extents.c | 50 ++++++++++++++++++++++++++++++++++++++--
include/linux/ext4_fs_extents.h | 1
2 files changed, 49 insertions(+), 2 deletions(-)
Index: linux-2.6.20-rc5/fs/ext4/extents.c
===================================================================
--- linux-2.6.20-rc5.orig/fs/ext4/extents.c
+++ linux-2.6.20-rc5/fs/ext4/extents.c
@@ -1129,6 +1129,45 @@ ext4_can_extents_be_merged(struct inode
}
/*
+ * ext4_ext_check_overlap:
+ * check if a portion of the "newext" extent overlaps with an
+ * existing extent.
+ *
+ * If there is an overlap discovered, it updates the length of the newext
+ * such that there will be no overlap, and then returns 1.
+ * If there is no overlap found, it returns 0.
+ */
+unsigned int ext4_ext_check_overlap(struct inode *inode,
+ struct ext4_extent *newext,
+ struct ext4_ext_path *path)
+{
+ unsigned long b1, b2;
+ unsigned int depth, len1;
+
+ 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) {
+ newext->ee_len = cpu_to_le16(b2 - b1);
+ return 1;
+ }
+out:
+ return 0;
+}
+
+/*
* ext4_ext_insert_extent:
* tries to merge requsted extent into the existing extent or
* inserts requested extent as new one into the tree,
@@ -2032,7 +2071,15 @@ 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);
+ newex.ee_len = cpu_to_le16(max_blocks);
+ err = ext4_ext_check_overlap(inode, &newex, path);
+ if (err)
+ allocated = le16_to_cpu(newex.ee_len);
+ else
+ allocated = max_blocks;
newblock = ext4_new_blocks(handle, inode, goal, &allocated, &err);
if (!newblock)
goto out2;
@@ -2040,7 +2087,6 @@ 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-2.6.20-rc5/include/linux/ext4_fs_extents.h
===================================================================
--- linux-2.6.20-rc5.orig/include/linux/ext4_fs_extents.h
+++ linux-2.6.20-rc5/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 *);
--
Regards,
Amit Arora
-
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