[PATCH] discard an inode's preallocated blocks after failed allocation
Date:	Tue, 30 Nov 2010 15:22:38 -0800 (PST)
From:	jiayingz@google.com (Jiaying Zhang)

We have seen kernel crashes caused by the BUG_ON in
ext4_mb_return_to_preallocation() that checks whether the inode's
i_prealloc_list is empty. As I understand, the assumption is that
when ext4_mb_return_to_preallocation() is called from ext4_free_blocks(),
the inode's preallocation list should have been already discarded.
However, although we call ext4_discard_preallocations() during ext4
truncate, we don't always call that function in various failure
cases before calling ext4_free_blocks(). So it is likely to hit this
BUG_ON with disk errors or corrupted fs etc.

To fix the problem, the following patch adds ext4_discard_preallocation()
before ext4_free_blocks() in failed allocation cases. This will discard
any preallocated block extent attached to the inode, but I think it is
probably what we should be doing with failed allocation.

I am also curious whether we can drop the ext4_mb_return_to_preallocation()
call from ext4_free_blocks(). From the comments above that function, it
seems to intent to discard the specified blocks only but all it is currently
doing is the BUG_ON check on whether the inode's preallocation list is empty.
Is there any plan to extend this function later?


ext4: discard an inode's preallocated blocks after failed allocation so that
we won't hit on the BUG_ON check in ext4_mb_return_to_preallocation() later.

Signed-off-by: Jiaying Zhang <jiayingz@google.com>

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 29a4adf..2164df6 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -1056,6 +1056,7 @@ cleanup:
       }

       if (err) {
+		ext4_discard_preallocations(inode);
               /* free all allocated blocks in error case */
               for (i = 0; i < depth; i++) {
                       if (!ablocks[i])
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 4f4362c..456cb4a 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -690,6 +690,7 @@ allocated:
       *err = 0;
       return ret;
 failed_out:
+	ext4_discard_preallocations(inode);
       for (i = 0; i < index; i++)
               ext4_free_blocks(handle, inode, 0, new_blocks[i], 1, 0);
       return ret;
@@ -787,6 +788,7 @@ static int ext4_alloc_branch(handle_t *handle, struct inode *inode,
       return err;
 failed:
       /* Allocation failed, free what we already allocated */
+	ext4_discard_preallocations(inode);
       for (i = 1; i <= n ; i++) {
		BUFFER_TRACE(branch[i].bh, "call jbd2_journal_forget");
		ext4_journal_forget(handle, branch[i].bh);
@@ -878,6 +880,7 @@ static int ext4_splice_branch(handle_t *handle, struct inode *inode,
       return err;

 err_out:
+	ext4_discard_preallocations(inode);
       for (i = 1; i <= num; i++) {
               /*
                * branch[i].bh is newly allocated, so there is no
--
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