linux-ext4 - [PATCH] discard an inode's preallocated blocks after failed allocation

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Message-Id: <20101130232238.4F5A74274C@ruihe.smo.corp.google.com>
Date:	Tue, 30 Nov 2010 15:22:38 -0800 (PST)
From:	jiayingz@...gle.com (Jiaying Zhang)
To:	adilger.kernel@...ger.ca, tytso@....ed
Cc:	linux-ext4@...r.kernel.org
Subject: [PATCH] discard an inode's preallocated blocks after failed allocation

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@...gle.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);
 	ext4_free_blocks(handle, inode, 0, new_blocks[0], 1, 0);
 	for (i = 1; i <= n ; i++) {
 		/*
@@ -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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