While running btrfs/011 in a loop I would often ASSERT() while trying to
add a new free space entry that already existed, or get an -EEXIST while
adding a new block to the extent tree, which is another indication of
double allocation.
This occurs because when we do the free space tree population, we create
the new root and then populate the tree and commit the transaction.
The problem is when you create a new root, the root node and commit root
node are the same. This means that caching a block group before the
transaction is committed can race with other operations modifying the
free space tree, and thus you can get double adds and other sort of
shenanigans. This is only a problem for the first transaction, once
we've committed the transaction we created the free space tree in we're
OK to use the free space tree to cache block groups.
Fix this by marking the fs_info as unsafe to load the free space tree,
and fall back on the old slow method. We could be smarter than this,
for example caching the block group while we're populating the free
space tree, but since this is a serious problem I've opted for the
simplest solution.
cc: stable@xxxxxxxxxxxxxxx
Fixes: a5ed91828518 ("Btrfs: implement the free space B-tree")
Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
---
fs/btrfs/block-group.c | 11 ++++++++++-
fs/btrfs/ctree.h | 3 +++
fs/btrfs/free-space-tree.c | 9 ++++++++-
3 files changed, 21 insertions(+), 2 deletions(-)
diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
index 52f2198d44c9..b8bbdd95743e 100644
--- a/fs/btrfs/block-group.c
+++ b/fs/btrfs/block-group.c
@@ -673,7 +673,16 @@ static noinline void caching_thread(struct btrfs_work *work)
wake_up(&caching_ctl->wait);
}
- if (btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE))
+ /*
+ * If we are in the transaction that populated the free space tree we
+ * can't actually cache from the free space tree as our commit root and
+ * real root are the same, so we could change the contents of the blocks
+ * while caching. Instead do the slow caching in this case, and after
+ * the transaction has committed we will be safe.
+ */
+ if (btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE) &&
+ !(test_bit(BTRFS_FS_FREE_SPACE_TREE_UNTRUSTED,
+ &fs_info->flags)))
ret = load_free_space_tree(caching_ctl);
else
ret = load_extent_tree_free(caching_ctl);
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 3935d297d198..4a60d81da5cb 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -562,6 +562,9 @@ enum {
/* Indicate that we need to cleanup space cache v1 */
BTRFS_FS_CLEANUP_SPACE_CACHE_V1,
+
+ /* Indicate that we can't trust the free space tree for caching yet. */
+ BTRFS_FS_FREE_SPACE_TREE_UNTRUSTED,
};
/*
diff --git a/fs/btrfs/free-space-tree.c b/fs/btrfs/free-space-tree.c
index e33a65bd9a0c..8fbda221f4b5 100644
--- a/fs/btrfs/free-space-tree.c
+++ b/fs/btrfs/free-space-tree.c
@@ -1150,6 +1150,7 @@ int btrfs_create_free_space_tree(struct btrfs_fs_info *fs_info)
return PTR_ERR(trans);
set_bit(BTRFS_FS_CREATING_FREE_SPACE_TREE, &fs_info->flags);
+ set_bit(BTRFS_FS_FREE_SPACE_TREE_UNTRUSTED, &fs_info->flags);
free_space_root = btrfs_create_tree(trans,
BTRFS_FREE_SPACE_TREE_OBJECTID);
if (IS_ERR(free_space_root)) {
@@ -1171,8 +1172,14 @@ int btrfs_create_free_space_tree(struct btrfs_fs_info *fs_info)
btrfs_set_fs_compat_ro(fs_info, FREE_SPACE_TREE);
btrfs_set_fs_compat_ro(fs_info, FREE_SPACE_TREE_VALID);
clear_bit(BTRFS_FS_CREATING_FREE_SPACE_TREE, &fs_info->flags);
+ ret = btrfs_commit_transaction(trans);
- return btrfs_commit_transaction(trans);
+ /*
+ * Now that we've committed the transaction any reading of our commit
+ * root will be safe, so we can caching from the free space tree now.
+ */
+ clear_bit(BTRFS_FS_FREE_SPACE_TREE_UNTRUSTED, &fs_info->flags);
+ return ret;
abort:
clear_bit(BTRFS_FS_CREATING_FREE_SPACE_TREE, &fs_info->flags);
--
2.26.2
