[PATCH 5.10 588/717] btrfs: update last_byte_to_unpin in switch_commit_roots

Greg Kroah-Hartman
2020-12-28T14:42:52Z

From: Josef Bacik <josef@toxicpanda.com>

commit 27d56e62e4748c2135650c260024e9904b8c1a0a upstream.

While writing an explanation for the need of the commit_root_sem for
btrfs_prepare_extent_commit, I realized we have a slight hole that could
result in leaked space if we have to do the old style caching. Consider
the following scenario

commit root
+----+----+----+----+----+----+----+
|\\\\| |\\\\|\\\\| |\\\\|\\\\|
+----+----+----+----+----+----+----+
0 1 2 3 4 5 6 7

new commit root
+----+----+----+----+----+----+----+
| | | |\\\\| | |\\\\|
+----+----+----+----+----+----+----+
0 1 2 3 4 5 6 7

Prior to this patch, we run btrfs_prepare_extent_commit, which updates
the last_byte_to_unpin, and then we subsequently run
switch_commit_roots. In this example lets assume that
caching_ctl->progress == 1 at btrfs_prepare_extent_commit() time, which
means that cache->last_byte_to_unpin == 1. Then we go and do the
switch_commit_roots(), but in the meantime the caching thread has made
some more progress, because we drop the commit_root_sem and re-acquired
it. Now caching_ctl->progress == 3. We swap out the commit root and
carry on to unpin.

The race can happen like:

1) The caching thread was running using the old commit root when it
found the extent for [2, 3);

2) Then it released the commit_root_sem because it was in the last
item of a leaf and the semaphore was contended, and set ->progress
to 3 (value of 'last'), as the last extent item in the current leaf
was for the extent for range [2, 3);

3) Next time it gets the commit_root_sem, will start using the new
commit root and search for a key with offset 3, so it never finds
the hole for [2, 3).

So the caching thread never saw [2, 3) as free space in any of the
commit roots, and by the time finish_extent_commit() was called for
the range [0, 3), ->last_byte_to_unpin was 1, so it only returned the
subrange [0, 1) to the free space cache, skipping [2, 3).

In the unpin code we have last_byte_to_unpin == 1, so we unpin [0,1),
but do not unpin [2,3). However because caching_ctl->progress == 3 we
do not see the newly freed section of [2,3), and thus do not add it to
our free space cache. This results in us missing a chunk of free space
in memory (on disk too, unless we have a power failure before writing
the free space cache to disk).

Fix this by making sure the ->last_byte_to_unpin is set at the same time
that we swap the commit roots, this ensures that we will always be
consistent.

CC: stable@vger.kernel.org # 5.8+
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
[ update changelog with Filipe's review comments ]
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/btrfs/ctree.h | 1 -
fs/btrfs/extent-tree.c | 25 -------------------------
fs/btrfs/transaction.c | 42 ++++++++++++++++++++++++++++++++++++++++--
3 files changed, 40 insertions(+), 28 deletions(-)

--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -2593,7 +2593,6 @@ int btrfs_free_reserved_extent(struct bt
u64 start, u64 len, int delalloc);
int btrfs_pin_reserved_extent(struct btrfs_trans_handle *trans, u64 start,
u64 len);
-void btrfs_prepare_extent_commit(struct btrfs_fs_info *fs_info);
int btrfs_finish_extent_commit(struct btrfs_trans_handle *trans);
int btrfs_inc_extent_ref(struct btrfs_trans_handle *trans,
struct btrfs_ref *generic_ref);
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -2730,31 +2730,6 @@ btrfs_inc_block_group_reservations(struc
atomic_inc(&bg->reservations);
}

-void btrfs_prepare_extent_commit(struct btrfs_fs_info *fs_info)
-{
- struct btrfs_caching_control *next;
- struct btrfs_caching_control *caching_ctl;
- struct btrfs_block_group *cache;
-
- down_write(&fs_info->commit_root_sem);
-
- list_for_each_entry_safe(caching_ctl, next,
- &fs_info->caching_block_groups, list) {
- cache = caching_ctl->block_group;
- if (btrfs_block_group_done(cache)) {
- cache->last_byte_to_unpin = (u64)-1;
- list_del_init(&caching_ctl->list);
- btrfs_put_caching_control(caching_ctl);
- } else {
- cache->last_byte_to_unpin = caching_ctl->progress;
- }
- }
-
- up_write(&fs_info->commit_root_sem);
-
- btrfs_update_global_block_rsv(fs_info);
-}
-
/*
* Returns the free cluster for the given space info and sets empty_cluster to
* what it should be based on the mount options.
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -155,6 +155,7 @@ static noinline void switch_commit_roots
struct btrfs_transaction *cur_trans = trans->transaction;
struct btrfs_fs_info *fs_info = trans->fs_info;
struct btrfs_root *root, *tmp;
+ struct btrfs_caching_control *caching_ctl, *next;

down_write(&fs_info->commit_root_sem);
list_for_each_entry_safe(root, tmp, &cur_trans->switch_commits,
@@ -180,6 +181,45 @@ static noinline void switch_commit_roots
spin_lock(&cur_trans->dropped_roots_lock);
}
spin_unlock(&cur_trans->dropped_roots_lock);
+
+ /*
+ * We have to update the last_byte_to_unpin under the commit_root_sem,
+ * at the same time we swap out the commit roots.
+ *
+ * This is because we must have a real view of the last spot the caching
+ * kthreads were while caching. Consider the following views of the
+ * extent tree for a block group
+ *
+ * commit root
+ * +----+----+----+----+----+----+----+
+ * |\\\\| |\\\\|\\\\| |\\\\|\\\\|
+ * +----+----+----+----+----+----+----+
+ * 0 1 2 3 4 5 6 7
+ *
+ * new commit root
+ * +----+----+----+----+----+----+----+
+ * | | | |\\\\| | |\\\\|
+ * +----+----+----+----+----+----+----+
+ * 0 1 2 3 4 5 6 7
+ *
+ * If the cache_ctl->progress was at 3, then we are only allowed to
+ * unpin [0,1) and [2,3], because the caching thread has already
+ * processed those extents. We are not allowed to unpin [5,6), because
+ * the caching thread will re-start it's search from 3, and thus find
+ * the hole from [4,6) to add to the free space cache.
+ */
+ list_for_each_entry_safe(caching_ctl, next,
+ &fs_info->caching_block_groups, list) {
+ struct btrfs_block_group *cache = caching_ctl->block_group;
+
+ if (btrfs_block_group_done(cache)) {
+ cache->last_byte_to_unpin = (u64)-1;
+ list_del_init(&caching_ctl->list);
+ btrfs_put_caching_control(caching_ctl);
+ } else {
+ cache->last_byte_to_unpin = caching_ctl->progress;
+ }
+ }
up_write(&fs_info->commit_root_sem);
}

@@ -2293,8 +2333,6 @@ int btrfs_commit_transaction(struct btrf
goto unlock_tree_log;
}

- btrfs_prepare_extent_commit(fs_info);
-
cur_trans = fs_info->running_transaction;

btrfs_set_root_node(&fs_info->tree_root->root_item,