[PATCH 4/4] btrfs: fix bytes_may_use underflow in prealloc error condtition

[Josef Bacik <josef@toxicpanda.com>]

I hit the following warning while running my error injection stress testing

------------[ cut here ]------------
WARNING: CPU: 3 PID: 1453 at fs/btrfs/space-info.h:108 btrfs_free_reserved_data_space_noquota+0xfd/0x160 [btrfs]
RIP: 0010:btrfs_free_reserved_data_space_noquota+0xfd/0x160 [btrfs]
Call Trace:
btrfs_free_reserved_data_space+0x4f/0x70 [btrfs]
__btrfs_prealloc_file_range+0x378/0x470 [btrfs]
elfcorehdr_read+0x40/0x40
? elfcorehdr_read+0x40/0x40
? btrfs_commit_transaction+0xca/0xa50 [btrfs]
? dput+0xb4/0x2a0
? btrfs_log_dentry_safe+0x55/0x70 [btrfs]
? btrfs_sync_file+0x30e/0x420 [btrfs]
? do_fsync+0x38/0x70
? __x64_sys_fdatasync+0x13/0x20
? do_syscall_64+0x5b/0x1b0
? entry_SYSCALL_64_after_hwframe+0x44/0xa9
---[ end trace 70ccb5d0fe51151c ]---

This happens if we fail to insert our reserved file extent.  At this
point we've already converted our reservation from ->bytes_may_use to
->bytes_reserved.  However once we break we will attempt to free
everything from [cur_offset, end] from ->bytes_may_use, but our extent
reservation will overlap part of this.

Fix this problem by adding ins.offset (our extent allocation size) to
cur_offset so we remove the actual remaining part from ->bytes_may_use.

I validated this fix using my inject-error.py script

python inject-error.py -o should_fail_bio -t cache_save_setup -t \
	__btrfs_prealloc_file_range \
	-t insert_reserved_file_extent.constprop.0 \
	-r "-5" ./run-fsstress.sh

where run-fsstress.sh simply mounts and runs fsstress on a disk.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/inode.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 84e649724549..747d860aedf6 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -9919,6 +9919,14 @@ static int __btrfs_prealloc_file_range(struct inode *inode, int mode,
 						  ins.offset, 0, 0, 0,
 						  BTRFS_FILE_EXTENT_PREALLOC);
 		if (ret) {
+			/*
+			 * We've reserved this space, and thus converted it from
+			 * ->bytes_may_use to ->bytes_reserved, which we cleanup
+			 * here.  We need to adjust cur_offset so that we only
+			 * drop the ->bytes_may_use for the area we still have
+			 * remaining in ->>bytes_may_use.
+			 */
+			cur_offset += ins.objectid;
 			btrfs_free_reserved_extent(fs_info, ins.objectid,
 						   ins.offset, 0);
 			btrfs_abort_transaction(trans, ret);
-- 
2.24.1