[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.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/inode.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 84e649724549..ffc6fcfe805c 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -9876,6 +9876,7 @@ static int __btrfs_prealloc_file_range(struct inode *inode, int mode,
 	struct btrfs_root *root = BTRFS_I(inode)->root;
 	struct btrfs_key ins;
 	u64 cur_offset = start;
+	u64 clear_offset = start;
 	u64 i_size;
 	u64 cur_bytes;
 	u64 last_alloc = (u64)-1;
@@ -9910,6 +9911,15 @@ static int __btrfs_prealloc_file_range(struct inode *inode, int mode,
 				btrfs_end_transaction(trans);
 			break;
 		}
+
+		/*
+		 * We've reserved this space, and thus converted it from
+		 * ->bytes_may_use to ->bytes_reserved.  Any error that happens
+		 * from here on out we will only need to clear our reservation
+		 * for the remaining unreserved area, so advance our
+		 * clear_offset by our extent size.
+		 */
+		clear_offset += ins.offset;
 		btrfs_dec_block_group_reservations(fs_info, ins.objectid);
 
 		last_alloc = ins.offset;
@@ -9989,9 +9999,9 @@ static int __btrfs_prealloc_file_range(struct inode *inode, int mode,
 		if (own_trans)
 			btrfs_end_transaction(trans);
 	}
-	if (cur_offset < end)
-		btrfs_free_reserved_data_space(inode, NULL, cur_offset,
-			end - cur_offset + 1);
+	if (clear_offset < end)
+		btrfs_free_reserved_data_space(inode, NULL, clear_offset,
+			end - clear_offset + 1);
 	return ret;
 }
 
-- 
2.24.1