f2fs: Fix accounting for unusable blocks

Fixes possible underflows when dealing with unusable blocks.
Signed-off-by: Daniel Rosenberg <drosen@google.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/f2fs.h

@@ -1769,8 +1769,12 @@ static inline int inc_valid_block_count(struct f2fs_sb_info *sbi,
 
 	if (!__allow_reserved_blocks(sbi, inode, true))
 		avail_user_block_count -= F2FS_OPTION(sbi).root_reserved_blocks;
-	if (unlikely(is_sbi_flag_set(sbi, SBI_CP_DISABLED)))
-		avail_user_block_count -= sbi->unusable_block_count;
+	if (unlikely(is_sbi_flag_set(sbi, SBI_CP_DISABLED))) {
+		if (avail_user_block_count > sbi->unusable_block_count)
+			avail_user_block_count -= sbi->unusable_block_count;
+		else
+			avail_user_block_count = 0;
+	}
 	if (unlikely(sbi->total_valid_block_count > avail_user_block_count)) {
 		diff = sbi->total_valid_block_count - avail_user_block_count;
 		if (diff > *count)
@@ -1970,7 +1974,7 @@ static inline int inc_valid_node_count(struct f2fs_sb_info *sbi,
 					struct inode *inode, bool is_inode)
 {
 	block_t	valid_block_count;
-	unsigned int valid_node_count;
+	unsigned int valid_node_count, user_block_count;
 	int err;
 
 	if (is_inode) {
@@ -1997,10 +2001,11 @@ static inline int inc_valid_node_count(struct f2fs_sb_info *sbi,
 
 	if (!__allow_reserved_blocks(sbi, inode, false))
 		valid_block_count += F2FS_OPTION(sbi).root_reserved_blocks;
+	user_block_count = sbi->user_block_count;
 	if (unlikely(is_sbi_flag_set(sbi, SBI_CP_DISABLED)))
-		valid_block_count += sbi->unusable_block_count;
+		user_block_count -= sbi->unusable_block_count;
 
-	if (unlikely(valid_block_count > sbi->user_block_count)) {
+	if (unlikely(valid_block_count > user_block_count)) {
 		spin_unlock(&sbi->stat_lock);
 		goto enospc;
 	}