f2fs: avoid race between zero_range and background GC

Thread A				Background GC
- f2fs_zero_range
 - truncate_pagecache_range
					- gc_data_segment
					 - get_read_data_page
					  - move_data_page
					   - set_page_dirty
					   - set_cold_data
 - f2fs_do_zero_range
  - dn->data_blkaddr = NEW_ADDR;
  - f2fs_set_data_blkaddr

Actually, we don't need to set dirty & checked flag on the page, since
all valid data in the page should be zeroed by zero_range().

Use i_gc_rwsem[WRITE] to avoid such race condition.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/file.c

@@ -1314,8 +1314,6 @@ static int f2fs_zero_range(struct inode *inode, loff_t offset, loff_t len,
 	if (ret)
 		goto out_sem;
 
-	truncate_pagecache_range(inode, offset, offset + len - 1);
-
 	pg_start = ((unsigned long long) offset) >> PAGE_SHIFT;
 	pg_end = ((unsigned long long) offset + len) >> PAGE_SHIFT;
 
@@ -1345,12 +1343,19 @@ static int f2fs_zero_range(struct inode *inode, loff_t offset, loff_t len,
 			unsigned int end_offset;
 			pgoff_t end;
 
+			down_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
+
+			truncate_pagecache_range(inode,
+				(loff_t)index << PAGE_SHIFT,
+				((loff_t)pg_end << PAGE_SHIFT) - 1);
+
 			f2fs_lock_op(sbi);
 
 			set_new_dnode(&dn, inode, NULL, NULL, 0);
 			ret = f2fs_get_dnode_of_data(&dn, index, ALLOC_NODE);
 			if (ret) {
 				f2fs_unlock_op(sbi);
+				up_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
 				goto out;
 			}
 
@@ -1359,7 +1364,9 @@ static int f2fs_zero_range(struct inode *inode, loff_t offset, loff_t len,
 
 			ret = f2fs_do_zero_range(&dn, index, end);
 			f2fs_put_dnode(&dn);
+
 			f2fs_unlock_op(sbi);
+			up_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
 
 			f2fs_balance_fs(sbi, dn.node_changed);