f2fs: call f2fs_balance_fs outside of locked page

Otherwise, we can hit deadlock by waiting for the locked page in
move_data_block in GC.

 Thread A                     Thread B
 - do_page_mkwrite
  - f2fs_vm_page_mkwrite
   - lock_page
                              - f2fs_balance_fs
                                  - mutex_lock(gc_mutex)
                               - f2fs_gc
                                - do_garbage_collect
                                 - ra_data_block
                                  - grab_cache_page
   - f2fs_balance_fs
    - mutex_lock(gc_mutex)

Fixes: 39a86958 ("f2fs: refactor ->page_mkwrite() flow")
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/file.c

@@ -50,7 +50,7 @@ static vm_fault_t f2fs_vm_page_mkwrite(struct vm_fault *vmf)
 	struct page *page = vmf->page;
 	struct inode *inode = file_inode(vmf->vma->vm_file);
 	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
-	struct dnode_of_data dn = { .node_changed = false };
+	struct dnode_of_data dn;
 	int err;
 
 	if (unlikely(f2fs_cp_error(sbi))) {
@@ -63,6 +63,9 @@ static vm_fault_t f2fs_vm_page_mkwrite(struct vm_fault *vmf)
 		goto err;
 	}
 
+	/* should do out of any locked page */
+	f2fs_balance_fs(sbi, true);
+
 	sb_start_pagefault(inode->i_sb);
 
 	f2fs_bug_on(sbi, f2fs_has_inline_data(inode));
@@ -120,8 +123,6 @@ static vm_fault_t f2fs_vm_page_mkwrite(struct vm_fault *vmf)
 out_sem:
 	up_read(&F2FS_I(inode)->i_mmap_sem);
 
-	f2fs_balance_fs(sbi, dn.node_changed);
-
 	sb_end_pagefault(inode->i_sb);
 err:
 	return block_page_mkwrite_return(err);