Hi Damien,

I love your patch! Yet something to improve:

[auto build test ERROR on linus/master]
[cannot apply to v5.3-rc6 next-20190830]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Damien-Le-Moal/fs-New-zonefs-file-system/20190826-212833
config: s390-allmodconfig (attached as .config)
compiler: s390-linux-gcc (GCC) 7.4.0
reproduce:
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # save the attached .config to linux build tree
        GCC_VERSION=7.4.0 make.cross ARCH=s390 

If you fix the issue, kindly add following tag
Reported-by: kbuild test robot <lkp@intel.com>

All errors (new ones prefixed by >>):

   fs//zonefs/super.c:84:37: warning: 'struct iomap_writepage_ctx' declared inside parameter list will not be visible outside of this definition or declaration
    static int zonefs_map_blocks(struct iomap_writepage_ctx *wpc,
                                        ^~~~~~~~~~~~~~~~~~~
   fs//zonefs/super.c: In function 'zonefs_map_blocks':
>> fs//zonefs/super.c:87:19: error: dereferencing pointer to incomplete type 'struct iomap_writepage_ctx'
     if (offset >= wpc->iomap.offset &&
                      ^~
   fs//zonefs/super.c: At top level:
>> fs//zonefs/super.c:96:21: error: variable 'zonefs_writeback_ops' has initializer but incomplete type
    static const struct iomap_writeback_ops zonefs_writeback_ops = {
                        ^~~~~~~~~~~~~~~~~~~
>> fs//zonefs/super.c:97:3: error: 'const struct iomap_writeback_ops' has no member named 'map_blocks'
     .map_blocks  = zonefs_map_blocks,
      ^~~~~~~~~~
   fs//zonefs/super.c:97:17: warning: excess elements in struct initializer
     .map_blocks  = zonefs_map_blocks,
                    ^~~~~~~~~~~~~~~~~
   fs//zonefs/super.c:97:17: note: (near initialization for 'zonefs_writeback_ops')
   fs//zonefs/super.c: In function 'zonefs_writepage':
>> fs//zonefs/super.c:102:9: error: variable 'wpc' has initializer but incomplete type
     struct iomap_writepage_ctx wpc = { };
            ^~~~~~~~~~~~~~~~~~~
>> fs//zonefs/super.c:102:29: error: storage size of 'wpc' isn't known
     struct iomap_writepage_ctx wpc = { };
                                ^~~
>> fs//zonefs/super.c:104:9: error: implicit declaration of function 'iomap_writepage'; did you mean 'iomap_readpage'? [-Werror=implicit-function-declaration]
     return iomap_writepage(page, wbc, &wpc, &zonefs_writeback_ops);
            ^~~~~~~~~~~~~~~
            iomap_readpage
   fs//zonefs/super.c:102:29: warning: unused variable 'wpc' [-Wunused-variable]
     struct iomap_writepage_ctx wpc = { };
                                ^~~
   fs//zonefs/super.c: In function 'zonefs_writepages':
   fs//zonefs/super.c:110:9: error: variable 'wpc' has initializer but incomplete type
     struct iomap_writepage_ctx wpc = { };
            ^~~~~~~~~~~~~~~~~~~
   fs//zonefs/super.c:110:29: error: storage size of 'wpc' isn't known
     struct iomap_writepage_ctx wpc = { };
                                ^~~
>> fs//zonefs/super.c:112:9: error: implicit declaration of function 'iomap_writepages'; did you mean 'do_writepages'? [-Werror=implicit-function-declaration]
     return iomap_writepages(mapping, wbc, &wpc, &zonefs_writeback_ops);
            ^~~~~~~~~~~~~~~~
            do_writepages
   fs//zonefs/super.c:110:29: warning: unused variable 'wpc' [-Wunused-variable]
     struct iomap_writepage_ctx wpc = { };
                                ^~~
   fs//zonefs/super.c: At top level:
>> fs//zonefs/super.c:96:41: error: storage size of 'zonefs_writeback_ops' isn't known
    static const struct iomap_writeback_ops zonefs_writeback_ops = {
                                            ^~~~~~~~~~~~~~~~~~~~
   fs//zonefs/super.c: In function 'zonefs_writepage':
   fs//zonefs/super.c:105:1: warning: control reaches end of non-void function [-Wreturn-type]
    }
    ^
   fs//zonefs/super.c: In function 'zonefs_writepages':
   fs//zonefs/super.c:113:1: warning: control reaches end of non-void function [-Wreturn-type]
    }
    ^
   cc1: some warnings being treated as errors

vim +87 fs//zonefs/super.c

    83	
  > 84	static int zonefs_map_blocks(struct iomap_writepage_ctx *wpc,
    85				     struct inode *inode, loff_t offset)
    86	{
  > 87		if (offset >= wpc->iomap.offset &&
    88		    offset < wpc->iomap.offset + wpc->iomap.length)
    89			return 0;
    90	
    91		memset(&wpc->iomap, 0, sizeof(wpc->iomap));
    92		return zonefs_iomap_begin(inode, offset, zonefs_file_max_size(inode),
    93					  0, &wpc->iomap);
    94	}
    95	
  > 96	static const struct iomap_writeback_ops zonefs_writeback_ops = {
  > 97		.map_blocks		= zonefs_map_blocks,
    98	};
    99	
   100	static int zonefs_writepage(struct page *page, struct writeback_control *wbc)
   101	{
 > 102		struct iomap_writepage_ctx wpc = { };
   103	
 > 104		return iomap_writepage(page, wbc, &wpc, &zonefs_writeback_ops);
   105	}
   106	
   107	static int zonefs_writepages(struct address_space *mapping,
   108				     struct writeback_control *wbc)
   109	{
 > 110		struct iomap_writepage_ctx wpc = { };
   111	
 > 112		return iomap_writepages(mapping, wbc, &wpc, &zonefs_writeback_ops);
   113	}
   114	

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