Hi Qu,

Thank you for the patch! Perhaps something to improve:

[auto build test WARNING on v5.9-rc4]
[also build test WARNING on next-20200903]
[cannot apply to kdave/for-next btrfs/next]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch]

url:    https://github.com/0day-ci/linux/commits/Qu-Wenruo/btrfs-add-read-only-support-for-subpage-sector-size/20200908-155601
base:    f4d51dffc6c01a9e94650d95ce0104964f8ae822
config: xtensa-allyesconfig (attached as .config)
compiler: xtensa-linux-gcc (GCC) 9.3.0
reproduce (this is a W=1 build):
        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
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-9.3.0 make.cross ARCH=xtensa 

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

All warnings (new ones prefixed by >>):

>> fs/btrfs/extent_io.c:6309:5: warning: no previous prototype for 'try_release_subpage_ebs' [-Wmissing-prototypes]
    6309 | int try_release_subpage_ebs(struct page *page)
         |     ^~~~~~~~~~~~~~~~~~~~~~~

# https://github.com/0day-ci/linux/commit/3ef1cb4eb96ce4dce4dc94e3f06c4dd41879e977
git remote add linux-review https://github.com/0day-ci/linux
git fetch --no-tags linux-review Qu-Wenruo/btrfs-add-read-only-support-for-subpage-sector-size/20200908-155601
git checkout 3ef1cb4eb96ce4dce4dc94e3f06c4dd41879e977
vim +/try_release_subpage_ebs +6309 fs/btrfs/extent_io.c

  6308	
> 6309	int try_release_subpage_ebs(struct page *page)
  6310	{
  6311		struct subpage_eb_mapping *mapping;
  6312		int i;
  6313	
  6314		assert_spin_locked(&page->mapping->private_lock);
  6315		if (!PagePrivate(page))
  6316			return 1;
  6317	
  6318		mapping = (struct subpage_eb_mapping *)page->private;
  6319		for (i = 0; i < SUBPAGE_NR_EXTENT_BUFFERS && PagePrivate(page); i++) {
  6320			struct btrfs_fs_info *fs_info = page_to_fs_info(page);
  6321			struct extent_buffer *eb;
  6322			int ret;
  6323	
  6324			if (!test_bit(i, &mapping->bitmap))
  6325				continue;
  6326	
  6327			eb = mapping->buffers[i];
  6328			spin_unlock(&page->mapping->private_lock);
  6329			spin_lock(&eb->refs_lock);
  6330			ret = release_extent_buffer(eb);
  6331			spin_lock(&page->mapping->private_lock);
  6332	
  6333			/*
  6334			 * Extent buffer can't be freed yet, must jump to next slot
  6335			 * and avoid calling release_extent_buffer().
  6336			 */
  6337			if (!ret)
  6338				i += (fs_info->nodesize / fs_info->sectorsize - 1);
  6339		}
  6340		/*
  6341		 * detach_subpage_mapping() from release_extent_buffer() has detached
  6342		 * all ebs from this page. All related ebs are released.
  6343		 */
  6344		if (!PagePrivate(page))
  6345			return 1;
  6346		return 0;
  6347	}
  6348	

---
0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org