ext4: fix BUG_ON in fs/ext4/page_io.c:ext4_release_io_end()

The function ext4_release_io_end() can be called by
ext4_put_io_end_defer() with the EXT4_IO_UNWRITTEN flag set and
io_end->size is 0.  In that case, it's safe to release the io_end
structure, since if io_end->size is zero, there is no unwritten region
to release.

This can be reproduced using generic/300, although not very reliably,
and almost never using a freshly rebooted kernel.

Google-Bug-Id: 15054006
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
---
 fs/ext4/page-io.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

On Mon, Apr 13, 2020 at 10:28:42PM -0400, Theodore Ts'o wrote:
> The function ext4_release_io_end() can be called by
> ext4_put_io_end_defer() with the EXT4_IO_UNWRITTEN flag set and
> io_end->size is 0.  In that case, it's safe to release the io_end
> structure, since if io_end->size is zero, there is no unwritten region
> to release.
> 
> This can be reproduced using generic/300, although not very reliably,
> and almost never using a freshly rebooted kernel.
> 
> Google-Bug-Id: 15054006
> Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

NACK; this patch no longer applies given the move to use iomap for direct I/O.

      	   	    	   	   	 - Ted

Hi Theodore,

I love your patch! Yet something to improve:

[auto build test ERROR on ext4/dev]
[also build test ERROR on v5.7-rc1 next-20200413]
[if your patch is applied to the wrong git tree, please drop us a note to help
improve the system. BTW, we also suggest to use '--base' option to specify the
base tree in git format-patch, please see https://stackoverflow.com/a/37406982]

url:    https://github.com/0day-ci/linux/commits/Theodore-Ts-o/ext4-fix-BUG_ON-in-fs-ext4-page_io-c-ext4_release_io_end/20200414-105422
base:   https://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4.git dev
config: x86_64-defconfig (attached as .config)
compiler: gcc-7 (Ubuntu 7.5.0-6ubuntu2) 7.5.0
reproduce:
        # save the attached .config to linux build tree
        make ARCH=x86_64 

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

All error/warnings (new ones prefixed by >>):

   In file included from include/linux/export.h:43:0,
                    from include/linux/linkage.h:7,
                    from include/linux/fs.h:5,
                    from fs/ext4/page-io.c:10:
   fs/ext4/page-io.c: In function 'ext4_release_io_end':
>> fs/ext4/page-io.c:158:57: error: 'ext4_io_end_t {aka struct ext4_io_end}' has no member named 'size'
     BUG_ON((io_end->flag & EXT4_IO_END_UNWRITTEN) && io_end->size);
                                                            ^
   include/linux/compiler.h:78:42: note: in definition of macro 'unlikely'
    # define unlikely(x) __builtin_expect(!!(x), 0)
                                             ^
>> fs/ext4/page-io.c:158:2: note: in expansion of macro 'BUG_ON'
     BUG_ON((io_end->flag & EXT4_IO_END_UNWRITTEN) && io_end->size);
     ^~~~~~

vim +158 fs/ext4/page-io.c

   152	
   153	static void ext4_release_io_end(ext4_io_end_t *io_end)
   154	{
   155		struct bio *bio, *next_bio;
   156	
   157		BUG_ON(!list_empty(&io_end->list));
 > 158		BUG_ON((io_end->flag & EXT4_IO_END_UNWRITTEN) && io_end->size);
   159		WARN_ON(io_end->handle);
   160	
   161		for (bio = io_end->bio; bio; bio = next_bio) {
   162			next_bio = bio->bi_private;
   163			ext4_finish_bio(bio);
   164			bio_put(bio);
   165		}
   166		ext4_free_io_end_vec(io_end);
   167		kmem_cache_free(io_end_cachep, io_end);
   168	}
   169	

---
0-DAY CI Kernel Test Service, Intel Corporation
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