Re: kernel BUG in ext4_writepages

[Tadeusz Struk <tadeusz.struk@linaro.org>]

On 5/19/22 05:23, Jan Kara wrote:
> Hi!
> 
> On Tue 10-05-22 15:28:38, Tadeusz Struk wrote:
>> Syzbot found another BUG in ext4_writepages [1].
>> This time it complains about inode with inline data.
>> C reproducer can be found here [2]
>> I was able to trigger it on 5.18.0-rc6
>>
>> [1] https://syzkaller.appspot.com/bug?id=a1e89d09bbbcbd5c4cb45db230ee28c822953984
>> [2] https://syzkaller.appspot.com/text?tag=ReproC&x=129da6caf00000
> 
> Thanks for report. This should be fixed by:
> 
> https://lore.kernel.org/all/20220516012752.17241-1-yebin10@huawei.com/


In case of the syzbot bug there is something messed up with PAGE DIRTY flags
and the way syzbot sets up the write. This is what triggers the crash:

$ ftrace -f ./repro
...
[pid  2395] open("./bus", O_RDWR|O_CREAT|O_SYNC|O_NOATIME, 000 <unfinished ...>
[pid  2395] <... open resumed> )        = 6
...
[pid  2395] write(6, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0", 22 <unfinished ...>
...
[pid  2395] <... write resumed> )       = 22

One way I could fix it was to clear the PAGECACHE_TAG_DIRTY on the mapping in
ext4_try_to_write_inline_data() after the page has been updated:

diff --git a/fs/ext4/inline.c b/fs/ext4/inline.c
index 9c076262770d..e4bbb53fa26f 100644
--- a/fs/ext4/inline.c
+++ b/fs/ext4/inline.c
@@ -715,6 +715,7 @@ int ext4_try_to_write_inline_data(struct address_space *mapping,
  			put_page(page);
  			goto out_up_read;
  		}
+		__xa_clear_mark(&mapping->i_pages, 0, PAGECACHE_TAG_DIRTY);
  	}
  
  	ret = 1;

Please let me know it if makes sense any I will send a proper patch.

-- 
Thanks,
Tadeusz