[Devel] Re: [PATCH] incorrect error handling inside generic_file_direct_write

[Dmitriy Monakhov]

Andrew Morton <akpm at osdl.org> writes:

> On Mon, 11 Dec 2006 16:34:27 +0300
> Dmitriy Monakhov <dmonakhov at openvz.org> wrote:
>
>> OpenVZ team has discovered error inside generic_file_direct_write()
>> If generic_file_direct_IO() has fail (ENOSPC condition) it may have instantiated
>> a few blocks outside i_size. And fsck will complain about wrong i_size
>> (ext2, ext3 and reiserfs interpret i_size and biggest block difference as error),
>> after fsck will fix error i_size will be increased to the biggest block,
>> but this blocks contain gurbage from previous write attempt, this is not 
>> information leak, but its silence file data corruption. 
>> We need truncate any block beyond i_size after write have failed , do in simular
>> generic_file_buffered_write() error path.
>> 
>> Exampe:
>> open("mnt2/FILE3", O_WRONLY|O_CREAT|O_DIRECT, 0666) = 3
>> write(3, "aaaaaa"..., 4096) = -1 ENOSPC (No space left on device)
>> 
>> stat mnt2/FILE3
>> File: `mnt2/FILE3'
>> Size: 0               Blocks: 4          IO Block: 4096   regular empty file
>> >>>>>>>>>>>>>>>>>>>>>>^^^^^^^^^^ file size is less than biggest block idx
>> Device: 700h/1792d      Inode: 14          Links: 1
>> Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
>> 
>> fsck.ext2 -f -n  mnt1/fs_img
>> Pass 1: Checking inodes, blocks, and sizes
>> Inode 14, i_size is 0, should be 2048.  Fix? no
>> 
>> Signed-off-by: Dmitriy Monakhov <dmonakhov at openvz.org>
>> ----------
>>
>> diff --git a/mm/filemap.c b/mm/filemap.c
>> index 7b84dc8..bf7cf6c 100644
>> --- a/mm/filemap.c
>> +++ b/mm/filemap.c
>> @@ -2041,6 +2041,14 @@ generic_file_direct_write(struct kiocb *
>>  			mark_inode_dirty(inode);
>>  		}
>>  		*ppos = end;
>> +	} else if (written < 0) {
>> +		loff_t isize = i_size_read(inode);
>> +		/*
>> +		 * generic_file_direct_IO() may have instantiated a few blocks
>> +		 * outside i_size.  Trim these off again.
>> +		 */
>> +		if (pos + count > isize)
>> +			vmtruncate(inode, isize);
>>  	}
>>  
>
> XFS (at least) can call generic_file_direct_write() with i_mutex not held. 
How could it be ?

from mm/filemap.c:2046 generic_file_direct_write() comment right after 
place where i want to add vmtruncate()
/*
	 * Sync the fs metadata but not the minor inode changes and
	 * of course not the data as we did direct DMA for the IO.
	 * i_mutex is held, which protects generic_osync_inode() from
	 * livelocking.
	 */

> And vmtruncate() expects i_mutex to be held.
generic_file_direct_IO must called under i_mutex too
from mm/filemap.c:2388
  /*
   * Called under i_mutex for writes to S_ISREG files.   Returns -EIO if something
   * went wrong during pagecache shootdown.
   */
  static ssize_t
  generic_file_direct_IO(int rw, struct kiocb *iocb, const struct iovec *iov,

This means XFS generic_file_direct_write() call generic_file_direct_IO() without
i_mutex held too?
>
> I guess a suitable solution would be to push this problem back up to the
> callers: let them decide whether to run vmtruncate() and if so, to ensure
> that i_mutex is held.
>
> The existence of generic_file_aio_write_nolock() makes that rather messy
> though.