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Re: [PATCH] incorrect error handling inside generic_file_direct_write

To: Andrew Morton <akpm@xxxxxxxx>
Subject: Re: [PATCH] incorrect error handling inside generic_file_direct_write
From: Dmitriy Monakhov <dmonakhov@xxxxx>
Date: Tue, 12 Dec 2006 12:22:14 +0300
Cc: Dmitriy Monakhov <dmonakhov@xxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, Linux Memory Management <linux-mm@xxxxxxxxx>, <devel@xxxxxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <20061211124052.144e69a0.akpm@xxxxxxxx> (Andrew Morton's message of "Mon, 11 Dec 2006 12:40:52 -0800")
References: <87k60y1rq4.fsf@xxxxx> <20061211124052.144e69a0.akpm@xxxxxxxx>
Sender: xfs-bounce@xxxxxxxxxxx
User-agent: Gnus/5.1008 (Gnus v5.10.8) Emacs/21.4 (gnu/linux)
Andrew Morton <akpm@xxxxxxxx> writes:

> On Mon, 11 Dec 2006 16:34:27 +0300
> Dmitriy Monakhov <dmonakhov@xxxxxxxxxx> 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@xxxxxxxxxx>
>> ----------
>>
>> 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.


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