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SubjectRe: [PATCH] incorrect error handling inside generic_file_direct_write
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Andrew Morton <akpm@osdl.org> writes:

> On Mon, 11 Dec 2006 16:34:27 +0300
> Dmitriy Monakhov <dmonakhov@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@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.
> And vmtruncate() expects i_mutex to be held.
>
> 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.
This means we may call generic_file_aio_write_nolock() without i_mutex, right?
but call trace is :
generic_file_aio_write_nolock()
->generic_file_buffered_write() /* i_mutex not held here */
but according to filemaps locking rules: mm/filemap.c:77
..
* ->i_mutex (generic_file_buffered_write)
* ->mmap_sem (fault_in_pages_readable->do_page_fault)
..
I'm confused a litle bit, where is the truth?
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