[Ocfs2-devel] [PATCH] ocfs2: avoid direct write if we fall back to buffered

[Tao Ma]

Hi Dongyang,

Tao Ma wrote:
> 
> Li Dongyang wrote:
>> Hi, Tao
>> On Monday 12 April 2010 13:16:43 Tao Ma wrote:
>>> Hi dong yang,
>>>
>>> Dong Yang Li wrote:
>>>> I still get a bug with this check and without my patch:
>>> yes, the check doesn't work actually in this case.
>>>
>>>> [16179.955148] (13400,1):ocfs2_truncate_file:465 ERROR: bug expression:
>>>> le64_to_cpu(fe->i_size) != i_size_read(inode) [16179.955157]
>>>> (13400,1):ocfs2_truncate_file:465 ERROR: Inode 254789, inode i_size =
>>>> 811008 != di i_size = 809011, i_flags = 0x1 the call trace is the same.
>>>>
>>>>
>>>> the problem is this check in ocfs2_direct_IO_get_blocks just check if we
>>>> are going beyond the blocks right now, so if a direct write won't play
>>>> with new blocks but extending the i_size still get a pass, like the error
>>>> above said, di->i_size is 809011, using 198 blocks and the direct write
>>>> end up with i_size 811008, just same 198 blocks.
>>> yeah, you are right.
>>>
>> Thanks for the script,
>> and a stupid question: why we still try to call __generic_file_aio_write and 
>> let it try direct write first in ocfs2_file_aio_write even we decided we could 
>> not do the direct write?
>>>> IMHO, we can add this check back and fix this check, or we don't try to
>>>> do direct write if we decided we can't in ocfs2_file_aio_write, after
>>>> calling ocfs2_prepare_inode_for_write as my patch said.
>>> I think we only need to check this condition in get_blocks. So would you
>>> mind providing a patch? You old method is too aggressive actually.
>>>
>> what about add this check in ocfs2_direct_IO? if we see we are extending just 
>> return 0. right now we only check if we are appending.
> As for the 2 questions, I just want to do buffered write as small as 
> possible since it has to lock inode, create pages and then sync pages 
> etc(you can check ocfs2_write_begin/end for details. ;) ). So say this 
> question, actually only the last block needed to be buffered ioed and 
> i_size get updated accordingly.
> 
> I just checked ext4_direct_IO and actually it updated the disk size at 
> the end of direct_IO. So maybe we can work like that also.
sorry, I mislead you.
Joel pointed out that except the problem my little script exposed, there 
is another problem about ip_alloc_sem locking. So we have to fall back 
to buffer write from the very beginning. I just saw that Joel has 
commented your original patch, so do please revise it.

Regards,
Tao
> 
> Regards,
> Tao
>>> btw, I have created a small test script which will expose this bug
>>> easily. So you don't need to use the time-consuming fsstress test now.
>>> Just use it to test your fix.
>>>
>>> echo 'y'|mkfs.ocfs2 --fs-features=local,noinline-data -b 4K -C 4K
>>> $DEVICE 1000000
>>> mount -t ocfs2 $DEVICE $MNT_DIR
>>> echo "foo" > $MNT_DIR/foo
>>> dd if=/dev/zero of=$MNT_DIR/foo bs=4K count=1 conv=notrunc oflag=direct
>>> echo "foo" > $MNT_DIR/foo
>>> # The kernel should panic here.
>>>
>>> Regards,
>>> Tao
>>>
>>>> Comments? ;-)
>>>>
>>>>
>>>> Br,
>>>> Li Dongyang