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

[Li Dongyang]

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.
> 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
> >
> >>>> Sunil Mushran  04/10/10 1:42 AM >>>
> >
> >   Li Dongyang wrote:
> >> On Friday 09 April 2010 11:32:10 Tao Ma wrote:
> >>> Hi Dongyang,
> >>>
> >>> Li Dongyang wrote:
> >>>> Hi, Tao,
> >>>>
> >>>> On Friday 09 April 2010 10:38:33 Tao Ma wrote:
> >>>>> Hi Dongyang,
> >>>>>
> >>>>> Li Dongyang wrote:
> >>>>>> This is because ocfs2_file_aio_write calls
> >>>>>> ocfs2_prepare_inode_for_write which sets direct_io to 0 if it finds
> >>>>>> out that direct IO would extend the file. But later we call
> >>>>>> __generic_file_aio_write which end's up calling
> >>>>>> generic_file_direct_write because the file has O_DIRECT flag.So
> >>>>>> every time we do a direct write extending the file, the
> >>>>>> inode->i_size gets inconsistent with the i_size on disk because we
> >>>>>> call
> >>>>>> generic_file_direct_write, and if we do a truncate after this, we
> >>>>>> will meet a bug in ocfs2_truncate_file.
> >>>>>
> >>>>> yes we have O_DIRECT flag set and in __generic_file_aio_write it will
> >>>>> call generic_file_direct_write first and then trigger to
> >>>>> ocfs2_direct_IO. In this function we will check again and return 0.
> >>>>> And _generic_file_aio_write will fall back to buffered write if the
> >>>>> directIO can't write. Am I wrong somehow?
> >>>>
> >>>> yes ocfs2_direct_IO has some check, but it just check if we are
> >>>> appending(the i_size <= offset), if the offset < i_size and offset +
> >>>> count > i_size, it will do direct io anyway. seems we also can fix
> >>>> this by adding a check to ocfs2_direct_IO.
> >>>
> >>> It is done by ocfs2_direct_IO_get_blocks. Just debug the kernel and you
> >>> will get what I mean. ;)
> >>
> >> Do you mean this section in ocfs2_direct_IO_get_blocks:?
> >> /*
> >>  * Any write past EOF is not allowed because we'd be extending.
> >>  */
> >> if (create && (iblock + max_blocks) > inode_blocks) {
> >>     ret = -EIO;
> >>     goto bail;
> >> }
> >>
> >> I was using the linus tree
> >> git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git
> >> and we don't have that check, but I can find this in the
> >> git://git.kernel.org/pub/scm/linux/kernel/git/jlbec/ocfs2.git,
> >> introduced by commit 564f8a3228879d6962edb3432d01bcd7499a67ec
> >>
> >> and now with this check I got what you mean, you are right, but I wonder
> >> why the linus tree doesn't have this check? and are we suppose to do
> >> with this? IMHO we can just push this commit to linus tree.
> >
> > commit 5fe878ae7f82fbf0830dbfaee4c5ca18f3aee442
> > Author: Christoph Hellwig
> > Date:   Tue Dec 15 16:47:50 2009 -0800
> >
> >     direct-io: cleanup blockdev_direct_IO locking
> >
> > This check was removed recently by the above patch.
>