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Message-ID: <20130731141340.GB22930@quack.suse.cz>
Date: Wed, 31 Jul 2013 16:13:40 +0200
From: Jan Kara <jack@...e.cz>
To: Zhao Hongjiang <zhaohongjiang@...wei.com>
Cc: Jan Kara <jack@...e.cz>, Theodore Ts'o <tytso@....edu>,
linux-ext4@...r.kernel.org, hch@....de, khoroshilov@...ras.ru
Subject: Re: xfstests failure generic/239
On Wed 31-07-13 10:42:37, Zhao Hongjiang wrote:
> On 2013/7/30 23:48, Jan Kara wrote:
> > On Tue 30-07-13 11:28:58, Zhao Hongjiang wrote:
> >> Hi, jack
> >>
> >> I test the latest kernel 3.11-rc2 and it seems the problem is fix by the
> >> follow patch: commit id:97a851ed71cd9cc2542955e92a001c6ea3d21d35 (ext4:
> >> use io_end for multiple bios). But it's so difficult to backport to
> >> kernel 3.4-stable, any suggestion for this?
> > Backporting that patch to stable kernels is no-go. It is far to intrusive
> > for stable kernels. I was looking for a while how that patch could fix the
> > problem you were observing. I think there is a subtle race possible when
> > AIO DIO write completes before __blockdev_direct_IO() returns. In that case
> > we set iocb->private to NULL in ext4_end_io_dio() but we also key off
> > iocb->private in ext4_ext_direct_IO() as:
> > if (iocb->private)
> > ext4_inode_aio_set(inode, NULL);
> >
> > So in the case above we forget to reset inode's AIO pointer. That can then
> > cause strange effects with unwritten extent handling (although I admit I'm
> > not sure whether it can also cause the failure you observe) and
> > 97a851ed71cd9cc2542955e92a001c6ea3d21d35 actually fixes that bug. You can
> > easily check whether you are hitting that bug or not by changing the above
> > condition from testing iocb->private to testing some private variable...
> > E.g. you could declare io_end and set it to NULL one level up in
> > ext4_ext_direct_IO() and then test io_end != NULL in that condition.
> >
> Thanks for your reply first.
> I change the code like the follow:
>
> @@ -2921,6 +2921,7 @@ static ssize_t ext4_ext_direct_IO(int rw, struct kiocb *iocb,
> struct inode *inode = file->f_mapping->host;
> ssize_t ret;
> size_t count = iov_length(iov, nr_segs);
> + ext4_io_end_t *io_end = NULL;
>
> loff_t final_size = offset + count;
> if (rw == WRITE && final_size <= inode->i_size) {
> @@ -2947,8 +2948,7 @@ static ssize_t ext4_ext_direct_IO(int rw, struct kiocb *iocb,
> iocb->private = NULL;
> EXT4_I(inode)->cur_aio_dio = NULL;
> if (!is_sync_kiocb(iocb)) {
> - ext4_io_end_t *io_end =
> - ext4_init_io_end(inode, GFP_NOFS);
> + io_end = ext4_init_io_end(inode, GFP_NOFS);
> if (!io_end)
> return -ENOMEM;
> io_end->flag |= EXT4_IO_END_DIRECT;
> @@ -2970,8 +2970,10 @@ static ssize_t ext4_ext_direct_IO(int rw, struct kiocb *iocb,
> ext4_end_io_dio,
> NULL,
> DIO_LOCKING);
> - if (iocb->private)
> + if (io_end != NULL) {
> + printk("Zhao Hongjiang Ext4 test!\n");
> EXT4_I(inode)->cur_aio_dio = NULL;
> + }
> /*
> * The io_end structure takes a reference to the inode,
> * that structure needs to be destroyed and the
>
> And the print come out when i run the test everytime. So i think the test
> hit the bug that you mentioned, Am i right or miss something?
It is not a bug that you hit the branch with printk(). It would be a bug
if the debug check looked like:
if (io_end != NULL) {
if (iocb->private == NULL)
printk("Bug happened!\n");
EXT4_I(inode)->cur_aio_dio = NULL;
}
Honza
> >> On 2013/6/9 6:30, Theodore Ts'o wrote:
> >>> On Sat, Jun 08, 2013 at 11:13:35AM +0800, Zhao Hongjiang wrote:
> >>>>
> >>>> I run xfstests #239 against mainline 3.10.0-rc3, unfortunately it failure in my QEMU. I run the
> >>>> case a hundred times, it certainly hit the failure several times. The failure msg is as follow:
> >>>>
> >>>> FSTYP -- ext4
> >>>> PLATFORM -- Linux/x86_64 3.10.0-rc3-mainline
> >>>>
> >>>> generic/239 1s ... - output mismatch (see /home/zhj/xfstests/results/generic/239.out.bad)
> >>>> --- tests/generic/239.out 2013-06-07 22:04:09.000000000 -0400
> >>>> +++ /home/zff/xfstests/results/generic/239.out.bad 2013-06-07 22:04:09.000000000 -0400
> >>>> @@ -1,2 +1,515 @@
> >>>> QA output created by 239
> >>>> +hostname: Host name lookup failure
> >>>
> >>> OK, so this hostname failure is weird; I'm not sure what's causing
> >>> this, but this I presume unrelated to the failure at hand.
> >>>
> >>>> Silence is golden
> >>>> +0: 0x0
> >>>> +1: 0x0
> >>>> +2: 0x0
> >>>> +3: 0x0
> >>>
> >>> This indicates a problem. Test generic/239 is running
> >>> aio-dio-hole-filling-race.c, which submits an asynchronous, direct I/O
> >>> 4k write with a buffer containing non-zero contents to a sparse file,
> >>> and once the I/O has completed, it uses pread to read it back, using
> >>> the same descriptor, so it is doing the read using direct I/O. It
> >>> then checks to see if the read returns zero or not.
> >>>
> >>> The "XX: 0x0" lines indicates that buffer is zero, which implies that
> >>> somehow aio_complete() is getting called before the uninitialized to
> >>> initialized conversion is taking place. I'm not seeing how this is
> >>> happening, though, so I'm a bit puzzled. If there are any unwritten
> >>> extents, we don't call aio_complete() in ext4_end_io_dio(), but
> >>> instead the conversion is queued via a call to ext4_add_compete_io(),
> >>> and and aio_done() is only called on the iocb after the conversion is
> >>> complete.
> >>>
> >>> Can anyone see something that I might be missing?
> >>>
> >>> - Ted
> >>>
> >>> P.S. Zhao, what was the hardware that you using to find this failure?
> >>> I'm not seeing it, but then again if the failure is only happening
> >>> once every few hundred runs that might explain it. I'm perhaps
> >>> wondering if we should add a mode to aio-dio-hole-filling-race.c which
> >>> allows it to try the race a large number of times, instead of just
> >>> once.
> >>>
> >>> P.P.S. One thought.... perhaps it might be useful to have a debug
> >>> mode where we use queue_delayed_work() to submit the conversion
> >>> request to the workqueue. It will of course make certain workloads
> >>> run slow as molasses, but it might expose some races so we can see
> >>> them more easily.
> >>>
> >>> .
>
>
>
--
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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