From: Jan Kara Subject: Re: xfstests failure generic/239 Date: Thu, 1 Aug 2013 11:10:46 +0200 Message-ID: <20130801091046.GD19219@quack.suse.cz> References: <51B2A15F.1060704@huawei.com> <20130608223038.GA19229@thunk.org> <51F732FA.9090307@huawei.com> <20130730154801.GA22013@quack.suse.cz> <51F8799D.7070202@huawei.com> <20130731141340.GB22930@quack.suse.cz> <51F9C254.1000207@huawei.com> <20130801084941.GB19219@quack.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Jan Kara , Theodore Ts'o , linux-ext4@vger.kernel.org, hch@lst.de, khoroshilov@ispras.ru To: Zhao Hongjiang Return-path: Received: from cantor2.suse.de ([195.135.220.15]:46541 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751744Ab3HAJKs (ORCPT ); Thu, 1 Aug 2013 05:10:48 -0400 Content-Disposition: inline In-Reply-To: <20130801084941.GB19219@quack.suse.cz> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Thu 01-08-13 10:49:41, Jan Kara wrote: > Hi, > > On Thu 01-08-13 10:05:08, Zhao Hongjiang wrote: > > It hit this bug, the "Bug happened!" is come out everytime while the test > > is fail. Any suggestion for fix this? > OK, so the test is still failing after using io_end instead of > iocb->private? If yes, I'm not sure where the problem exactly is, sorry. Just one addition: But from the fact that debug printk triggers we see that really the problem happens when DIO is completed before __blockdev_direct_IO() returns and thus extent conversion can race with parts of __blockdev_direct_IO() or ext4_ext_direct_IO(). That is impossible after my patch because ext4_ext_direct_IO() holds a reference to io_end so it gets queued for completion only after __blockdev_direct_IO() returns. So at least I somewhat understand why my patch makes a difference. Honza > > On 2013/7/31 22:13, Jan Kara wrote: > > > 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 > SUSE Labs, CR -- Jan Kara SUSE Labs, CR