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Date:   Wed, 2 Jan 2019 15:46:32 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        syzbot <syzbot+9933e4476f365f5d5a1b@...kaller.appspotmail.com>,
        Linux-MM <linux-mm@...ck.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>, jlayton@...hat.com,
        LKML <linux-kernel@...r.kernel.org>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        tim.c.chen@...ux.intel.com,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: INFO: task hung in generic_file_write_iter

On Wed, Jan 2, 2019 at 3:40 PM Jan Kara <jack@...e.cz> wrote:
>
> On Fri 28-12-18 22:34:13, Tetsuo Handa wrote:
> > On 2018/08/06 19:09, Jan Kara wrote:
> > > On Tue 31-07-18 00:07:22, Tetsuo Handa wrote:
> > >> On 2018/07/21 5:06, Andrew Morton wrote:
> > >>> On Fri, 20 Jul 2018 19:36:23 +0900 Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp> wrote:
> > >>>
> > >>>>>
> > >>>>> This report is stalling after mount() completed and process used remap_file_pages().
> > >>>>> I think that we might need to use debug printk(). But I don't know what to examine.
> > >>>>>
> > >>>>
> > >>>> Andrew, can you pick up this debug printk() patch?
> > >>>> I guess we can get the result within one week.
> > >>>
> > >>> Sure, let's toss it in -next for a while.
> > >>>
> > >>>> >From 8f55e00b21fefffbc6abd9085ac503c52a302464 Mon Sep 17 00:00:00 2001
> > >>>> From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> > >>>> Date: Fri, 20 Jul 2018 19:29:06 +0900
> > >>>> Subject: [PATCH] fs/buffer.c: add debug print for __getblk_gfp() stall problem
> > >>>>
> > >>>> Among syzbot's unresolved hung task reports, 18 out of 65 reports contain
> > >>>> __getblk_gfp() line in the backtrace. Since there is a comment block that
> > >>>> says that __getblk_gfp() will lock up the machine if try_to_free_buffers()
> > >>>> attempt from grow_dev_page() is failing, let's start from checking whether
> > >>>> syzbot is hitting that case. This change will be removed after the bug is
> > >>>> fixed.
> > >>>
> > >>> I'm not sure that grow_dev_page() is hanging.  It has often been
> > >>> suspected, but always is proven innocent.  Lets see.
> > >>
> > >> syzbot reproduced this problem ( https://syzkaller.appspot.com/text?tag=CrashLog&x=11f2fc44400000 ) .
> > >> It says that grow_dev_page() is returning 1 but __find_get_block() is failing forever. Any idea?
> > >
> > > Looks like some kind of a race where device block size gets changed while
> > > getblk() runs (and creates buffers for underlying page). I don't have time
> > > to nail it down at this moment can have a look into it later unless someone
> > > beats me to it.
> >
> > I feel that the frequency of hitting this problem was decreased
> > by merging loop module's ioctl() serialization patches. But this
> > problem is still there, and syzbot got a new line in
> > https://syzkaller.appspot.com/text?tag=CrashLog&x=177f889f400000 .
> >
> >   [  615.881781] __loop_clr_fd: partition scan of loop5 failed (rc=-22)
> >   [  619.059920] syz-executor4(2193): getblk(): executed=cd bh_count=0 bh_state=29
> >   [  622.069808] syz-executor4(2193): getblk(): executed=9 bh_count=0 bh_state=0
> >   [  625.080013] syz-executor4(2193): getblk(): executed=9 bh_count=0 bh_state=0
> >   [  628.089900] syz-executor4(2193): getblk(): executed=9 bh_count=0 bh_state=0
> >
> > I guess that loop module is somehow related to this problem.
>
> I had a look into this and the only good explanation for this I have is
> that sb->s_blocksize is different from (1 << sb->s_bdev->bd_inode->i_blkbits).
> If that would happen, we'd get exactly the behavior syzkaller observes
> because grow_buffers() would populate different page than
> __find_get_block() then looks up.
>
> However I don't see how that's possible since the filesystem has the block
> device open exclusively and blkdev_bszset() makes sure we also have
> exclusive access to the block device before changing the block device size.
> So changing block device block size after filesystem gets access to the
> device should be impossible.

If this is that critical and impossible to fire, maybe it makes sense
to add a corresponding debug check to some code paths?
syzkaller will immediately catch any violations if they happen.


> Anyway, could you perhaps add to your debug patch a dump of 'size' passed
> to __getblk_slow() and bdev->bd_inode->i_blkbits? That should tell us
> whether my theory is right or not. Thanks!

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