linux-ext4 - Re: [v4.14-rc3 bug] scheduling while atomic in generic/451 test on extN

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Message-ID: <20171016081708.GF32738@quack2.suse.cz>
Date:   Mon, 16 Oct 2017 10:17:08 +0200
From:   Jan Kara <jack@...e.cz>
To:     Lukas Czerner <lczerner@...hat.com>
Cc:     Jan Kara <jack@...e.cz>, Eryu Guan <eguan@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: [v4.14-rc3 bug] scheduling while atomic in generic/451 test on
 extN

On Fri 13-10-17 15:22:00, Lukas Czerner wrote:
> On Fri, Oct 13, 2017 at 12:28:42PM +0200, Lukas Czerner wrote:
> > On Thu, Oct 12, 2017 at 05:07:40PM +0200, Jan Kara wrote:
> > > Hi Eryu!
> > > 
> > > On Thu 05-10-17 14:07:00, Eryu Guan wrote:
> > > > I hit "scheduling while atomic" bug by running fstests generic/451 on
> > > > extN filesystems in v4.14-rc3 testing, but it didn't reproduce for me on
> > > > every host I tried, but I've seen it multiple times on multiple hosts. A
> > > > test vm of mine with 4 vcpus and 8G memory reproduced the bug reliably,
> > > > while a bare metal host with 8 cpus and 8G mem couldn't.
> > > > 
> > > > This is due to commit 332391a9935d ("fs: Fix page cache inconsistency
> > > > when mixing buffered and AIO DIO"), which defers AIO DIO io completion
> > > > to a workqueue if the inode has mapped pages and does page cache
> > > > invalidation in process context. I think that the problem is that the
> > > > pages can be mapped after the dio->inode->i_mapping->nrpages check, so
> > > > we're doing page cache invalidation, which could sleep, in interrupt
> > > > context, thus "scheduling while atomic" bug happens.
> > > > 
> > > > Defering all AIO DIO completion to workqueue unconditionally (as what
> > > > the iomap based path does) fixed the problem for me. But there're
> > > > performance concerns to do so in the original discussions.
> > > > 
> > > > https://www.spinics.net/lists/linux-fsdevel/msg112669.html
> > > 
> > > Thanks for report and the detailed analysis. I think your analysis is
> > > correct and the nrpages check in dio_bio_end_aio() is racy. My solution to
> > > this would be to pass to dio_complete() as an argument whether invalidation
> > > is required or not (and set it to true for deferred completion and to false
> > > when we decide not to defer completion since nrpages is 0 at that moment).
> > > Lukas?
> 
> Btw, instead of changing the arguments, can't we just use
> 
> if (current->flags & PF_WQ_WORKER)
> 
> to make sure we're called from the workqueue ?

I don't think that would be ideal since dio_complete() can be also called
in task's context where this check would fail...

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR