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

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Date:   Fri, 13 Oct 2017 00:57:07 +0800
From:   Eryu Guan <eguan@...hat.com>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        lczerner@...hat.com
Subject: Re: [v4.14-rc3 bug] scheduling while atomic in generic/451 test on
 extN

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?

But wouldn't that bring the original bug back? i.e. read the stale data
from pagecache, because it's possible that we need to invalidate the
caches but we didn't.

Thanks,
Eryu