linux-kernel - Re: kernel BUG at mm/page-writeback.c:2241 [ BUG

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Date:   Mon, 26 Oct 2020 13:13:53 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Jan Kara <jack@...e.cz>
Cc:     Qian Cai <cai@....pw>, Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        linux-mm@...ck.org
Subject: Re: kernel BUG at mm/page-writeback.c:2241 [
 BUG_ON(PageWriteback(page); ]

On Mon, Oct 26, 2020 at 10:49:48AM +0100, Jan Kara wrote:
> On Thu 22-10-20 01:49:06, Matthew Wilcox wrote:
> > On Wed, Oct 21, 2020 at 08:30:18PM -0400, Qian Cai wrote:
> > > Today's linux-next starts to trigger this wondering if anyone has any clue.
> > 
> > I've seen that occasionally too.  I changed that BUG_ON to VM_BUG_ON_PAGE
> > to try to get a clue about it.  Good to know it's not the THP patches
> > since they aren't in linux-next.
> > 
> > I don't understand how it can happen.  We have the page locked, and then we do:
> > 
> >                         if (PageWriteback(page)) {
> >                                 if (wbc->sync_mode != WB_SYNC_NONE)
> >                                         wait_on_page_writeback(page);
> >                                 else
> >                                         goto continue_unlock;
> >                         }
> > 
> >                         VM_BUG_ON_PAGE(PageWriteback(page), page);
> > 
> > Nobody should be able to put this page under writeback while we have it
> > locked ... right?  The page can be redirtied by the code that's supposed
> > to be writing it back, but I don't see how anyone can make PageWriteback
> > true while we're holding the page lock.
> 
> FWIW here's very similar report for ext4 [1] and I strongly suspect this
> started happening after Linus' rewrite of the page bit waiting logic. Linus
> thinks it's preexisting bug which just got exposed by his changes (which is
> possible). I've been searching a culprit for some time but so far I failed.
> It's good to know it isn't ext4 specific so we should be searching in the
> generic code ;). So far I was concentrating more on ext4 bits...
> 
> 								Honza
> 
> [1] https://lore.kernel.org/lkml/000000000000d3a33205add2f7b2@google.com/

Oh good, I was wondering if it was an XFS bug ;-)

I hope Qian gets it to reproduce soon with the assert because that will
tell us whether it's a spurious wakeup or someone calling SetPageWriteback
without holding the page lock.