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From: Matthew Wilcox <willy@infradead.org>
To: Jan Kara <jack@suse.cz>
Cc: Qian Cai <cai@lca.pw>, Christoph Hellwig <hch@infradead.org>,
	"Darrick J. Wong" <darrick.wong@oracle.com>,
	linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org, Jens Axboe <axboe@kernel.dk>,
	linux-mm@kvack.org
Subject: Re: kernel BUG at mm/page-writeback.c:2241 [ BUG_ON(PageWriteback(page); ]
Date: Mon, 26 Oct 2020 13:13:53 +0000	[thread overview]
Message-ID: <20201026131353.GP20115@casper.infradead.org> (raw)
In-Reply-To: <20201026094948.GA29758@quack2.suse.cz>

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.


  reply	other threads:[~2020-10-26 13:14 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <645a3f332f37e09057c10bc32f4f298ce56049bb.camel@lca.pw>
2020-10-22  0:49 ` kernel BUG at mm/page-writeback.c:2241 [ BUG_ON(PageWriteback(page); ] Matthew Wilcox
2020-10-22 13:23   ` William Kucharski
2020-10-22 16:46     ` Matthew Wilcox
     [not found]   ` <7ec15e2710db02be81a6c47afc57abed4bf8016c.camel@lca.pw>
2020-10-22 17:12     ` Matthew Wilcox
2020-10-30 12:08       ` Qian Cai
2020-10-26  9:49   ` Jan Kara
2020-10-26 13:13     ` Matthew Wilcox [this message]
2020-10-26 13:55       ` Jens Axboe
2020-10-26 14:26         ` Qian Cai
2020-11-04 15:16           ` Jan Kara
2020-11-04 15:40             ` Qian Cai
2020-10-26 14:51         ` Qian Cai

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