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

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Date:   Thu, 22 Oct 2020 17:46:24 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     William Kucharski <william.kucharski@...cle.com>
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 Thu, Oct 22, 2020 at 07:23:33AM -0600, William Kucharski wrote:
> 
> 
> > On Oct 21, 2020, at 6:49 PM, Matthew Wilcox <willy@...radead.org> 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.
> 
> Looking at __test_set_page_writeback(), I see that it (and most other
> callers to lock_page_memcg()) do the following:

lock_page_memcg() is, unfortunately, completely unrelated to lock_page().
I believe all callers of __test_set_page_writeback() have the page lock
held already, but I'm going to put in an assert to that effect.