linux-ext4 - Re: [REPORT] kernel BUG at fs/ext4/inode.c:2620

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Date:   Fri, 18 Feb 2022 08:51:54 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Theodore Ts'o <tytso@....edu>
Cc:     John Hubbard <jhubbard@...dia.com>,
        Lee Jones <lee.jones@...aro.org>, linux-ext4@...r.kernel.org,
        Christoph Hellwig <hch@....de>,
        Dave Chinner <dchinner@...hat.com>,
        Goldwyn Rodrigues <rgoldwyn@...e.com>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        Bob Peterson <rpeterso@...hat.com>,
        Damien Le Moal <damien.lemoal@....com>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        Ritesh Harjani <riteshh@...ux.ibm.com>,
        Johannes Thumshirn <jth@...nel.org>, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, cluster-devel@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [REPORT] kernel BUG at fs/ext4/inode.c:2620 - page_buffers()

On Thu, Feb 17, 2022 at 11:08:38PM -0500, Theodore Ts'o wrote:
> On Thu, Feb 17, 2022 at 05:06:45PM -0800, John Hubbard wrote:
> > Yes. And looking at the pair of backtraces below, this looks very much
> > like another aspect of the "get_user_pages problem" [1], originally
> > described in Jan Kara's 2018 email [2].
> 
> Hmm... I just posted my analysis, which tracks with yours; but I had
> forgotten about Jan's 2018 e-mail on the matter.
> 
> > I'm getting close to posting an RFC for the direct IO conversion to
> > FOLL_PIN, but even after that, various parts of the kernel (reclaim,
> > filesystems/block layer) still need to be changed so as to use
> > page_maybe_dma_pinned() to help avoid this problem. There's a bit
> > more than that, actually.
> 
> The challenge is that fixing this "the right away" is probably not
> something we can backport into an LTS kernel, whether it's 5.15 or
> 5.10... or 4.19.

Don't worry about stable backports to start with.  Do it the "right way"
first and then we can consider if it needs to be backported or not.

thanks,

greg k-h