linux-ext4 - Re: [PATCH v4 0/2] ext4: fix DAX dma vs truncate/hole-punch

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Date:   Wed, 11 Jul 2018 10:17:41 +0200
From:   Jan Kara <jack@...e.cz>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:     Jan Kara <jack@...e.cz>, Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Christoph Hellwig <hch@....de>, linux-nvdimm@...ts.01.org,
        Jeff Moyer <jmoyer@...hat.com>, linux-ext4@...r.kernel.org,
        Lukas Czerner <lczerner@...hat.com>
Subject: Re: [PATCH v4 0/2] ext4: fix DAX dma vs truncate/hole-punch

On Tue 10-07-18 13:10:29, Ross Zwisler wrote:
> Changes since v3:
>  * Added an ext4_break_layouts() call to ext4_insert_range() to ensure
>    that the {ext4,xfs}_break_layouts() calls have the same meaning.
>    (Dave, Darrick and Jan)

How about the occasional WARN_ON_ONCE you mention below. Were you able to
hunt them down?

								Honza
> ---
> 
> This series from Dan:
> 
> https://lists.01.org/pipermail/linux-nvdimm/2018-March/014913.html
> 
> added synchronization between DAX dma and truncate/hole-punch in XFS.
> This short series adds analogous support to ext4.
> 
> I've added calls to ext4_break_layouts() everywhere that ext4 removes
> blocks from an inode's map.
> 
> The timings in XFS are such that it's difficult to hit this race.  Dan
> was able to show the race by manually introducing delays in the direct
> I/O path.
> 
> For ext4, though, its trivial to hit this race, and a hit will result in
> a trigger of this WARN_ON_ONCE() in dax_disassociate_entry():
> 
>         WARN_ON_ONCE(trunc && page_ref_count(page) > 1);
> 
> I've made an xfstest which tests all the paths where we now call
> ext4_break_layouts(). Each of the four paths easily hits this race many
> times in my test setup with the xfstest.  You can find that test here:
> 
> https://lists.01.org/pipermail/linux-nvdimm/2018-June/016435.html
> 
> With these patches applied, I've still seen occasional hits of the above
> WARN_ON_ONCE(), which tells me that we still have some work to do.  I'll
> continue looking at these more rare hits.
> 
> Ross Zwisler (2):
>   dax: dax_layout_busy_page() warn on !exceptional
>   ext4: handle layout changes to pinned DAX mappings
> 
>  fs/dax.c           | 10 +++++++++-
>  fs/ext4/ext4.h     |  1 +
>  fs/ext4/extents.c  | 17 +++++++++++++++++
>  fs/ext4/inode.c    | 46 ++++++++++++++++++++++++++++++++++++++++++++++
>  fs/ext4/truncate.h |  4 ++++
>  5 files changed, 77 insertions(+), 1 deletion(-)
> 
> -- 
> 2.14.4
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR