linux-kernel - Re: [PATCH v6 12/15] xfs: require mmap lock for xfs_break

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Date:   Mon, 19 Mar 2018 10:57:55 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 12/15] xfs: require mmap lock for xfs_break_layouts()

On Mon, Mar 19, 2018 at 10:33 AM, Darrick J. Wong
<darrick.wong@...cle.com> wrote:
> On Thu, Mar 15, 2018 at 08:52:29AM -0700, Dan Williams wrote:
>> In preparation for adding coordination between truncate operations and
>> busy dax-pages, extend xfs_break_layouts() to assume it must be called
>> with the mmap lock held. This locking scheme will be required for
>> coordinating the break of 'dax layouts' (non-idle dax (ZONE_DEVICE)
>> pages mapped into the file's address space).
>
> If I'm reading this right, you've added a requirement (for xfs anyway)
> that we have to have grabbed MMAPLOCK_EXCL before calling break_layout()
> so that the layout breaking process will block until active dmas have
> finished.
>
> In 4.16 we added xfs_iolock_two_inodes_and_break_layout (in
> xfs_reflink.c) to break pnfs leases for files that are about to be
> reflinked (since pnfs and reflink aren't compatible either).  I think
> that function will have to be adapted to take the appropriate mmap locks
> too -- definitely the exclusive mmap lock for the destination file
> because we anticipate punching out blocks.  I'm not sure about the
> source file; I think taking the shared mmap lock is fine for that?

I don't see anything to adapt with respect to mmap locks since reflink
and dax are mutually exclusive. The new lock coordination and layout
breaking in xfs_break_dax_layouts() only applies to dax vs truncate,
so xfs_iolock_two_inodes_and_break_layout() looks ok to me as is.