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

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Date:   Fri, 16 Mar 2018 20:04:43 +0100
From:   Christoph Hellwig <hch@....de>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-nvdimm@...ts.01.org,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, jack@...e.cz,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 12/15] xfs: require mmap lock for xfs_break_layouts()

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).

This requirement wasn't really there in the last series, why do we
require it now?

As far as I can tell all we'd need is to just drop this assert:

> -	ASSERT(xfs_isilocked(ip, XFS_IOLOCK_SHARED|XFS_IOLOCK_EXCL));
> +	ASSERT(xfs_isilocked(ip, XFS_IOLOCK_SHARED | XFS_IOLOCK_EXCL
> +				| XFS_MMAPLOCK_EXCL));

entirely.

>  	while ((error = break_layout(inode, false) == -EWOULDBLOCK)) {
>  		xfs_iunlock(ip, *iolock);
>  		error = break_layout(inode, true);
> -		*iolock = XFS_IOLOCK_EXCL;
> +		*iolock &= ~XFS_IOLOCK_SHARED;
> +		*iolock |= XFS_IOLOCK_EXCL;
>  		xfs_ilock(ip, *iolock);

And take this one hunk from your patch.

To enable the DAX use case.