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Date: Fri, 27 Oct 2017 08:45:27 +0200
From: Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v3 00/13] dax: fix dma vs truncate and remove
        'page-less' support
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To: Jan Kara <jack@suse.cz>
Cc: Christoph Hellwig <hch@lst.de>, Dan Williams <dan.j.williams@intel.com>, akpm@linux-foundation.org, Michal Hocko <mhocko@suse.com>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, Dave Hansen <dave.hansen@linux.intel.com>, Dave Chinner <david@fromorbit.com>, "J. Bruce Fields" <bfields@fieldses.org>, linux-mm@kvack.org, Paul Mackerras <paulus@samba.org>, Sean Hefty <sean.hefty@intel.com>, Jeff Layton <jlayton@poochiereds.net>, Matthew Wilcox <mawilcox@microsoft.com>, linux-rdma@vger.kernel.org, Michael Ellerman <mpe@ellerman.id.au>, Jeff Moyer <jmoyer@redhat.com>, Jason Gunthorpe <jgunthorpe@obsidianresearch.com>, Doug Ledford <dledford@redhat.com>, Ross Zwisler <ross.zwisler@linux.intel.com>, Hal Rosenstock <hal.rosenstock@gmail.com>, Heiko Carstens <heiko.carstens@de.ibm.com>, linux-nvdimm@lists.01.org, Alexander Viro <viro@zeniv.linux.org.uk>, Gerald Schaefer <gerald.schaefer@de.ibm.com>, "Darrick J. Wong" <darrick.wong@oracle.com>, linux-kernel@vger.kernel.org, linux-xfs@vger.kernel.org, Martin Schwidefsky <schwidefsky@de.ibm.com>, linux-fsdevel@vger.kernel.org, "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>

On Thu, Oct 26, 2017 at 12:58:50PM +0200, Jan Kara wrote:
> But are we guaranteed page refs are short term? E.g. if someone creates
> v4l2 videobuf in MAP_SHARED mapping of a file on DAX filesystem, page refs
> can be rather long-term similarly as in RDMA case. Also freeing of blocks
> on page reference drop is another async entry point into the filesystem
> which could unpleasantly surprise us but I guess workqueues would solve
> that reasonably fine.

The point is that we need to prohibit long term elevated page counts
with DAX anyway - we can't just let people grab allocated blocks forever
while ignoring file system operations.  For stage 1 we'll just need to
fail those, and in the long run they will have to use a mechanism
similar to FL_LAYOUT locks to deal with file system allocation changes.