linux-kernel - Re: [PATCH v3 00/13] dax: fix dma vs truncate and remove 'page-less' support

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

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.