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

[Christoph Hellwig]

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.