Re: [PATCH v3 1/4] mm: introduce get_user_pages_longterm

[Michal Hocko]

On Thu 30-11-17 08:39:51, Dan Williams wrote:
> On Thu, Nov 30, 2017 at 1:53 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > On Wed 29-11-17 10:05:35, Dan Williams wrote:
> >> Until there is a solution to the dma-to-dax vs truncate problem it is
> >> not safe to allow long standing memory registrations against
> >> filesytem-dax vmas. Device-dax vmas do not have this problem and are
> >> explicitly allowed.
> >>
> >> This is temporary until a "memory registration with layout-lease"
> >> mechanism can be implemented for the affected sub-systems (RDMA and
> >> V4L2).
> >
> > One thing is not clear to me. Who is allowed to pin pages for ever?
> > Is it possible to pin LRU pages that way as well? If yes then there
> > absolutely has to be a limit for that. Sorry I could have studied the
> > code much more but from a quick glance it seems to me that this is not
> > limited to dax (or non-LRU in general) pages.
> 
> I would turn this question around. "who can not tolerate a page being
> pinned forever?".

Any struct page on the movable zone or anything that is living on the
LRU list because such a memory is unreclaimable.

> In the case of filesytem-dax a page is
> one-in-the-same object as a filesystem-block, and a filesystem expects
> that its operations will not be blocked indefinitely. LRU pages can
> continue to be pinned indefinitely because operations can continue
> around the pinned page, i.e. every agent, save for the dma agent,
> drops their reference to the page and its tolerable that the final
> put_page() never arrives.

I do not understand. Are you saying that a user triggered IO can pin LRU
pages indefinitely. This would be _really_ wrong. It would be basically
an mlock without any limit. So I must be misreading you here

> As far as I can tell it's only filesystems
> and dax that have this collision of wanting to revoke dma access to a
> page combined with not being able to wait indefinitely for dma to
> quiesce.

-- 
Michal Hocko
SUSE Labs