linux-kernel - Re: [PATCH 2/2] mm: set PG_dma_pinned on get_user

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Date:   Wed, 27 Jun 2018 13:32:21 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Jan Kara <jack@...e.cz>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        John Hubbard <jhubbard@...dia.com>,
        Christoph Hellwig <hch@....de>, Jason Gunthorpe <jgg@...pe.ca>,
        John Hubbard <john.hubbard@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Christopher Lameter <cl@...ux.com>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm: set PG_dma_pinned on get_user_pages*()

On Tue 26-06-18 18:48:25, Jan Kara wrote:
> On Tue 26-06-18 15:47:57, Michal Hocko wrote:
> > On Mon 18-06-18 12:21:46, Dan Williams wrote:
> > [...]
> > > I do think we should explore a page flag for pages that are "long
> > > term" pinned. Michal asked for something along these lines at LSF / MM
> > > so that the core-mm can give up on pages that the kernel has lost
> > > lifetime control. Michal, did I capture your ask correctly?
> > 
> > I am sorry to be late. I didn't ask for a page flag exactly. I've asked
> > for a way to query for the pin to be temporal or permanent. How that is
> > achieved is another question. Maybe we have some more spare room after
> > recent struct page reorganization but I dunno, to be honest. Maybe we
> > can have an _count offset for these longterm pins. It is not like we are
> > using the whole ref count space, right?
> 
> Matthew had an interesting idea to pull pinned pages completely out from
> any LRU and reuse that space in struct page for pinned refcounts. From some
> initial investigation (read on elsewhere in this thread) it looks doable. I
> was considering offsetting in refcount as well but on 32-bit architectures
> there's not that many bits that I'd be really comfortable with that
> solution...

I am really slow at following up this discussion. The problem I would
see with off-lru pages is that this can quickly turn into a weird
reclaim behavior. Especially when we are talking about a lot of memory.
It is true that such pages wouldn't be reclaimable directly but could
poke them in some way if we see too many of them while scanning LRU.

Not that this is a fundamental block stopper but this is the first thing
that popped out when thinking about such a solution. Maybe it is a good
start though.

Appart from that, do we really care about 32b here? Big DIO, IB users
seem to be 64b only AFAIU.
-- 
Michal Hocko
SUSE Labs