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From:   Pavel Tatashin <pasha.tatashin@soleen.com>
Date:   Fri, 4 Dec 2020 15:16:54 -0500
Message-ID: <CA+CK2bARYtSgkZTtMMFGB9kT4QZ_RgASW2wQ0Ro5_TXPddSung@mail.gmail.com>
Subject: Re: [PATCH 6/6] mm/gup: migrate pinned pages out of movable zone
To:     Daniel Jordan <daniel.m.jordan@oracle.com>
Cc:     Jason Gunthorpe <jgg@ziepe.ca>,
        Alex Williamson <alex.williamson@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Michal Hocko <mhocko@suse.com>,
        David Hildenbrand <david@redhat.com>,
        Oscar Salvador <osalvador@suse.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Sasha Levin <sashal@kernel.org>,
        Tyler Hicks <tyhicks@linux.microsoft.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>, mike.kravetz@oracle.com,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Mel Gorman <mgorman@suse.de>,
        Matthew Wilcox <willy@infradead.org>,
        David Rientjes <rientjes@google.com>,
        John Hubbard <jhubbard@nvidia.com>
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On Fri, Dec 4, 2020 at 3:06 PM Daniel Jordan <daniel.m.jordan@oracle.com> wrote:
>
> Jason Gunthorpe <jgg@ziepe.ca> writes:
>
> > On Wed, Dec 02, 2020 at 08:34:32PM -0500, Pavel Tatashin wrote:
> >> What I meant is the users of the interface do it incrementally not in
> >> large chunks. For example:
> >>
> >> vfio_pin_pages_remote
> >>    vaddr_get_pfn
> >>         ret = pin_user_pages_remote(mm, vaddr, 1, flags |
> >> FOLL_LONGTERM, page, NULL, NULL);
> >> 1 -> pin only one pages at a time
> >
> > I don't know why vfio does this, it is why it so ridiculously slow at
> > least.
>
> Well Alex can correct me, but I went digging and a comment from the
> first type1 vfio commit says the iommu API didn't promise to unmap
> subpages of previous mappings, so doing page at a time gave flexibility
> at the cost of inefficiency.
>
> Then 166fd7d94afd allowed the iommu to use larger pages in vfio, but
> vfio kept pinning pages at a time.  I couldn't find an explanation for
> why that stayed the same.
>
> Yesterday I tried optimizing vfio to skip gup calls for tail pages after
> Matthew pointed out this same issue to me by coincidence last week.
> Currently debugging, but if there's a fundamental reason this won't work
> on the vfio side, it'd be nice to know.

Hi Daniel,

I do not think there are any fundamental reasons why it won't work. I
have also thinking increasing VFIO chunking for a different reason:

If a client touches pages before doing a VFIO DMA map, those pages
might be huge, and pinning a small page at a time and migrating a
small page at a time can break-up the huge pages. So, it is not only
inefficient to pin, but it can also inadvertently slow down the
runtime.

Thank you,
Pasha