linux-kernel - Re: [PATCH 6/6] mm/gup: migrate pinned pages out of movable zone

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Date:   Mon, 07 Dec 2020 21:27:20 -0500
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>,
        Alex Williamson <alex.williamson@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...e.com>,
        David Hildenbrand <david@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Sasha Levin <sashal@...nel.org>,
        Tyler Hicks <tyhicks@...ux.microsoft.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, mike.kravetz@...cle.com,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        David Rientjes <rientjes@...gle.com>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH 6/6] mm/gup: migrate pinned pages out of movable zone

Pavel Tatashin <pasha.tatashin@...een.com> writes:

> On Fri, Dec 4, 2020 at 3:06 PM Daniel Jordan <daniel.m.jordan@...cle.com> wrote:
>>
>> Jason Gunthorpe <jgg@...pe.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.

Hi Pasha,

I see, and I'm curious, do you experience this case where a user has
touched the pages before doing a VFIO DMA map, and if so where?

The usual situation on my side is that the pages are faulted in during
pinning.

Daniel