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

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Message-ID: <87360lnxph.fsf@oracle.com>
Date:   Fri, 04 Dec 2020 15:05:46 -0500
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     Jason Gunthorpe <jgg@...pe.ca>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Alex Williamson <alex.williamson@...hat.com>
Cc:     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

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.