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

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Date:   Tue, 8 Dec 2020 09:24:35 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Daniel Jordan <daniel.m.jordan@...cle.com>
Cc:     Pavel Tatashin <pasha.tatashin@...een.com>,
        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

On Mon, Dec 07, 2020 at 09:48:48PM -0500, Daniel Jordan wrote:
> Jason Gunthorpe <jgg@...pe.ca> writes:
> > On Fri, Dec 04, 2020 at 03:05:46PM -0500, Daniel Jordan wrote:
> >> 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.
> >
> > iommu restrictions are not related to with gup. vfio needs to get the
> > page list from the page tables as efficiently as possible, then you
> > break it up into what you want to feed into the IOMMU how the iommu
> > wants.
> >
> > vfio must maintain a page list to call unpin_user_pages() anyhow, so
> 
> It does in some cases but not others, namely the expensive
> VFIO_IOMMU_MAP_DMA/UNMAP_DMA path where the iommu page tables are used
> to find the pfns when unpinning.

Oh, I see.. Well, that is still possible, but vfio really needs to
batch operations, eg call pin_user_pages() with some larger buffer and
store those into the iommu and then reverse this to build up
contiguous runs of pages to unpin

> I don't see why vfio couldn't do as you say, though, and the worst case
> memory overhead of using scatterlist to remember the pfns of a 300g VM
> backed by huge but physically discontiguous pages is only a few meg, not
> bad at all.

Yes, but 0 is still better.. I would start by focusing on batching
pin_user_pages.

Jason