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From:   Daniel Jordan <daniel.m.jordan@oracle.com>
To:     Jason Gunthorpe <jgg@ziepe.ca>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Alex Williamson <alex.williamson@redhat.com>
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        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>
Subject: Re: [PATCH 6/6] mm/gup: migrate pinned pages out of movable zone
In-Reply-To: <20201203141729.GS5487@ziepe.ca>
References: <20201202052330.474592-1-pasha.tatashin@soleen.com>
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 <20201202163507.GL5487@ziepe.ca>
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 <20201203141729.GS5487@ziepe.ca>
Date:   Fri, 04 Dec 2020 15:05:46 -0500
Message-ID: <87360lnxph.fsf@oracle.com>
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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.