netdev - Re: [PATCH v11 00/25] mm/gup: track dma-pinned pages: FOLL

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Date:   Fri, 20 Dec 2019 16:02:15 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Jan Kara <jack@...e.cz>
CC:     Leon Romanovsky <leon@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Alex Williamson" <alex.williamson@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Dave Chinner <david@...morbit.com>,
        David Airlie <airlied@...ux.ie>,
        "David S . Miller" <davem@...emloft.net>,
        Ira Weiny <ira.weiny@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>,
        Jonathan Corbet <corbet@....net>,
        Jérôme Glisse <jglisse@...hat.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Paul Mackerras <paulus@...ba.org>,
        Shuah Khan <shuah@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, <bpf@...r.kernel.org>,
        <dri-devel@...ts.freedesktop.org>, <kvm@...r.kernel.org>,
        <linux-block@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <linux-fsdevel@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
        <linux-media@...r.kernel.org>, <linux-rdma@...r.kernel.org>,
        <linuxppc-dev@...ts.ozlabs.org>, <netdev@...r.kernel.org>,
        <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        Maor Gottlieb <maorg@...lanox.com>
Subject: Re: [PATCH v11 00/25] mm/gup: track dma-pinned pages: FOLL_PIN

On 12/20/19 1:21 AM, Jan Kara wrote:
...
>> So, ideas and next steps:
>>
>> 1. Assuming that you *are* hitting this, I think I may have to fall back to
>> implementing the "deferred" part of this design, as part of this series, after
>> all. That means:
>>
>>   For the pin/unpin calls at least, stop treating all pages as if they are
>>   a cluster of PAGE_SIZE pages; instead, retrieve a huge page as one page.
>>   That's not how it works now, and the need to hand back a huge array of
>>   subpages is part of the problem. This affects the callers too, so it's not
>>   a super quick change to make. (I was really hoping not to have to do this
>>   yet.)
> 
> Does that mean that you would need to make all GUP users huge page aware?
> Otherwise I don't see how what you suggest would work... And I don't think
> making all GUP users huge page aware is realistic (effort-wise) or even
> wanted (maintenance overhead in all those places).
> 

Well, pretty much yes. It's really just the pin_user_pages*() callers, but
the internals, follow_page() and such, are so interconnected right now that
it would probably blow up into a huge effort, as you point out.

> I believe there might be also a different solution for this: For
> transparent huge pages, we could find a space in 'struct page' of the
> second page in the huge page for proper pin counter and just account pins
> there so we'd have full width of 32-bits for it.
> 
> 								Honza
> 

OK, let me pursue that. Given that I shouldn't need to handle pages
splitting, it should be not *too* bad.

I am starting to think that I should just post the first 9 or so 
prerequisite patches (first 9 patches, plus the v4l2 fix that arguably should 
have been earlier in the sequence I guess), as 5.6 candidates, while I go
back to the drawing board here. 

thanks,
-- 
John Hubbard
NVIDIA