linux-kernel - Re: [PATCH] mm/gup: restore the ability to pin more than 2GB at a time

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Message-ID: <128d04dd-2d48-4a98-8537-49589b4db1c3@nvidia.com>
Date: Tue, 29 Oct 2024 23:50:05 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Alistair Popple <apopple@...dia.com>
Cc: Christoph Hellwig <hch@...radead.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
 linux-stable@...r.kernel.org, Vivek Kasireddy <vivek.kasireddy@...el.com>,
 David Hildenbrand <david@...hat.com>, Dave Airlie <airlied@...hat.com>,
 Gerd Hoffmann <kraxel@...hat.com>, Matthew Wilcox <willy@...radead.org>,
 Jason Gunthorpe <jgg@...dia.com>, Peter Xu <peterx@...hat.com>,
 Arnd Bergmann <arnd@...db.de>, Daniel Vetter <daniel.vetter@...ll.ch>,
 Dongwon Kim <dongwon.kim@...el.com>, Hugh Dickins <hughd@...gle.com>,
 Junxiao Chang <junxiao.chang@...el.com>,
 Mike Kravetz <mike.kravetz@...cle.com>, Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH] mm/gup: restore the ability to pin more than 2GB at a
 time

On 10/29/24 11:18 PM, Alistair Popple wrote:
> John Hubbard <jhubbard@...dia.com> writes:
>> On 10/29/24 9:42 PM, Christoph Hellwig wrote:
>>> On Tue, Oct 29, 2024 at 09:39:15PM -0700, John Hubbard wrote:
...
>>> Because pinning down these amounts of memoryt is completely insane.
>>> I don't mind the switch to kvmalloc, but we need to put in an upper
>>> bound of what can be pinned.
>>
>> I'm wondering though, how it is that we decide how much of the user's
>> system we prevent them from using? :)  People with hardware accelerators
>> do not always have page fault capability, and yet these troublesome
>> users insist on stacking their system full of DRAM and then pointing
>> the accelerator to it.
>>
>> How would we choose a value? Memory sizes keep going up...
> 
> The obvious answer is you let users decide. I did have a patch series to
> do that via a cgroup[1]. However I dropped that series mostly because I
> couldn't find any users of such a limit to provide feedback on how they
> would use it or how they wanted it to work.
> 

Trawling through the discussion there, I see that Jason Gunthorpe mentioned:

"Things like VFIO & KVM use cases effectively pin 90% of all system memory"

...which means that we'll be able to get that in-tree call trace that 
Christoph
is asking for, pretty soon. No GPUs required. :)


thanks,
-- 
John Hubbard

> - Alistair
> 
> [1] - https://lore.kernel.org/linux-mm/cover.c238416f0e82377b449846dbb2459ae9d7030c8e.1675669136.git-series.apopple@nvidia.com/
>