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

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Message-ID: <ea81f12a-95a3-4b9d-90e7-53a5d9c910be@nvidia.com>
Date: Tue, 29 Oct 2024 21:39:15 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Christoph Hellwig <hch@...radead.org>
Cc: 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 9:33 PM, Christoph Hellwig wrote:
> On Tue, Oct 29, 2024 at 09:30:41PM -0700, John Hubbard wrote:
>> I do, yes. And what happens is that when you use GPUs, drivers like
>> to pin system memory, and then point the GPU page tables to that
>> memory. For older GPUs that don't support replayable page faults,
>> that's required.
>>
>> So this behavior has been around forever.
>>
>> The customer was qualifying their software and noticed that before
>> Linux 6.10, they could allocate >2GB, and with 6.11, they could
>> not.
>>
>> Whether it is "wise" for user space to allocate that much at once
>> is a reasonable question, but at least one place is (or was!) doing
>> it.
> 
> Still missing a callchain, which make me suspect that it is your weird
> out of tree driver, in which case this simply does not matter.
> 

I expect I could piece together something with Nouveau, given enough
time and help from Ben Skeggs and Danillo and all...

Yes, this originated with the out of tree driver. But it never occurred
to me that upstream be uninterested in an obvious fix to an obvious
regression.


thanks,
-- 
John Hubbard