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

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Message-ID: <bfee966f-807d-4668-b353-159a6e8066f2@redhat.com>
Date: Wed, 30 Oct 2024 09:34:51 +0100
From: David Hildenbrand <david@...hat.com>
To: John Hubbard <jhubbard@...dia.com>, 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>,
 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 30.10.24 07:50, John Hubbard wrote:
> 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"

The unusual thing is not the amount of system memory we are pinning but 
*how many* pages we try pinning in the single call.

If you stare at vfio_pin_pages_remote, we seem to be batching it.

long req_pages = min_t(long, npage, batch->capacity);

Which is

#define VFIO_BATCH_MAX_CAPACITY (PAGE_SIZE / sizeof(struct page *))


So you can fix this in your driver ;)


We should maybe try a similar limit internally: if you call 
pin_user_pages_remote() with a large number, we'll cap it at some magic 
value (similar to above). The caller will simply realize that not all 
pages were pinned and will retry.

See get_user_pages_remote(): "Returns either number of pages pinned 
(which may be less than the number requested), or an error. Details 
about the return value:"


Alternatively, I recall there was a way to avoid the temporary 
allocation ... let me hack up a prototype real quick.
-- 
Cheers,

David / dhildenb