linux-kernel - Re: [RFC PATCH 00/19] mm: Introduce a cgroup to limit the amount of locked and pinned memory

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Date:   Tue, 31 Jan 2023 15:15:49 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Alistair Popple <apopple@...dia.com>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        jhubbard@...dia.com, tjmercier@...gle.com, hannes@...xchg.org,
        surenb@...gle.com, mkoutny@...e.com, daniel@...ll.ch
Subject: Re: [RFC PATCH 00/19] mm: Introduce a cgroup to limit the amount of
 locked and pinned memory

On 31.01.23 15:10, Jason Gunthorpe wrote:
> On Tue, Jan 31, 2023 at 03:06:10PM +0100, David Hildenbrand wrote:
>> On 31.01.23 15:03, Jason Gunthorpe wrote:
>>> On Tue, Jan 31, 2023 at 02:57:20PM +0100, David Hildenbrand wrote:
>>>
>>>>> I'm excited by this series, thanks for making it.
>>>>>
>>>>> The pin accounting has been a long standing problem and cgroups will
>>>>> really help!
>>>>
>>>> Indeed. I'm curious how GUP-fast, pinning the same page multiple times, and
>>>> pinning subpages of larger folios are handled :)
>>>
>>> The same as today. The pinning is done based on the result from GUP,
>>> and we charge every returned struct page.
>>>
>>> So duplicates are counted multiple times, folios are ignored.
>>>
>>> Removing duplicate charges would be costly, it would require storage
>>> to keep track of how many times individual pages have been charged to
>>> each cgroup (eg an xarray indexed by PFN of integers in each cgroup).
>>>
>>> It doesn't seem worth the cost, IMHO.
>>>
>>> We've made alot of investment now with iommufd to remove the most
>>> annoying sources of duplicated pins so it is much less of a problem in
>>> the qemu context at least.
>>
>> Wasn't there the discussion regarding using vfio+io_uring+rdma+$whatever on
>> a VM and requiring multiple times the VM size as memlock limit?
> 
> Yes, but iommufd gives us some more options to mitigate this.
> 
> eg it makes some of logical sense to point RDMA at the iommufd page
> table that is already pinned when trying to DMA from guest memory, in
> this case it could ride on the existing pin.

Right, I suspect some issue is that the address space layout for the 
RDMA device might be completely different. But I'm no expert on IOMMUs 
at all :)

I do understand that at least multiple VFIO containers could benefit by 
only pinning once (IIUC that mgiht have been an issue?).

> 
>> Would it be the same now, just that we need multiple times the pin
>> limit?
> 
> Yes

Okay, thanks.


It's all still a big improvement, because I also asked for TDX 
restrictedmem to be accounted somehow as unmovable.

-- 
Thanks,

David / dhildenb