Message-ID: <2e78d261-9ae9-d203-446e-eaa3c652ca6e@redhat.com>
Date:   Tue, 31 Jan 2023 15:06:10 +0100
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Subject: Re: [RFC PATCH 00/19] mm: Introduce a cgroup to limit the amount of
 locked and pinned memory
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To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     Alistair Popple <apopple@nvidia.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        jhubbard@nvidia.com, tjmercier@google.com, hannes@cmpxchg.org,
        surenb@google.com, mkoutny@suse.com, daniel@ffwll.ch
References: <cover.f52b9eb2792bccb8a9ecd6bc95055705cfe2ae03.1674538665.git-series.apopple@nvidia.com>
 <Y9A7kDjm3ZFAttRR@nvidia.com>
 <6369225e-3522-341b-cd20-d95b1f11ea71@redhat.com>
 <Y9kfn4YX59PIxj7+@nvidia.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <Y9kfn4YX59PIxj7+@nvidia.com>
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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? Would 
it be the same now, just that we need multiple times the pin limit?

-- 
Thanks,

David / dhildenb