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[2003:cb:c719:5b00:61af:900f:3aef:3af3]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4247d0b63b5sm36684105e9.7.2024.06.20.11.56.21 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 20 Jun 2024 11:56:22 -0700 (PDT) Message-ID: Date: Thu, 20 Jun 2024 20:56:20 +0200 Precedence: bulk X-Mailing-List: linux-kselftest@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC 0/5] mm/gup: Introduce exclusive GUP pinning To: Sean Christopherson Cc: Jason Gunthorpe , Fuad Tabba , Christoph Hellwig , John Hubbard , Elliot Berman , Andrew Morton , Shuah Khan , Matthew Wilcox , maz@kernel.org, kvm@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, pbonzini@redhat.com References: <7fb8cc2c-916a-43e1-9edf-23ed35e42f51@nvidia.com> <14bd145a-039f-4fb9-8598-384d6a051737@redhat.com> <20240619115135.GE2494510@nvidia.com> <20240620135540.GG2494510@nvidia.com> <6d7b180a-9f80-43a4-a4cc-fd79a45d7571@redhat.com> <20240620142956.GI2494510@nvidia.com> <385a5692-ffc8-455e-b371-0449b828b637@redhat.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 20.06.24 18:04, Sean Christopherson wrote: > On Thu, Jun 20, 2024, David Hildenbrand wrote: >> On 20.06.24 16:29, Jason Gunthorpe wrote: >>> On Thu, Jun 20, 2024 at 04:01:08PM +0200, David Hildenbrand wrote: >>>> On 20.06.24 15:55, Jason Gunthorpe wrote: >>>>> On Thu, Jun 20, 2024 at 09:32:11AM +0100, Fuad Tabba wrote: >>>> Regarding huge pages: assume the huge page (e.g., 1 GiB hugetlb) is shared, >>>> now the VM requests to make one subpage private. >>> >>> I think the general CC model has the shared/private setup earlier on >>> the VM lifecycle with large runs of contiguous pages. It would only >>> become a problem if you intend to to high rate fine granual >>> shared/private switching. Which is why I am asking what the actual >>> "why" is here. >> >> I am not an expert on that, but I remember that the way memory >> shared<->private conversion happens can heavily depend on the VM use case, > > Yeah, I forget the details, but there are scenarios where the guest will share > (and unshare) memory at 4KiB (give or take) granularity, at runtime. There's an > RFC[*] for making SWIOTLB operate at 2MiB is driven by the same underlying problems. > > But even if Linux-as-a-guest were better behaved, we (the host) can't prevent the > guest from doing suboptimal conversions. In practice, killing the guest or > refusing to convert memory isn't an option, i.e. we can't completely push the > problem into the guest Agreed! > > https://lore.kernel.org/all/20240112055251.36101-1-vannapurve@google.com > >> and that under pKVM we might see more frequent conversion, without even >> going to user space. >> >>> >>>> How to handle that without eventually running into a double >>>> memory-allocation? (in the worst case, allocating a 1GiB huge page >>>> for shared and for private memory). >>> >>> I expect you'd take the linear range of 1G of PFNs and fragment it >>> into three ranges private/shared/private that span the same 1G. >>> >>> When you construct a page table (ie a S2) that holds these three >>> ranges and has permission to access all the memory you want the page >>> table to automatically join them back together into 1GB entry. >>> >>> When you construct a page table that has only access to the shared, >>> then you'd only install the shared hole at its natural best size. >>> >>> So, I think there are two challenges - how to build an allocator and >>> uAPI to manage this sort of stuff so you can keep track of any >>> fractured pfns and ensure things remain in physical order. >>> >>> Then how to re-consolidate this for the KVM side of the world. >> >> Exactly! >> >>> >>> guest_memfd, or something like it, is just really a good answer. You >>> have it obtain the huge folio, and keep track on its own which sub >>> pages can be mapped to a VMA because they are shared. KVM will obtain >>> the PFNs directly from the fd and KVM will not see the shared >>> holes. This means your S2's can be trivially constructed correctly. >>> >>> No need to double allocate.. >> >> Yes, that's why my thinking so far was: >> >> Let guest_memfd (or something like that) consume huge pages (somehow, let it >> access the hugetlb reserves). Preallocate that memory once, as the VM starts >> up: just like we do with hugetlb in VMs. >> >> Let KVM track which parts are shared/private, and if required, let it map >> only the shared parts to user space. KVM has all information to make these >> decisions. >> >> If we could disallow pinning any shared pages, that would make life a lot >> easier, but I think there were reasons for why we might require it. To >> convert shared->private, simply unmap that folio (only the shared parts >> could possibly be mapped) from all user page tables. >> >> Of course, there might be alternatives, and I'll be happy to learn about >> them. The allcoator part would be fairly easy, and the uAPI part would >> similarly be comparably easy. So far the theory :) >> >>> >>> I'm kind of surprised the CC folks don't want the same thing for >>> exactly the same reason. It is much easier to recover the huge >>> mappings for the S2 in the presence of shared holes if you track it >>> this way. Even CC will have this problem, to some degree, too. >> >> Precisely! RH (and therefore, me) is primarily interested in existing >> guest_memfd users at this point ("CC"), and I don't see an easy way to get >> that running with huge pages in the existing model reasonably well ... > > This is the general direction guest_memfd is headed, but getting there is easier > said than done. E.g. as alluded to above, "simply unmap that folio" is quite > difficult, bordering on infeasible if the kernel is allowed to gup() shared > guest_memfd memory. Right. I think ways forward are the ones stated in my mail to Jason: disallow long-term GUP or expose the huge page as unmovable small folios to core-mm. Maybe there are other alternatives, but it all feels like we want the MM to track in granularity of small pages, but map it into the KVM/IOMMU page tables in large pages. -- Cheers, David / dhildenb