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[2003:cb:c725:e600:4063:2059:fd18:9d65]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-366d5cbd5bbsm313262f8f.97.2024.06.21.02.37.51 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 21 Jun 2024 02:37:52 -0700 (PDT) Message-ID: <3fc39c4f-d4e6-4f63-96d6-7c24cad16612@redhat.com> Date: Fri, 21 Jun 2024 11:37:50 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC 0/5] mm/gup: Introduce exclusive GUP pinning To: Quentin Perret Cc: Jason Gunthorpe , Elliot Berman , Fuad Tabba , Christoph Hellwig , John Hubbard , 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: <20240619115135.GE2494510@nvidia.com> <20240620135540.GG2494510@nvidia.com> <6d7b180a-9f80-43a4-a4cc-fd79a45d7571@redhat.com> <20240620142956.GI2494510@nvidia.com> <20240620140516768-0700.eberman@hu-eberman-lv.qualcomm.com> <20240620231814.GO2494510@nvidia.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 8A85418000E X-Stat-Signature: dfkxs989em4a47yx15i1sihpkroazm8s X-HE-Tag: 1718962676-453944 X-HE-Meta: U2FsdGVkX18ttmBdVCNJtkjQm2r7URaInrOHZwVnM9gAcPXhw4qg9z4fc2NpKCT6xIL34FmlARffDOBruq/EDJfztCJqrXYjpQnk4ynzpunPWMM7cOjsRO/F6EsarGZXd+j5RBjN4NzgXttOUKNQh6ShljbbADlDEtI2zjZWWPaIjZLQzxsTqUmS9yJa4dV3UmrFfWJWrF8Ah2PDvr370KgRlYvlU0y4vXadLxEt03hBUoYQ0/YkgOl+SLHMc0I7I7vJN+PIWqMOp0FwgPPuY85siQf4Eclk4pbHFAmRnTkaZrohXLP7X2ai4YYTuXFFUdwyDUOjWEixf6MdsmOvPlMiG0dtnBHc0rm6Xuk9VFO+LkUZxUKgq5sEcZsl+t9AZIm4eSs/LGcxZzm75EeVo0K5vSexMvLZXmfDiy14ezAlG5nCnBhHja/3nA7CNnTdsE43ynh6WPK2hCCRFMZSLpioWVNt18TNHwmQVfTSQjqzFssFB+QhxUL8nn8kGOrna4FF+vlW+pUkYntbNDIsNms0bmzp5afKhA2ZkyyZLnyc+0Kan7ohGdZ5JyX0iW5hfFzjgk59uhVaWpCs7otymmxffSdZIhRNTE/qVxiRgEuO5GzIpttwhcM4UXfg2RpQylvdJbawl9ZDeUaWU4HgSVFy0pWprzLtki0w8YE3oqrk8VoJC/vy4acjo/OKq7MeAEVCjuxKQ/VCFhDnq0ybWtOEs9d/2d2BtOcuXTwKo5wp6nZcxtRPDrXX2ii4YBOab2JEBb8ySty4Xd/L1lsE/kPFycUGgr3nKENih+cXSCAPfXvww0NbVYpSZ4L5FF4YOB6zYtVgqxKEZtvgQf/WJuHmclGeXWz/B3Mb8u/CpKgE4qb8VCh/rESKjP2MliLtmRBDlveMDzrXwjExjKiwpfVHq3hjzA1peHD3LFaFAL/lYMCRXsSdI5+eM0fJsMqwPn2xd4v/o0HwcaM0sfM +6NZL+gG M6NF4yWHeKK9hxRaeljbiJlzPCtB8JcF9TSGYh+xT7kkMjzJOzNfTKk3uliYHaUjKWfnPuqcVX/fbB9caFKniOgVpqkYzyXLw/+GEzkZK6zrRr7kTD5shXMHuuqnTlSdsNcBi6VboORFhSa0Nk+j38E414s49TQQFYFw6qA3JLIu8BhZZyMGJkBr/AR2D9Q4WB1nR/nIDOxijpDi8vMkNPCNozYeg1x9qTV0LJ8KxKPlXxVD62VT8xokOOtdTQs1nRklkx2fHm5CPyoExWMmIDWJxXCoeRGhaYiOwIQ0ck1QPjC20RdpSmNFfNSBquy/VkuVDnKsTJvfgbCQtgwvS0xQ2mDLCqHzcOWLPBxMyVFcjzywm4yFhjD4jPg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 21.06.24 11:25, Quentin Perret wrote: > On Friday 21 Jun 2024 at 10:02:08 (+0200), David Hildenbrand wrote: >> Thanks for the information. IMHO we really should try to find a common >> ground here, and FOLL_EXCLUSIVE is likely not it :) > > That's OK, IMO at least :-). > >> Thanks for reviving this discussion with your patch set! >> >> pKVM is interested in in-place conversion, I believe there are valid use >> cases for in-place conversion for TDX and friends as well (as discussed, I >> think that might be a clean way to get huge/gigantic page support in). >> >> This implies the option to: >> >> 1) Have shared+private memory in guest_memfd >> 2) Be able to mmap shared parts >> 3) Be able to convert shared<->private in place >> >> and later in my interest >> >> 4) Have huge/gigantic page support in guest_memfd with the option of >> converting individual subpages >> >> We might not want to make use of that model for all of CC -- as you state, >> sometimes the destructive approach might be better performance wise -- but >> having that option doesn't sound crazy to me (and maybe would solve real >> issues as well). > > Cool. > >> After all, the common requirement here is that "private" pages are not >> mapped/pinned/accessible. >> >> Sure, there might be cases like "pKVM can handle access to private pages in >> user page mappings", "AMD-SNP will not crash the host if writing to private >> pages" but there are not factors that really make a difference for a common >> solution. > > Sure, there isn't much value in differentiating on these things. One > might argue that we could save one mmap() on the private->shared > conversion path by keeping all of guest_memfd mapped in userspace > including private memory, but that's most probably not worth the > effort of re-designing the whole thing just for that, so let's forget > that. In a world where we can mmap() the whole (sparse "shared") thing, and dynamically map/unmap the shared parts only it would be saving a page fault on private->shared conversion, correct. But that's sounds more like a CC-specific optimization for frequent conversions, which we should just ignore initially. > > The ability to handle stage-2 faults in the kernel has implications in > other places however. It means we don't need to punch holes in the > kernel linear map when donating memory to a guest for example, even with > 'crazy' access patterns like load_unaligned_zeropad(). So that's good. > >> private memory: not mapped, not pinned >> shared memory: maybe mapped, maybe pinned >> granularity of conversion: single pages >> >> Anything I am missing? > > That looks good to me. And as discussed in previous threads, we have the > ambition of getting page-migration to work, including for private memory, > mostly to get kcompactd to work better when pVMs are running. Android > makes extensive use of compaction, and pVMs currently stick out like a > sore thumb. Yes, I think migration for compaction has to be supported at some point (at least for small pages that can be either private or shared, not a mixture), and I suspect we should be able to integrate it with core-mm in a not-too-horrible fashion. For example, we do have a non-lru page migration infrastructure in place already if the LRU-based one is not a good fit. Memory swapping and all other currently-strictly LRU-based mechanisms should be out of scope for now: as Sean says, we don't want to go down that path. > > We can trivially implement a hypercall to have pKVM swap a private > page with another without the guest having to know. The difficulty is > obviously to hook that in Linux, and I've personally not looked into it > properly, so that is clearly longer term. We don't want to take anybody > by surprise if there is a need for some added complexity in guest_memfd > to support this use-case though. I don't expect folks on the receiving > end of that to agree to it blindly without knowing _what_ this > complexity is FWIW. But at least our intentions are clear :-) Agreed. -- Cheers, David / dhildenb