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Mon, 18 Sep 2023 11:07:46 -0700 (PDT) Date: Mon, 18 Sep 2023 11:07:44 -0700 In-Reply-To: Mime-Version: 1.0 References: Message-ID: Subject: Re: KVM nonblocking MMU notifier with KVM_GUEST_USES_PFN looks racy [but is currently unused] From: Sean Christopherson To: Jann Horn Cc: Paolo Bonzini , David Woodhouse , kernel list , KVM list , Linux-MM , Michal Hocko Content-Type: text/plain; charset="us-ascii" X-Rspamd-Queue-Id: 58A63C0018 X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: mzd3nwymjp6113u68qq66sf3g83419hp X-HE-Tag: 1695060467-217217 X-HE-Meta: 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 MUB55vZZ 7U/QpyQttulXH0YcycilizP68fuiJ93qUDF96faT3hlwZcjVclOiLdYsZWDc7gMLLLxVr32nXWBhDYau15BbJjP1Tl/7zU6jJ9NnGgZFa6bcDQFcM76o8bUAUUN12JmtTyG6/TS7qdQdkWlEelEaiQYNdIdvnq4fnunCQg67LOcXuV5gfIXVKTEo+IPcKhT8NHiDJ20ow0hP3R2zDr+QgHzJj/Iu48nTfMNK9zX+0aQU7hyR7bLqnvArL8QGrFcnGWQfma4SYD/0bQzF2zJHoYnH9U6VN9aLCMpXWjZxoUKLP2Ztz60oNQqfvlaUNfPeoD5bomZYNbv7TM6p2Zhm9Qbgs5ZyjSYfruBlawYpEtgXMUtYxc6fofOEHmyan54uTeo263hKwJQKN8pshjEFLpkOSnB1KhYiYpJYBltDMrhAeerNmqESLs2mKl8DjmWSOeOAxrDUFwmhbBkhVtLVFJS6TMp+t8m5gzGmHr5zBVXXuvxObuWZ4BCaExhCpmRlM8ro1snUth2fHnrx/2RMMPc0iSChCONAqODlZaG/zT6RlEsNyfJTnxnMNQu0wNDQIknWyIlfpqmO0DirTMvJrub6+xDkRXxgWy8/FYgtqg2Xaia12QQ8JnmQ0KMfCb3o8sU6OWU+OwMs9gXqqGYDRQSICyHbtSThIXLNPZdoUUOI6aWOLc1FFjxcWaWYKgpXDvbX8pQdHF4mMLbnmkniiTO1k6A== X-Bogosity: Ham, tests=bogofilter, spamicity=0.374533, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Sep 18, 2023, Jann Horn wrote: > Hi! > > I haven't tested this and might be missing something, but I think that > the MMU notifier for KVM_GUEST_USES_PFN pfncache is currently a bit > broken. Except that nothing seems to actually use KVM_GUEST_USES_PFN, > so currently it's not actually a problem? Yeah, the implementation is busted, and IMO the entire concept is a dead-end[1]. David Steven's series[2] doesn't actually rip out KVM_GUEST_USES_PFN, I'll add that to the todo list. There are no users, and I don't expect any to come along, precisely because it's busted :-) [1] https://lore.kernel.org/all/ZBEEQtmtNPaEqU1i@google.com [2] https://lkml.kernel.org/r/20230911021637.1941096-1-stevensd%40google.com