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Tue, 24 Jan 2023 07:53:35 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id E4c3MH+Oz2OlHgAAMHmgww (envelope-from ); Tue, 24 Jan 2023 07:53:35 +0000 Message-ID: <8ac234a3-9dc3-3ebf-309a-b4a6bcb72d2d@suse.cz> Date: Tue, 24 Jan 2023 08:51:43 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.6.1 Subject: Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to create restricted user memory Content-Language: en-US To: Sean Christopherson , "Huang, Kai" Cc: "chao.p.peng@linux.intel.com" , "tglx@linutronix.de" , "linux-arch@vger.kernel.org" , "kvm@vger.kernel.org" , "jmattson@google.com" , "Lutomirski, Andy" , "ak@linux.intel.com" , "kirill.shutemov@linux.intel.com" , "Hocko, Michal" , "tabba@google.com" , "qemu-devel@nongnu.org" , "david@redhat.com" , "michael.roth@amd.com" , "corbet@lwn.net" , "linux-kernel@vger.kernel.org" , "dhildenb@redhat.com" , "bfields@fieldses.org" , "linux-fsdevel@vger.kernel.org" , "x86@kernel.org" , "bp@alien8.de" , "ddutile@redhat.com" , "rppt@kernel.org" , "shuah@kernel.org" , "vkuznets@redhat.com" , "naoya.horiguchi@nec.com" , "linux-api@vger.kernel.org" , "qperret@google.com" , "arnd@arndb.de" , "pbonzini@redhat.com" , "Annapurve, Vishal" , "mail@maciej.szmigiero.name" , "wanpengli@tencent.com" , "yu.c.zhang@linux.intel.com" , "hughd@google.com" , "aarcange@redhat.com" , "mingo@redhat.com" , "hpa@zytor.com" , "Nakajima, Jun" , "jlayton@kernel.org" , "joro@8bytes.org" , "linux-mm@kvack.org" , "Wang, Wei W" , "steven.price@arm.com" , "linux-doc@vger.kernel.org" , "Hansen, Dave" , "akpm@linux-foundation.org" , "linmiaohe@huawei.com" References: <20221202061347.1070246-2-chao.p.peng@linux.intel.com> <5c6e2e516f19b0a030eae9bf073d555c57ca1f21.camel@intel.com> <20221219075313.GB1691829@chaop.bj.intel.com> <20221220072228.GA1724933@chaop.bj.intel.com> <126046ce506df070d57e6fe5ab9c92cdaf4cf9b7.camel@intel.com> <20221221133905.GA1766136@chaop.bj.intel.com> <010a330c-a4d5-9c1a-3212-f9107d1c5f4e@suse.cz> <0959c72ec635688f4b6c1b516815f79f52543b31.camel@intel.com> From: Vlastimil Babka In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 1/24/23 00:38, Sean Christopherson wrote: > On Mon, Jan 23, 2023, Huang, Kai wrote: >> On Mon, 2023-01-23 at 15:03 +0100, Vlastimil Babka wrote: >>> On 12/22/22 01:37, Huang, Kai wrote: >>>>>> I argue that this page pinning (or page migration prevention) is not >>>>>> tied to where the page comes from, instead related to how the page will >>>>>> be used. Whether the page is restrictedmem backed or GUP() backed, once >>>>>> it's used by current version of TDX then the page pinning is needed. So >>>>>> such page migration prevention is really TDX thing, even not KVM generic >>>>>> thing (that's why I think we don't need change the existing logic of >>>>>> kvm_release_pfn_clean()).  >>>>>> >>>> This essentially boils down to who "owns" page migration handling, and sadly, >>>> page migration is kinda "owned" by the core-kernel, i.e. KVM cannot handle page >>>> migration by itself -- it's just a passive receiver. >>>> >>>> For normal pages, page migration is totally done by the core-kernel (i.e. it >>>> unmaps page from VMA, allocates a new page, and uses migrate_pape() or a_ops- >>>>> migrate_page() to actually migrate the page). >>>> In the sense of TDX, conceptually it should be done in the same way. The more >>>> important thing is: yes KVM can use get_page() to prevent page migration, but >>>> when KVM wants to support it, KVM cannot just remove get_page(), as the core- >>>> kernel will still just do migrate_page() which won't work for TDX (given >>>> restricted_memfd doesn't have a_ops->migrate_page() implemented). >>>> >>>> So I think the restricted_memfd filesystem should own page migration handling, >>>> (i.e. by implementing a_ops->migrate_page() to either just reject page migration >>>> or somehow support it). >>> >>> While this thread seems to be settled on refcounts already,  >>> >> >> I am not sure but will let Sean/Paolo to decide. > > My preference is whatever is most performant without being hideous :-) > >>> just wanted >>> to point out that it wouldn't be ideal to prevent migrations by >>> a_ops->migrate_page() rejecting them. It would mean cputime wasted (i.e. >>> by memory compaction) by isolating the pages for migration and then >>> releasing them after the callback rejects it (at least we wouldn't waste >>> time creating and undoing migration entries in the userspace page tables >>> as there's no mmap). Elevated refcount on the other hand is detected >>> very early in compaction so no isolation is attempted, so from that >>> aspect it's optimal. >> >> I am probably missing something, > > Heh, me too, I could have sworn that using refcounts was the least efficient way > to block migration. Well I admit that due to my experience with it, I do mostly consider migration through memory compaction POV, which is a significant user of migration on random pages that's not requested by userspace actions on specific ranges. And compaction has in isolate_migratepages_block(): /* * Migration will fail if an anonymous page is pinned in memory, * so avoid taking lru_lock and isolating it unnecessarily in an * admittedly racy check. */ mapping = page_mapping(page); if (!mapping && page_count(page) > page_mapcount(page)) goto isolate_fail; so that prevents migration of pages with elevated refcount very early, before they are even isolated, so before migrate_pages() is called. But it's true there are other sources of "random pages migration" - numa balancing, demotion in lieu of reclaim... and I'm not sure if all have such early check too. Anyway, whatever is decided to be a better way than elevated refcounts, would ideally be checked before isolation as well, as that's the most efficient way. >> but IIUC the checking of refcount happens at very last stage of page migration too