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Wed, 2 Oct 2024 19:30:30 +0000 Received: from MW6PR10MB7660.namprd10.prod.outlook.com ([fe80::41fa:92d3:28b9:2a15]) by MW6PR10MB7660.namprd10.prod.outlook.com ([fe80::41fa:92d3:28b9:2a15%7]) with mapi id 15.20.8026.016; Wed, 2 Oct 2024 19:30:30 +0000 Message-ID: <2dffaf2e-8a27-44bb-8d54-ef4cc0b08dc5@oracle.com> Date: Wed, 2 Oct 2024 12:30:22 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v3 00/10] Add support for shared PTEs across processes To: Dave Hansen , akpm@linux-foundation.org, willy@infradead.org, markhemm@googlemail.com, viro@zeniv.linux.org.uk, david@redhat.com, khalid@kernel.org Cc: andreyknvl@gmail.com, luto@kernel.org, brauner@kernel.org, arnd@arndb.de, ebiederm@xmission.com, catalin.marinas@arm.com, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, mhiramat@kernel.org, rostedt@goodmis.org, vasily.averin@linux.dev, xhao@linux.alibaba.com, pcc@google.com, neilb@suse.de, maz@kernel.org, David Rientjes References: <20240903232241.43995-1-anthony.yznaga@oracle.com> <9927f9a3-efba-4053-8384-cc69c7949ea6@intel.com> Content-Language: en-US From: Anthony Yznaga In-Reply-To: <9927f9a3-efba-4053-8384-cc69c7949ea6@intel.com> Content-Type: text/plain; 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KVM has a primary > mm: the QEMU (or whatever) process mm. The virtualization (EPT/NPT) > tables get entries that effectively mirror the primary mm page tables > and constitute a secondary MMU. If the primary page tables change, > mmu_notifiers ensure that the changes get reflected into the > virtualization tables and also that the virtualization paging structure > caches are flushed. > > msharefs is doing something very similar. But, in the msharefs case, > the secondary MMUs are actually normal CPU MMUs. The page tables are > normal old page tables and the caches are the normal old TLB. That's > what makes it so confusing: we have lots of infrastructure for dealing > with that "stuff" (CPU page tables and TLB), but msharefs has > short-circuited the infrastructure and it doesn't work any more. > > Basically, I think it makes a lot of sense to check what KVM (or another > mmu_notifier user) is doing and make sure that msharefs is following its > lead. For instance, KVM _should_ have the exact same "page free" > flushing issue where it gets the MMU notifier call but the page may > still be in the secondary MMU. I _think_ KVM fixes it with an extra > page refcount that it takes when it first walks the primary page tables. > > But the short of it is that the msharefs host mm represents a "secondary > MMU". I don't think it is really that special of an MMU other than the > fact that it has an mm_struct. Thanks, Dave. This is helpful. I'll look at what other mmu notifier users are doing. This does align with the comments in mmu_notifier.h regarding invalidate_range_start/end. Anthony