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Shutemov" Cc: akpm@linux-foundation.org, willy@infradead.org, markhemm@googlemail.com, viro@zeniv.linux.org.uk, david@redhat.com, khalid@kernel.org, andreyknvl@gmail.com, dave.hansen@intel.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 References: <20240903232241.43995-1-anthony.yznaga@oracle.com> Content-Language: en-US From: Anthony Yznaga In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: LO2P265CA0444.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:e::24) To SJ2PR10MB7653.namprd10.prod.outlook.com (2603:10b6:a03:542::22) Precedence: bulk X-Mailing-List: linux-arch@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SJ2PR10MB7653:EE_|SJ0PR10MB6399:EE_ X-MS-Office365-Filtering-Correlation-Id: 6501bd4c-44cd-4e46-fe5e-08dce7058c70 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|376014|1800799024|366016; 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Shutemov wrote: > On Tue, Sep 03, 2024 at 04:22:31PM -0700, Anthony Yznaga wrote: >> This patch series implements a mechanism that allows userspace >> processes to opt into sharing PTEs. It adds a new in-memory >> filesystem - msharefs. A file created on msharefs represents a >> shared region where all processes mapping that region will map >> objects within it with shared PTEs. When the file is created, >> a new host mm struct is created to hold the shared page tables >> and vmas for objects later mapped into the shared region. This >> host mm struct is associated with the file and not with a task. > Taskless mm_struct can be problematic. Like, we don't have access to it's > counters because it is not represented in /proc. For instance, there's no > way to check its smaps. Definitely needs exposure in /proc. One of the things I'm looking into is the feasibility of showing the mappings in maps/smaps/etc.. > > Also, I *think* it is immune to oom-killer because oom-killer looks for a > victim task, not mm. > I hope it is not an intended feature :P oom-killer would have to kill all sharers of an mshare region before the mshare region itself could be freed, but I'm not sure that oom-killer would be the one to free the region. An mshare region is essentially a shared memory object not unlike a tmpfs or hugetlb file. I think some higher level intelligence would have to be involved to release it if appropriate when under oom conditions. > >> When a process mmap's the shared region, a vm flag VM_SHARED_PT >> is added to the vma. On page fault the vma is checked for the >> presence of the VM_SHARED_PT flag. > I think it is wrong approach. > > Instead of spaying VM_SHARED_PT checks across core-mm, we need to add a > generic hooks that can be used by mshare and hugetlb. And remove > is_vm_hugetlb_page() check from core-mm along the way. > > BTW, is_vm_hugetlb_page() callsites seem to be the indicator to check if > mshare has to do something differently there. I feel you miss a lot of > such cases. Good point about is_vm_hugetlb_page(). I'll review the callsites (there are only ~60 of them :-). Thanks, Anthony