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Mon, 7 Oct 2024 05:01:36 -0400 (EDT) Date: Mon, 7 Oct 2024 12:01:32 +0300 From: "Kirill A. Shutemov" To: Anthony Yznaga 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 Subject: Re: [RFC PATCH v3 00/10] Add support for shared PTEs across processes Message-ID: References: <20240903232241.43995-1-anthony.yznaga@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240903232241.43995-1-anthony.yznaga@oracle.com> X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 0CD144001F X-Stat-Signature: redwexwjpdiecb9e4f64sfscz38mpq8o X-Rspam-User: X-HE-Tag: 1728291705-782288 X-HE-Meta: U2FsdGVkX19jIrdySa9RX0zuS89W5GvIHF3PlRCmubm14bmIUkKS/7084FoR5pmF3zEbxg7NCpPn+EmshSrf3+oWO7aTJ8pqtn49bweN1tLz5uMTJ3JlGEqkT69ZYPbmBLtaVdHotHF40439sWUuRlALpzkvwP5Jqbc/SCQZ32UFhVVJhhhK6xQk/3x/WGXtKVgR9bPXNW8C+COyswSHzB3aPANr9rNJoy9TudewLT+PjbwBle6oetkn7+PJNvEhUFGLzgHV0nuRkn0oyYpqwLFn2zsAu3y5xbD1Dda78K81u+9wUHv/HRPFs+7INFv1qWJRkmX0LYVPIMCrkXEUo4fYOWcwr3753i0Rlq9l8SUj8KTYxVcn0xNJObiJaZTzp4xDQAfJDvb1c+d58dogaapKWwzJiR4EOfE33THEics5aCZ9IK8cDOT4u41Y12nTfoB/SLqja8b/XzojPDftZ6Bcjf8hkqGKuRCFOs2XY0I3LSQ4WUB2FZOSEGm+hP45Xc/EeBXJ5fLtjcT3EaUqrLeUh/+kDspiqB9m9pBgWontuP6RDV+BjrAthIW4GEl+hUV8E0nSCpATReQqR7aFAWiIRaF1ZyQeJBMoRtKs47ClUsRxA+bU1s5YexA66MFcZi+zBxlpiRhrbRcApuLxhbzvmSxw2fESndsTgDW5wQVEnc/VNX4OwyWCWamzpjXnGn+Kz2vfQ+V31iA3zRHl2ZLOweFUQ4bKeuIck76qc7E/pTmfsZ0zBnu8Zt/LjMhbnji5tWVuPrla/+y2J6vdT5jwHP9gigiadcuCRt3F7a6ZQqMkuPamSTkycnvWe9j3CmFvPzW2301xdq6SNTl66+TgKjmjVxvYS9F/iwghecbnkkKvrVzjgH6i40B4B2WhkTpz3IP+eanb6Te0uIgSC/XR4AljD5rz5NvwpMppq5iVDpN+RtM8jR1JOD0tdwPTHvz4m7zP5k2w3xIpk/+ Rds0LnqM C/DwkrK6vvOE2yYyqMjOf7g47oSaiOHnkPgkn6pg90dS7bwAntey/u1dg7RSRHcHwhPmzQohcS4GRKg546pcc19tcoPef6al/3ZhbQJiQXGqjWUUbREurnPmBMqVKmzmxkLgFU+wfMfQ7ROWGhFkXBK2nwsp60hYNGGw/gl0oDztO0g3siZzy86MlhpcGqAoIXUYpM3W9UzbjCkEbgKaAKMonLRWT1y9jQ3mWIz+5W+ZFnkwDlQAZdXsNIr5qmmCdHvO8u3EoQVLsjw08wdQdbE/f73XDQQlF4Gw0xP/JFPBUIr3bUpl+b+Tnvubk0hsY1Bqj+pNdQRJzEiPVQLMeq0JfJU1bQ2/qFf4K+MZ2fGfk+O8= 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 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. 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 > 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. -- Kiryl Shutsemau / Kirill A. Shutemov