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[87.161.174.152]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3d701622b92sm31349052f8f.58.2025.09.08.13.32.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 08 Sep 2025 13:32:25 -0700 (PDT) Message-ID: <5b7e71e8-4e31-4699-b656-c35dce678a80@redhat.com> Date: Mon, 8 Sep 2025 22:32:22 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 00/22] Add support for shared PTEs across processes To: Anthony Yznaga , linux-mm@kvack.org Cc: akpm@linux-foundation.org, andreyknvl@gmail.com, arnd@arndb.de, bp@alien8.de, brauner@kernel.org, bsegall@google.com, corbet@lwn.net, dave.hansen@linux.intel.com, dietmar.eggemann@arm.com, ebiederm@xmission.com, hpa@zytor.com, jakub.wartak@mailbox.org, jannh@google.com, juri.lelli@redhat.com, khalid@kernel.org, liam.howlett@oracle.com, linyongting@bytedance.com, lorenzo.stoakes@oracle.com, luto@kernel.org, markhemm@googlemail.com, maz@kernel.org, mhiramat@kernel.org, mgorman@suse.de, mhocko@suse.com, mingo@redhat.com, muchun.song@linux.dev, neilb@suse.de, osalvador@suse.de, pcc@google.com, peterz@infradead.org, pfalcato@suse.de, rostedt@goodmis.org, rppt@kernel.org, shakeel.butt@linux.dev, surenb@google.com, tglx@linutronix.de, vasily.averin@linux.dev, vbabka@suse.cz, vincent.guittot@linaro.org, viro@zeniv.linux.org.uk, vschneid@redhat.com, willy@infradead.org, x86@kernel.org, xhao@linux.alibaba.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org References: <20250820010415.699353-1-anthony.yznaga@oracle.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 20.08.25 03:03, Anthony Yznaga wrote: > Memory pages shared between processes require page table entries > (PTEs) for each process. Each of these PTEs consume some of > the memory and as long as the number of mappings being maintained > is small enough, this space consumed by page tables is not > objectionable. When very few memory pages are shared between > processes, the number of PTEs to maintain is mostly constrained by > the number of pages of memory on the system. As the number of shared > pages and the number of times pages are shared goes up, amount of > memory consumed by page tables starts to become significant. This > issue does not apply to threads. Any number of threads can share the > same pages inside a process while sharing the same PTEs. Extending > this same model to sharing pages across processes can eliminate this > issue for sharing across processes as well. > > Some of the field deployments commonly see memory pages shared > across 1000s of processes. On x86_64, each page requires a PTE that > is 8 bytes long which is very small compared to the 4K page > size. When 2000 processes map the same page in their address space, > each one of them requires 8 bytes for its PTE and together that adds > up to 8K of memory just to hold the PTEs for one 4K page. On a > database server with 300GB SGA, a system crash was seen with > out-of-memory condition when 1500+ clients tried to share this SGA > even though the system had 512GB of memory. On this server, in the > worst case scenario of all 1500 processes mapping every page from > SGA would have required 878GB+ for just the PTEs. If these PTEs > could be shared, the a substantial amount of memory saved. > > 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. > When a process mmap's the shared region, a vm flag VM_MSHARE > is added to the vma. On page fault the vma is checked for the > presence of the VM_MSHARE flag. If found, the host mm is > searched for a vma that covers the fault address. Fault handling > then continues using that host vma which establishes PTEs in the > host mm. Fault handling in a shared region also links the shared > page table to the process page table if the shared page table > already exists. Regarding the overall design, two important questions: In the context of this series, how do we handle VMA-modifying functions like mprotect/some madvise/mlock/mempolicy/...? Are they currently blocked when applied to a mshare VMA? And how are we handling other page table walkers that don't modify VMAs like MADV_DONTNEED, smaps, migrate_pages, ... etc? -- Cheers David / dhildenb