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[2003:cb:c722:3700:6501:8925:6f9:fcdc]) by smtp.gmail.com with ESMTPSA id iv11-20020a05600c548b00b0040b4cb14d40sm18172398wmb.19.2023.12.04.02.21.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 04 Dec 2023 02:21:04 -0800 (PST) Message-ID: <1c68ee91-1b6a-41e8-b96f-bcaf9faffa08@redhat.com> Date: Mon, 4 Dec 2023 11:21:03 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 2/6] mm/gmem: add arch-independent abstraction to track address mapping status To: Pedro Falcato Cc: Weixi Zhu , linux-mm@kvack.org, linux-kernel@vger.kernel.org, akpm@linux-foundation.org, weixi.zhu@openeuler.sh, mgorman@suse.de, jglisse@redhat.com, rcampbell@nvidia.com, jhubbard@nvidia.com, apopple@nvidia.com, mhairgrove@nvidia.com, ziy@nvidia.com, alexander.deucher@amd.com, christian.koenig@amd.com, Xinhui.Pan@amd.com, amd-gfx@lists.freedesktop.org, Felix.Kuehling@amd.com, ogabbay@kernel.org, dri-devel@lists.freedesktop.org, jgg@nvidia.com, leonro@nvidia.com, zhenyuw@linux.intel.com, zhi.a.wang@intel.com, intel-gvt-dev@lists.freedesktop.org, intel-gfx@lists.freedesktop.org, jani.nikula@linux.intel.com, joonas.lahtinen@linux.intel.com, rodrigo.vivi@intel.com, tvrtko.ursulin@linux.intel.com References: <20231128125025.4449-1-weixi.zhu@huawei.com> <20231128125025.4449-3-weixi.zhu@huawei.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: A740F1C0007 X-Stat-Signature: mgq7q8cpxn1t8fmm1qc9xxme43p4jp77 X-HE-Tag: 1701685268-349089 X-HE-Meta: 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 tk+dnayb 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 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 02.12.23 15:50, Pedro Falcato wrote: > On Fri, Dec 1, 2023 at 9:23 AM David Hildenbrand wrote: >> >> On 28.11.23 13:50, Weixi Zhu wrote: >>> This patch adds an abstraction layer, struct vm_object, that maintains >>> per-process virtual-to-physical mapping status stored in struct gm_mapping. >>> For example, a virtual page may be mapped to a CPU physical page or to a >>> device physical page. Struct vm_object effectively maintains an >>> arch-independent page table, which is defined as a "logical page table". >>> While arch-dependent page table used by a real MMU is named a "physical >>> page table". The logical page table is useful if Linux core MM is extended >>> to handle a unified virtual address space with external accelerators using >>> customized MMUs. >> >> Which raises the question why we are dealing with anonymous memory at >> all? Why not go for shmem if you are already only special-casing VMAs >> with a MMAP flag right now? >> >> That would maybe avoid having to introduce controversial BSD design >> concepts into Linux, that feel like going a step backwards in time to me >> and adding *more* MM complexity. >> >>> >>> In this patch, struct vm_object utilizes a radix >>> tree (xarray) to track where a virtual page is mapped to. This adds extra >>> memory consumption from xarray, but provides a nice abstraction to isolate >>> mapping status from the machine-dependent layer (PTEs). Besides supporting >>> accelerators with external MMUs, struct vm_object is planned to further >>> union with i_pages in struct address_mapping for file-backed memory. >> >> A file already has a tree structure (pagecache) to manage the pages that >> are theoretically mapped. It's easy to translate from a VMA to a page >> inside that tree structure that is currently not present in page tables. >> >> Why the need for that tree structure if you can just remove anon memory >> from the picture? >> >>> >>> The idea of struct vm_object is originated from FreeBSD VM design, which >>> provides a unified abstraction for anonymous memory, file-backed memory, >>> page cache and etc[1]. >> >> :/ >> >>> Currently, Linux utilizes a set of hierarchical page walk functions to >>> abstract page table manipulations of different CPU architecture. The >>> problem happens when a device wants to reuse Linux MM code to manage its >>> page table -- the device page table may not be accessible to the CPU. >>> Existing solution like Linux HMM utilizes the MMU notifier mechanisms to >>> invoke device-specific MMU functions, but relies on encoding the mapping >>> status on the CPU page table entries. This entangles machine-independent >>> code with machine-dependent code, and also brings unnecessary restrictions. >> >> Why? we have primitives to walk arch page tables in a non-arch specific >> fashion and are using them all over the place. >> >> We even have various mechanisms to map something into the page tables >> and get the CPU to fault on it, as if it is inaccessible (PROT_NONE as >> used for NUMA balancing, fake swap entries). >> >>> The PTE size and format vary arch by arch, which harms the extensibility. >> >> Not really. >> >> We might have some features limited to some architectures because of the >> lack of PTE bits. And usually the problem is that people don't care >> enough about enabling these features on older architectures. >> >> If we ever *really* need more space for sw-defined data, it would be >> possible to allocate auxiliary data for page tables only where required >> (where the features apply), instead of crafting a completely new, >> auxiliary datastructure with it's own locking. >> >> So far it was not required to enable the feature we need on the >> architectures we care about. >> >>> >>> [1] https://docs.freebsd.org/en/articles/vm-design/ >> >> In the cover letter you have: >> >> "The future plan of logical page table is to provide a generic >> abstraction layer that support common anonymous memory (I am looking at >> you, transparent huge pages) and file-backed memory." >> >> Which I doubt will happen; there is little interest in making anonymous >> memory management slower, more serialized, and wasting more memory on >> metadata. > > Also worth noting that: > > 1) Mach VM (which FreeBSD inherited, from the old BSD) vm_objects > aren't quite what's being stated here, rather they are somewhat > replacements for both anon_vma and address_space[1]. Very similarly to > Linux, they take pages from vm_objects and map them in page tables > using pmap (the big difference is anon memory, which has its > bookkeeping in page tables, on Linux) > > 2) These vm_objects were a horrendous mistake (see CoW chaining) and > FreeBSD has to go to horrendous lengths to make them tolerable. The > UVM paper/dissertation (by Charles Cranor) talks about these issues at > length, and 20 years later it's still true. > > 3) Despite Linux MM having its warts, it's probably correct to > consider it a solid improvement over FreeBSD MM or NetBSD UVM > > And, finally, randomly tacking on core MM concepts from other systems > is at best a *really weird* idea. Particularly when they aren't even > what was stated! Can you read my mind? :) thanks for noting all that, with which I 100% agree. -- Cheers, David / dhildenb