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Howlett" To: Lorenzo Stoakes Cc: lsf-pc@lists.linux-foundation.org, linux-mm@kvack.org, David Hildenbrand , Vlastimil Babka , Suren Baghdasaryan , Pedro Falcato , Ryan Roberts , Harry Yoo , Rik van Riel , Jann Horn , Chris Li , Barry Song Subject: Re: [LSM/MM/BPF TOPIC] The Future of the Anonymous Reverse Mapping Message-ID: <4qkgoulknoxxkefxyrpnugwmpwbwh324pnnpy6hg2tkdh7lbkc@2i3ep6aoh2g5> Mail-Followup-To: "Liam R. 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 ba+GIiY7 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mapping between folios and VMAs at a VMA level, > utilising a complicated and confusing combination of anon_vma objects and > anon_vma_chain's linking them, which must be updated when VMAs are split, > merged, remapped or forked. > > It's further complicated by various optimisations intended to avoid scalability > issues in locking and memory allocation. > > I have done recent work to improve the situation [0] which has also lead to a > reported improvement in lock scalability [1], but fundamentally the situation > remains the same. > > The logic is actually, when you think hard enough about it, is a fairly > reasonable means of implementing the reverse mapping at a VMA level. > > It is, however, a very broken abstraction as it stands. In order to work with > the logic, you have to essentially keep a broad understanding of the entire > implementation in your head at one time - that is, not much is really > abstracted. > > This results in confusion, mistakes, and bit rot. It's also very time-consuming > to work with - personally I've gone to the lengths of writing a private set of > slides for myself on the topic as a reminder each time I come back to it. > > There are also issues with lock scalability - the use of interval trees to > maintain a connection between an anon_vma and AVCs connected to VMAs requires > that a lock must be held across the entire 'CoW hierarchy' of parent and child > VMAs whenever performing an rmap walk or performing a merge, split, remap or > fork. > > This is because we tear down all interval tree mappings and reestablish them > each time we might see changes in VMA geometry. This is an issue Barry Song > identified as problematic in a real world use case [2]. > > So what do we do to improve the situation? > > Recently I have been working on an experimental new approach to the anonymous > reverse mapping, in which we instead track anonymous remaps, and then use the > VMA's virtual page offset to locate VMAs from the folio. > > I have got the implementation working to the point where it tracks the exact > same VMAs as the anon_vma implementation, and it seems a lot of it can be done > under RCU. > > It avoids the need to maintain expensive mappings at a VMA level, though it > incurs a cost in tracking remaps, and MAP_PRIVATE files are very much a TODO > (they maintain a file vma->vm_pgoff, even when CoW'd, so the remap tracking is > pretty sub-optimal). > > I am investigating whether I can change how MAP_PRIVATE file-backed mappings > work to avoid this issue, and will be developing tests to see how lock > scalability, throughput and memory usage compare to the anon_vma approach under > different workloads. > > This experiment may or may not work out, either way it will be interesting to > discuss it. Discussing alternatives to the anon_vma and anon_vma_chain would be interesting. Just to clarify, this is to look at the complexity of the data structures and not the locking, or both? > > By the time LSF/MM comes around I may even have already decided on a different > approach but that's what makes things interesting :) > > [0]:https://lore.kernel.org/all/cover.1767711638.git.lorenzo.stoakes@oracle.com/ > [1]:https://lore.kernel.org/all/202602061747.855f053f-lkp@intel.com/ > [2]:https://lore.kernel.org/linux-mm/CAGsJ_4x=YsQR=nNcHA-q=0vg0b7ok=81C_qQqKmoJ+BZ+HVduQ@mail.gmail.com/ > > Cheers, Lorenzo