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 oLAHztwu 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Baghdasaryan wrote: > On Thu, Feb 19, 2026 at 11:28 AM Lorenzo Stoakes > wrote: > > > > Currently we track the reverse 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. > > Do you have a link to the code we can look at before the discussion? The code is in a really early stage and being constantly changed so _not yet_ but I'll put it somewhere public once it's settled down. It's also currently just a case of the implementation is side-by-side the existing anon_vma stuff, with code in rmap_walk_anon() also kicking off a cow context walk then comparing the count of discovered matching folios. The code will of course eventually > > > > > 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. > > I'm interested in this discussion. Hopefully this will result in > simpler rmap code and reduced lock contention. > Thanks, > Suren. Thanks, I am keen to extract numbers from this and use that to guide the implementation - I want this to be an evidence-based improvement rather than simply a rework of some kind :) If the proposed solution causes meaningful regressions then I will look to an alternative approach, either way I think it's important to base as much as possible on actual observed numbers. I hope to implement benchmarking/test code as part of this work which should be useful regardless of the approach taken. When I have this more developed I may ask people to help test this on different hardware/clusters/etc. to ensure stability and assess impact, so don't be surprised if I ask for a favour at this point :) > > > > > 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 > Cheers, Lorenzo