On Mon, Sep 5, 2022 at 5:32 AM 'Michal Hocko' via kernel-team <kernel-team@xxxxxxxxxxx> wrote: > > Unless I am missing something, this is not based on the Maple tree > rewrite, right? Does the change in the data structure makes any > difference to the approach? I remember discussions at LSFMM where it has > been pointed out that some issues with the vma tree are considerably > simpler to handle with the maple tree. Correct, this does not use the Maple tree yet but once Maple tree transition happens and it supports RCU-safe lookups, my code in find_vma_under_rcu() becomes really simple. > > On Thu 01-09-22 10:34:48, Suren Baghdasaryan wrote: > [...] > > One notable way the implementation deviates from the proposal is the way > > VMAs are marked as locked. Because during some of mm updates multiple > > VMAs need to be locked until the end of the update (e.g. vma_merge, > > split_vma, etc). > > I think it would be really helpful to spell out those issues in a greater > detail. Not everybody is aware of those vma related subtleties. Ack. I'll expand the description of the cases when multiple VMAs need to be locked in the same update. The main difficulties are: 1. Multiple VMAs might need to be locked within one mmap_write_lock/mmap_write_unlock session (will call it an update transaction). 2. Figuring out when it's safe to unlock a previously locked VMA is tricky because that might be happening in different functions and at different call levels. So, instead of the usual lock/unlock pattern, the proposed solution marks a VMA as locked and provides an efficient way to: 1. Identify locked VMAs. 2. Unlock all locked VMAs in bulk. We also postpone unlocking the locked VMAs until the end of the update transaction, when we do mmap_write_unlock. Potentially this keeps a VMA locked for longer than is absolutely necessary but it results in a big reduction of code complexity. > > Thanks for working on this Suren! Thanks for reviewing! Suren. > -- > Michal Hocko > SUSE Labs > > -- > To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@xxxxxxxxxxx. >
