Re: [RFC PATCH RESEND 00/28] per-VMA locks proposal

[Kent Overstreet <kent.overstreet@linux.dev>]

On Mon, Sep 05, 2022 at 11:32:48AM -0700, Suren Baghdasaryan wrote:
> On Mon, Sep 5, 2022 at 5:32 AM 'Michal Hocko' via kernel-team
> <kernel-team@android.com> 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.

Correct me if I'm wrong, but it looks like any time multiple VMAs need to be
locked we need mmap_lock anyways, which is what makes your approach so sweet.

If however we ever want to lock multiple VMAs without taking mmap_lock, then
deadlock avoidance algorithms aren't that bad - there's the ww_mutex approach,
which is simple and works well when there isn't much expected contention (the
advantage of the ww_mutex approach is that it doesn't have to track all held
locks). I've also written full cycle detection; that approcah gets you fewer
restarts, at the cost of needing a list of all currently held locks.

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