Re: [PATCH 3/4] mm: replace rw_semaphore with atomic_t in vma_lock

Suren Baghdasaryan
2024-12-11T15:20:15Z

On Wed, Dec 11, 2024 at 12:25 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, Dec 10, 2024 at 03:37:50PM -0800, Suren Baghdasaryan wrote:
>
> > > Replace vm_lock with vm_refcnt. Replace vm_detached with vm_refcnt == 0
> > > -- that is, attach sets refcount to 1 to indicate it is part of the mas,
> > > detached is the final 'put'.
> >
> > I need to double-check if we ever write-lock a detached vma. I don't
> > think we do but better be safe. If we do then that wait-until() should
> > accept 0x8000'0001 as well.
>
> vma_start_write()
> __is_vma_write_locked()
> mmap_assert_write_locked(vma->vm_mm);
>
> So this really should hold afaict.
>
> > > RCU lookup does the inc_not_zero thing, when increment succeeds, compare
> > > mm/addr to validate.
> > >
> > > vma_start_write() already relies on mmap_lock being held for writing,
> > > and thus does not have to worry about writer-vs-writer contention, that
> > > is fully resolved by mmap_sem. This means we only need to wait for
> > > readers to drop out.
> > >
> > > vma_start_write()
> > > add(0x8000'0001); // could fetch_add and double check the high
> > > // bit wasn't already set.
> > > wait-until(refcnt == 0x8000'0002); // mas + writer ref
> > > WRITE_ONCE(vma->vm_lock_seq, mm_lock_seq);
> > > sub(0x8000'0000);
> > >
> > > vma_end_write()
> > > put();
> >
> > We don't really have vma_end_write(). Instead it's vma_end_write_all()
> > which increments mm_lock_seq unlocking all write-locked VMAs.
> > Therefore in vma_start_write() I think we can sub(0x8000'0001) at the
> > end.
>
> Right, I know you don't, but you should :-), I've suggested adding this
> before.

I'll look into adding it. IIRC there was some issue with that but I
can't recall...

>
> > > vma_start_read() then becomes something like:
> > >
> > > if (vm_lock_seq == mm_lock_seq)
> > > return false;
> > >
> > > cnt = fetch_inc(1);
> > > if (cnt & msb || vm_lock_seq == mm_lock_seq) {
> > > put();
> > > return false;
> > > }
> > >
> > > return true;
> > >
> > > vma_end_read() then becomes:
> > > put();
> > >
> > >
> > > and the down_read() from uffffffd requires mmap_read_lock() and thus
> > > does not have to worry about writers, it can simpy be inc() and put(),
> > > no?
> >
> > I think your proposal should work. Let me try to code it and see if
> > something breaks.
>
> Btw, for the wait-until() and put() you can use rcuwait; that is the
> simplest wait form we have. It's suitable because we only ever have the
> one waiter.

Yes, Davidlohr mentioned that before. I'll do that. Thanks!