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

Peter Zijlstra
2024-12-11T08:26:02Z

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.

> > 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.