linux-kernel - Re: [PATCH 3/4] mm: replace rw_semaphore with atomic_t in vma

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Message-ID: <CAJuCfpEMYhAmOPwjGO+j1t+069MJZxUs1O1co-zJ4+vEeXCtng@mail.gmail.com>
Date: Wed, 11 Dec 2024 07:20:03 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Matthew Wilcox <willy@...radead.org>, akpm@...ux-foundation.org, liam.howlett@...cle.com, 
	lorenzo.stoakes@...cle.com, mhocko@...e.com, vbabka@...e.cz, 
	hannes@...xchg.org, mjguzik@...il.com, oliver.sang@...el.com, 
	mgorman@...hsingularity.net, david@...hat.com, peterx@...hat.com, 
	oleg@...hat.com, dave@...olabs.net, paulmck@...nel.org, brauner@...nel.org, 
	dhowells@...hat.com, hdanton@...a.com, hughd@...gle.com, minchan@...gle.com, 
	jannh@...gle.com, shakeel.butt@...ux.dev, souravpanda@...gle.com, 
	pasha.tatashin@...een.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	kernel-team@...roid.com
Subject: Re: [PATCH 3/4] mm: replace rw_semaphore with atomic_t in vma_lock

On Wed, Dec 11, 2024 at 12:25 AM Peter Zijlstra <peterz@...radead.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!