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

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Message-ID: <20241211082541.GQ21636@noisy.programming.kicks-ass.net>
Date: Wed, 11 Dec 2024 09:25:41 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Suren Baghdasaryan <surenb@...gle.com>
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 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.