Re: [PATCH v9 11/17] mm: replace vm_lock and detached flag with a reference count

[Suren Baghdasaryan]

On Wed, Jan 15, 2025 at 3:13 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Wed, Jan 15, 2025 at 11:48:41AM +0100, Peter Zijlstra wrote:
> > On Sat, Jan 11, 2025 at 12:14:47PM -0800, Suren Baghdasaryan wrote:
> >
> > > > Replacing down_read_trylock() with the new routine loses an acquire
> > > > fence. That alone is not a problem, but see below.
> > >
> > > Hmm. I think this acquire fence is actually necessary. We don't want
> > > the later vm_lock_seq check to be reordered and happen before we take
> > > the refcount. Otherwise this might happen:
> > >
> > > reader             writer
> > > if (vm_lock_seq == mm_lock_seq) // check got reordered
> > >         return false;
> > >                        vm_refcnt += VMA_LOCK_OFFSET
> > >                        vm_lock_seq == mm_lock_seq
> > >                        vm_refcnt -= VMA_LOCK_OFFSET
> > > if (!__refcount_inc_not_zero_limited())
> > >         return false;
> > >
> > > Both reader's checks will pass and the reader would read-lock a vma
> > > that was write-locked.
> >
> > Hmm, you're right. That acquire does matter here.
>
> Notably, it means refcount_t is entirely unsuitable for anything
> SLAB_TYPESAFE_BY_RCU, since they all will need secondary validation
> conditions after the refcount succeeds.

Thanks for reviewing, Peter!
Yes, I'm changing the code to use atomic_t instead of refcount_t and
it comes out quite nicely I think. I had to add two small helper
functions:
vm_refcount_inc() - similar to refcount_add_not_zero() but with an
acquired fence.
vm_refcnt_sub() - similar to refcount_sub_and_test(). I could use
atomic_sub_and_test() but that would add unnecessary acquire fence in
the pagefault path, so I'm using refcount_sub_and_test() logic
instead.

For SLAB_TYPESAFE_BY_RCU I think we are ok with the
__vma_enter_locked()/__vma_exit_locked() transition in the
vma_mark_detached() before freeing the vma and would not need
secondary validation. In __vma_enter_locked(), vm_refcount gets
VMA_LOCK_OFFSET set, which prevents readers from taking the refcount.
In __vma_exit_locked() vm_refcnt transitions to 0, so again that
prevents readers from taking the refcount. IOW, the readers won't get
to the secondary validation and will fail early on
__refcount_inc_not_zero_limited(). I think this transition correctly
serves the purpose of waiting for current temporary readers to exit
and preventing new readers from read-locking and using the vma.

>
> And this is probably fine, but let me ponder this all a little more.

Thanks for taking the time!