Re: [PATCH v2 4/5] mm: make vma cache SLAB_TYPESAFE_BY_RCU

[Hugh Dickins <hughd@google.com>]

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On Tue, 12 Nov 2024, Suren Baghdasaryan wrote:
> On Tue, Nov 12, 2024 at 9:08 PM Hugh Dickins <hughd@google.com> wrote:
> > On Tue, 12 Nov 2024, Suren Baghdasaryan wrote:
> > >
> > > Thinking about this some more, I don't think this works. I'm relying
> > > on vma_start_read() to stabilize the vma, however the lock I'm taking
> > > is part of the vma which can be reused from under us. So, the lock I'm
> > > taking might be reinitialized after I take the lock...
> > > I need to figure out a way to stabilize the vma in some other manner
> > > before taking this lock.
> >
> > (I'm not paying attention and following the patches, I just happened
> > to notice this remark: forgive me if I'm out of context and have
> > misunderstood, but hope this might help:)
> >
> > But this is exactly the problem SLAB_TYPESAFE_BY_RCU was invented for.
> > You just have to be careful that the locks are initialized only when the
> > slab is first created (allocated from buddy), not reinitialized whenever
> > a new object is allocated from that slab.
> 
> Hi Hugh!
> I'm looking into SLAB_TYPESAFE_BY_RCU implementation and trying to
> figure out if initializing the lock in the ctor() of the cache as
> mentioned in the comment here:
> https://elixir.bootlin.com/linux/v6.12-rc7/source/include/linux/slab.h#L127
> would help my case. I assume that's what you are hinting here?

Yes, if I'm "hinting", it's because offhand I forget the right names:
"ctor", yes, that sounds right.

Just grep around for examples of how it is used: there must be plenty
now. but anon_vma is what it was first used for.

But given the title of this patch, I'm surprised it's new to you.

Hugh