linux-kernel - Re: [PATCH v4 2/5] mm: move per-vma lock into vm_area

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Message-ID: <iw34akksaz6wjlygwuztlkvto3aiduekrhw6rjlqq4lr7vzmug@tprkddvgrj3e>
Date: Wed, 20 Nov 2024 23:01:55 -0800
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: akpm@...ux-foundation.org, willy@...radead.org, 
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Subject: Re: [PATCH v4 2/5] mm: move per-vma lock into vm_area_struct

On Wed, Nov 20, 2024 at 04:33:37PM -0800, Suren Baghdasaryan wrote:
[...]
> > > >
> > > > Do we just want 'struct vm_area_struct' to be cacheline aligned or do we
> > > > want 'struct vma_lock vm_lock' to be on a separate cacheline as well?
> > >
> > > We want both to minimize cacheline sharing.
> > >
> >
> > For later, you will need to add a pad after vm_lock as well, so any
> > future addition will not share the cacheline with vm_lock. I would do
> > something like below. This is a nit and can be done later.
> >
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 7654c766cbe2..5cc4fff163a0 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -751,10 +751,12 @@ struct vm_area_struct {
> >  #endif
> >         struct vm_userfaultfd_ctx vm_userfaultfd_ctx;
> >  #ifdef CONFIG_PER_VMA_LOCK
> > +       CACHELINE_PADDING(__pad1__);
> >         /* Unstable RCU readers are allowed to read this. */
> > -       struct vma_lock vm_lock ____cacheline_aligned_in_smp;
> > +       struct vma_lock vm_lock;
> > +       CACHELINE_PADDING(__pad2__);
> >  #endif
> > -} __randomize_layout;
> > +} __randomize_layout ____cacheline_aligned_in_smp;
> 
> I thought SLAB_HWCACHE_ALIGN for vm_area_cachep added in this patch
> would have the same result, no?
> 

SLAB_HWCACHE_ALIGN is more about the slub allocator allocating cache
aligned memory. It does not say anything about the internals of the
struct for which the kmem_cache is being created. The
____cacheline_aligned_in_smp tag in your patch made sure that the field
vm_lock will be put in a new cacheline and there can be a hole between
vm_lock and the previous field if the previous field is not ending at
the cacheline boundary. Please note that if you add a new field after
vm_lock (without cacheline alignment tag), it will be on the same
cacheline as vm_lock. So, your code is achieving the vm_lock on its own
cacheline goal but vm_lock being the only field on that cacheline is not
being achieved.