Re: [PATCH v12 21/31] mm: Introduce find_vma_rcu()

[Peter Zijlstra]

On Tue, Apr 16, 2019 at 03:45:12PM +0200, Laurent Dufour wrote:
> This allows to search for a VMA structure without holding the mmap_sem.
> 
> The search is repeated while the mm seqlock is changing and until we found
> a valid VMA.
> 
> While under the RCU protection, a reference is taken on the VMA, so the
> caller must call put_vma() once it not more need the VMA structure.
> 
> At the time a VMA is inserted in the MM RB tree, in vma_rb_insert(), a
> reference is taken to the VMA by calling get_vma().
> 
> When removing a VMA from the MM RB tree, the VMA is not release immediately
> but at the end of the RCU grace period through vm_rcu_put(). This ensures
> that the VMA remains allocated until the end the RCU grace period.
> 
> Since the vm_file pointer, if valid, is released in put_vma(), there is no
> guarantee that the file pointer will be valid on the returned VMA.

What I'm missing here, and in the previous patch introducing the
refcount (also see refcount_t), is _why_ we need the refcount thing at
all.

My original plan was to use SRCU, which at the time was not complete
enough so I abused/hacked preemptible RCU, but that is no longer the
case, SRCU has all the required bits and pieces.

Also; the initial motivation was prefaulting large VMAs and the
contention on mmap was killing things; but similarly, the contention on
the refcount (I did try that) killed things just the same.

So I'm really sad to see the refcount return; and without any apparent
justification.