Re: [RFC PATCH RESEND 21/28] mm: introduce find_and_lock_anon_vma to be used from arch-specific code

[Laurent Dufour]

Le 01/09/2022 à 19:35, Suren Baghdasaryan a écrit :
> Introduce find_and_lock_anon_vma function to lookup and lock an anonymous
> VMA during page fault handling. When VMA is not found, can't be locked
> or changes after being locked, the function returns NULL. The lookup is
> performed under RCU protection to prevent the found VMA from being
> destroyed before the VMA lock is acquired. VMA lock statistics are
> updated according to the results.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> ---
>  include/linux/mm.h |  3 +++
>  mm/memory.c        | 45 +++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 48 insertions(+)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 7c3190eaabd7..a3cbaa7b9119 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -684,6 +684,9 @@ static inline void vma_assert_no_reader(struct vm_area_struct *vma)
>  		      vma);
>  }
>  
> +struct vm_area_struct *find_and_lock_anon_vma(struct mm_struct *mm,
> +					      unsigned long address);
> +
>  #else /* CONFIG_PER_VMA_LOCK */
>  
>  static inline void vma_init_lock(struct vm_area_struct *vma) {}
> diff --git a/mm/memory.c b/mm/memory.c
> index 29d2f49f922a..bf557f7056de 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5183,6 +5183,51 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
>  }
>  EXPORT_SYMBOL_GPL(handle_mm_fault);
>  
> +#ifdef CONFIG_PER_VMA_LOCK
> +static inline struct vm_area_struct *find_vma_under_rcu(struct mm_struct *mm,
> +							unsigned long address)
> +{
> +	struct vm_area_struct *vma = __find_vma(mm, address);
> +
> +	if (!vma || vma->vm_start > address)
> +		return NULL;
> +
> +	if (!vma_is_anonymous(vma))
> +		return NULL;
> +

It looks to me more natural to first check that the VMA is part of the RB
tree before try read locking it.

> +	if (!vma_read_trylock(vma)) {
> +		count_vm_vma_lock_event(VMA_LOCK_ABORT);
> +		return NULL;
> +	}
> +
> +	/* Check if the VMA got isolated after we found it */
> +	if (RB_EMPTY_NODE(&vma->vm_rb)) {
> +		vma_read_unlock(vma);
> +		count_vm_vma_lock_event(VMA_LOCK_MISS);
> +		return NULL;
> +	}
> +
> +	return vma;
> +}
> +
> +/*
> + * Lookup and lock and anonymous VMA. Returned VMA is guaranteed to be stable
> + * and not isolated. If the VMA is not found of is being modified the function
> + * returns NULL.
> + */
> +struct vm_area_struct *find_and_lock_anon_vma(struct mm_struct *mm,
> +					      unsigned long address)
> +{
> +	struct vm_area_struct *vma;
> +
> +	rcu_read_lock();
> +	vma = find_vma_under_rcu(mm, address);
> +	rcu_read_unlock();
> +
> +	return vma;
> +}
> +#endif /* CONFIG_PER_VMA_LOCK */
> +
>  #ifndef __PAGETABLE_P4D_FOLDED
>  /*
>   * Allocate p4d page table.