Re: [PATCH] mm: refactor __vunmap() to avoid duplicated call to find_vm_area()

[Anshuman Khandual]

On 05/15/2019 05:21 AM, Roman Gushchin wrote:
> __vunmap() calls find_vm_area() twice without an obvious reason:
> first directly to get the area pointer, second indirectly by calling
> vm_remove_mappings()->remove_vm_area(), which is again searching
> for the area.
> 
> To remove this redundancy, let's split remove_vm_area() into
> __remove_vm_area(struct vmap_area *), which performs the actual area
> removal, and remove_vm_area(const void *addr) wrapper, which can
> be used everywhere, where it has been used before. Let's pass
> a pointer to the vm_area instead of vm_struct to vm_remove_mappings(),
> so it can pass it to __remove_vm_area() and avoid the redundant area
> lookup.
> 
> On my test setup, I've got 5-10% speed up on vfree()'ing 1000000
> of 4-pages vmalloc blocks.
> 
> Perf report before:
>   29.44%  cat      [kernel.kallsyms]  [k] free_unref_page
>   11.88%  cat      [kernel.kallsyms]  [k] find_vmap_area
>    9.28%  cat      [kernel.kallsyms]  [k] __free_pages
>    7.44%  cat      [kernel.kallsyms]  [k] __slab_free
>    7.28%  cat      [kernel.kallsyms]  [k] vunmap_page_range
>    4.56%  cat      [kernel.kallsyms]  [k] __vunmap
>    3.64%  cat      [kernel.kallsyms]  [k] __purge_vmap_area_lazy
>    3.04%  cat      [kernel.kallsyms]  [k] __free_vmap_area
> 
> Perf report after:
>   32.41%  cat      [kernel.kallsyms]  [k] free_unref_page
>    7.79%  cat      [kernel.kallsyms]  [k] find_vmap_area
>    7.40%  cat      [kernel.kallsyms]  [k] __slab_free
>    7.31%  cat      [kernel.kallsyms]  [k] vunmap_page_range
>    6.84%  cat      [kernel.kallsyms]  [k] __free_pages
>    6.01%  cat      [kernel.kallsyms]  [k] __vunmap
>    3.98%  cat      [kernel.kallsyms]  [k] smp_call_function_single
>    3.81%  cat      [kernel.kallsyms]  [k] __purge_vmap_area_lazy
>    2.77%  cat      [kernel.kallsyms]  [k] __free_vmap_area
> 
> Signed-off-by: Roman Gushchin <guro@xxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> ---
>  mm/vmalloc.c | 52 +++++++++++++++++++++++++++++-----------------------
>  1 file changed, 29 insertions(+), 23 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index c42872ed82ac..8d4907865614 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2075,6 +2075,22 @@ struct vm_struct *find_vm_area(const void *addr)
>  	return NULL;
>  }
>  
> +static struct vm_struct *__remove_vm_area(struct vmap_area *va)
> +{
> +	struct vm_struct *vm = va->vm;
> +
> +	spin_lock(&vmap_area_lock);
> +	va->vm = NULL;
> +	va->flags &= ~VM_VM_AREA;
> +	va->flags |= VM_LAZY_FREE;
> +	spin_unlock(&vmap_area_lock);
> +
> +	kasan_free_shadow(vm);
> +	free_unmap_vmap_area(va);
> +
> +	return vm;
> +}
> +
>  /**
>   * remove_vm_area - find and remove a continuous kernel virtual area
>   * @addr:	    base address
> @@ -2087,26 +2103,14 @@ struct vm_struct *find_vm_area(const void *addr)
>   */
>  struct vm_struct *remove_vm_area(const void *addr)
>  {
> +	struct vm_struct *vm = NULL;
>  	struct vmap_area *va;
>  
> -	might_sleep();

Is not this necessary any more ?

> -
>  	va = find_vmap_area((unsigned long)addr);
> -	if (va && va->flags & VM_VM_AREA) {
> -		struct vm_struct *vm = va->vm;
> -
> -		spin_lock(&vmap_area_lock);
> -		va->vm = NULL;
> -		va->flags &= ~VM_VM_AREA;
> -		va->flags |= VM_LAZY_FREE;
> -		spin_unlock(&vmap_area_lock);
> -
> -		kasan_free_shadow(vm);
> -		free_unmap_vmap_area(va);
> +	if (va && va->flags & VM_VM_AREA)
> +		vm = __remove_vm_area(va);
>  
> -		return vm;
> -	}
> -	return NULL;
> +	return vm;
>  }

Other callers of remove_vm_area() cannot use __remove_vm_area() directly as well
to save a look up ?