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

[Roman Gushchin <guro@fb.com>]

On Wed, May 15, 2019 at 01:11:46PM +0530, Anshuman Khandual wrote:
> 
> 
> 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@fb.com>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Cc: Matthew Wilcox <willy@infradead.org>
> > Cc: Vlastimil Babka <vbabka@suse.cz>
> > ---
> >  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 ?

We've discussed it here: https://lkml.org/lkml/2019/4/17/1098 .
Tl;dr it's not that useful.

> 
> > -
> >  	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 ?
> 

I'll take a look. Good idea, thanks!

Roman