Re: [PATCH 1/2] mm: vmalloc: Avoid a double lookup of freed VA in a tree

[Uladzislau Rezki <urezki@gmail.com>]

On Tue, Dec 20, 2022 at 07:45:20PM +0100, Uladzislau Rezki wrote:
> On Tue, Dec 20, 2022 at 07:27:03PM +0100, Uladzislau Rezki (Sony) wrote:
> > When a VA is freed over a main path, for example by invoking
> > the vfree() function, a tree is accessed two times what is odd:
> > 
> > vfree():
> >   __vunmap()
> >     __find_vmap_area()
> >   vm_remove_mappings()
> >     remove_vm_area()
> >       __find_vmap_area()
> > 
> > __find_vmap_area() are called two times. Fix it by introducing
> > a find_unlink_vmap_area() helper that finds and un-links a VA
> > from a tree.
> > 
> > Performance test results on a single CPU:
> > 
> > - fix_size_alloc_test       loops: 1000000 avg: 476847   usec
> > - full_fit_alloc_test       loops: 1000000 avg: 806746   usec
> > - long_busy_list_alloc_test loops: 1000000 avg: 13552093 usec
> > - random_size_alloc_test    loops: 1000000 avg: 7441322  usec
> > - fix_align_alloc_test      loops: 1000000 avg: 1411132  usec
> > All test took worker0=87650866284 cycles
> > 
> > - fix_size_alloc_test       loops: 1000000 avg: 490713   usec
> > - full_fit_alloc_test       loops: 1000000 avg: 579162   usec
> > - long_busy_list_alloc_test loops: 1000000 avg: 10485448 usec
> > - random_size_alloc_test    loops: 1000000 avg: 5824449  usec
> > - fix_align_alloc_test      loops: 1000000 avg: 984735   usec
> > All test took worker0=67952362802 cycles
> > 
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > ---
> >  mm/vmalloc.c | 40 ++++++++++++++++++++++++++++------------
> >  1 file changed, 28 insertions(+), 12 deletions(-)
> > 
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 9e30f0b39203..0fc38c36e0df 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -1825,9 +1825,14 @@ static void free_vmap_area_noflush(struct vmap_area *va)
> >  	unsigned long va_start = va->va_start;
> >  	unsigned long nr_lazy;
> >  
> > -	spin_lock(&vmap_area_lock);
> > -	unlink_va(va, &vmap_area_root);
> > -	spin_unlock(&vmap_area_lock);
> > +	/*
> > +	 * A free_vmap_block() is left. It is NOT a main free path.
> > +	 */
> > +	if (!list_empty(&va->list)) {
> > +		spin_lock(&vmap_area_lock);
> > +		unlink_va(va, &vmap_area_root);
> > +		spin_unlock(&vmap_area_lock);
> > +	}
> >  
> >  	nr_lazy = atomic_long_add_return((va->va_end - va->va_start) >>
> >  				PAGE_SHIFT, &vmap_lazy_nr);
> > @@ -1871,6 +1876,19 @@ struct vmap_area *find_vmap_area(unsigned long addr)
> >  	return va;
> >  }
> >  
> > +static struct vmap_area *find_unlink_vmap_area(unsigned long addr)
> > +{
> > +	struct vmap_area *va;
> > +
> > +	spin_lock(&vmap_area_lock);
> > +	va = __find_vmap_area(addr, &vmap_area_root);
> > +	if (va)
> > +		unlink_va(va, &vmap_area_root);
> > +	spin_unlock(&vmap_area_lock);
> > +
> > +	return va;
> > +}
> > +
> >  /*** Per cpu kva allocator ***/
> >  
> >  /*
> > @@ -2236,7 +2254,7 @@ void vm_unmap_ram(const void *mem, unsigned int count)
> >  		return;
> >  	}
> >  
> > -	va = find_vmap_area(addr);
> > +	va = find_unlink_vmap_area(addr);
> >  	BUG_ON(!va);
> >  	debug_check_no_locks_freed((void *)va->va_start,
> >  				    (va->va_end - va->va_start));
> > @@ -2607,21 +2625,16 @@ struct vm_struct *remove_vm_area(const void *addr)
> >  
> >  	might_sleep();
> >  
> > -	spin_lock(&vmap_area_lock);
> > -	va = __find_vmap_area((unsigned long)addr, &vmap_area_root);
> > -	if (va && va->vm) {
> > +	va = find_unlink_vmap_area((unsigned long) addr);
> > +	if (va) {
> >  		struct vm_struct *vm = va->vm;
> >  
> > -		va->vm = NULL;
> > -		spin_unlock(&vmap_area_lock);
> > -
> >  		kasan_free_module_shadow(vm);
> >  		free_unmap_vmap_area(va);
> >  
> >  		return vm;
> >  	}
> >  
> > -	spin_unlock(&vmap_area_lock);
> >  	return NULL;
> >  }
> >  
> > @@ -2690,6 +2703,7 @@ static void vm_remove_mappings(struct vm_struct *area, int deallocate_pages)
> >  static void __vunmap(const void *addr, int deallocate_pages)
> >  {
> >  	struct vm_struct *area;
> > +	struct vmap_area *va;
> >  
> >  	if (!addr)
> >  		return;
> > @@ -2698,7 +2712,9 @@ static void __vunmap(const void *addr, int deallocate_pages)
> >  			addr))
> >  		return;
> >  
> > -	area = find_vm_area(addr);
> > +	va = find_unlink_vmap_area((unsigned long)addr);
> > +	area = va->vm;
> > +
> >  	if (unlikely(!area)) {
> >  		WARN(1, KERN_ERR "Trying to vfree() nonexistent vm area (%p)\n",
> >  				addr);
> > -- 
> > 2.30.2
> > 
Will send a v2.
 
--
Uladzislau Rezki