Re: [PATCH v2] mm: vmalloc: Clean up vunmap to avoid pgtable ops twice

[Chintan Pandya]

On 5/1/2018 4:22 AM, Andrew Morton wrote:
> On Mon, 16 Apr 2018 16:29:02 +0530 Chintan Pandya <cpandya@xxxxxxxxxxxxxx> wrote:
>
> > vunmap does page table clear operations twice in the
> > case when DEBUG_PAGEALLOC_ENABLE_DEFAULT is enabled.
> >
> > So, clean up the code as that is unintended.
> >
> > As a perf gain, we save few us. Below ftrace data was
> > obtained while doing 1 MB of vmalloc/vfree on ARM64
> > based SoC *without* this patch applied. After this
> > patch, we can save ~3 us (on 1 extra vunmap_page_range).
> >
> >    CPU  DURATION                  FUNCTION CALLS
> >    |     |   |                     |   |   |   |
> >   6)               |  __vunmap() {
> >   6)               |    vmap_debug_free_range() {
> >   6)   3.281 us    |      vunmap_page_range();
> >   6) + 45.468 us   |    }
> >   6)   2.760 us    |    vunmap_page_range();
> >   6) ! 505.105 us  |  }
>
> It's been a long time since I looked at the vmap code :(
>
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -603,26 +603,6 @@ static void unmap_vmap_area(struct vmap_area *va)
> >   	vunmap_page_range(va->va_start, va->va_end);
> >   }
> >   
> > -static void vmap_debug_free_range(unsigned long start, unsigned long end)
> > -{
> > -	/*
> > -	 * Unmap page tables and force a TLB flush immediately if pagealloc
> > -	 * debugging is enabled.  This catches use after free bugs similarly to
> > -	 * those in linear kernel virtual address space after a page has been
> > -	 * freed.
> > -	 *
> > -	 * All the lazy freeing logic is still retained, in order to minimise
> > -	 * intrusiveness of this debugging feature.
> > -	 *
> > -	 * This is going to be *slow* (linear kernel virtual address debugging
> > -	 * doesn't do a broadcast TLB flush so it is a lot faster).
> > -	 */
> > -	if (debug_pagealloc_enabled()) {
> > -		vunmap_page_range(start, end);
> > -		flush_tlb_kernel_range(start, end);
> > -	}
> > -}
> > -
> >   /*
> >    * lazy_max_pages is the maximum amount of virtual address space we gather up
> >    * before attempting to purge with a TLB flush.
> > @@ -756,6 +736,9 @@ static void free_unmap_vmap_area(struct vmap_area *va)
> >   {
> >   	flush_cache_vunmap(va->va_start, va->va_end);
> >   	unmap_vmap_area(va);
> > +	if (debug_pagealloc_enabled())
> > +		flush_tlb_kernel_range(va->va_start, va->va_end);
> > +
> >   	free_vmap_area_noflush(va);
> >   }
> >   
> > @@ -1142,7 +1125,6 @@ void vm_unmap_ram(const void *mem, unsigned int count)
> >   	BUG_ON(!PAGE_ALIGNED(addr));
> >   
> >   	debug_check_no_locks_freed(mem, size);
> > -	vmap_debug_free_range(addr, addr+size);
>
> This appears to be a functional change: if (count <= VMAP_MAX_ALLOC)
> and we're in debug mode then the
> vunmap_page_range/flush_tlb_kernel_range will no longer be performed.
> Why is this ok?

Yes, you are right. In vb_free(), we do vunmap_page_range() but not
flush_tlb_kernel_range(). I will add this stub for debug benefits and
share v3.

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 6729400..781ce02 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1036,6 +1036,10 @@ static void vb_free(const void *addr, unsigned 
long size)

        vunmap_page_range((unsigned long)addr, (unsigned long)addr + size);

+       if (debug_pagealloc_enabled())
+               flush_tlb_kernel_range((unsigned long)addr,
+                                       (unsigned long)addr + size);
+
        spin_lock(&vb->lock);

        /* Expand dirty range */



> >   	if (likely(count <= VMAP_MAX_ALLOC)) {
> >   		vb_free(mem, size);
> > @@ -1499,7 +1481,6 @@ struct vm_struct *remove_vm_area(const void *addr)
> >   		va->flags |= VM_LAZY_FREE;
> >   		spin_unlock(&vmap_area_lock);
> >   
> > -		vmap_debug_free_range(va->va_start, va->va_end);
> >   		kasan_free_shadow(vm);
> >   		free_unmap_vmap_area(va);
> >   

Chintan
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