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

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Date:   Tue, 1 May 2018 12:16:53 +0530
From:   Chintan Pandya <cpandya@...eaurora.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     vbabka@...e.cz, labbott@...hat.com, catalin.marinas@....com,
        hannes@...xchg.org, f.fainelli@...il.com, xieyisheng1@...wei.com,
        ard.biesheuvel@...aro.org, richard.weiyang@...il.com,
        byungchul.park@....com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm: vmalloc: Clean up vunmap to avoid pgtable ops
 twice



On 5/1/2018 4:22 AM, Andrew Morton wrote:
> On Mon, 16 Apr 2018 16:29:02 +0530 Chintan Pandya <cpandya@...eaurora.org> 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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