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SubjectRe: [PATCH v2] mm: vmalloc: Clean up vunmap to avoid pgtable ops twice
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On 5/1/2018 4:22 AM, Andrew Morton wrote:
> On Mon, 16 Apr 2018 16:29:02 +0530 Chintan Pandya <cpandya@codeaurora.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
--
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center,
Inc. is a member of the Code Aurora Forum, a Linux Foundation
Collaborative Project

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