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