Commit d5d4b0aa authored Mar 22, 2006 by Chen, Kenneth W Committed by Linus Torvalds Mar 22, 2006

[PATCH] optimize follow_hugetlb_page

follow_hugetlb_page() walks a range of user virtual address and then fills
in list of struct page * into an array that is passed from the argument
list. It also gets a reference count via get_page(). For compound page,
get_page() actually traverse back to head page via page_private() macro and
then adds a reference count to the head page. Since we are doing a virt to
pte look up, kernel already has a struct page pointer into the head page.
So instead of traverse into the small unit page struct and then follow a
link back to the head page, optimize that with incrementing the reference
count directly on the head page.

The benefit is that we don't take a cache miss on accessing page struct for
the corresponding user address and more importantly, not to pollute the
cache with a "not very useful" round trip of pointer chasing. This adds a
moderate performance gain on an I/O intensive database transaction
workload.

Signed-off-by: Ken Chen <kenneth.w.chen@intel.com>
Cc: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

parent bba1e9b2

mm/hugetlb.c

+17 −8

Original line number	Diff line number	Diff line
		@@ -661,10 +661,10 @@ int follow_hugetlb_page(struct mm_struct mm, struct vm_area_struct vma,
		struct page pages, struct vm_area_struct vmas,
		unsigned long position, int length, int i)
		{
		unsigned long vpfn, vaddr = *position;
		unsigned long pfn_offset;
		unsigned long vaddr = *position;
		int remainder = *length;

		vpfn = vaddr/PAGE_SIZE;
		spin_lock(&mm->page_table_lock);
		while (vaddr < vma->vm_end && remainder) {
		pte_t *pte;
		@@ -692,19 +692,28 @@ int follow_hugetlb_page(struct mm_struct mm, struct vm_area_struct vma,
		break;
		}

		if (pages) {
		page = &pte_page(*pte)[vpfn % (HPAGE_SIZE/PAGE_SIZE)];
		pfn_offset = (vaddr & ~HPAGE_MASK) >> PAGE_SHIFT;
		page = pte_page(*pte);
		same_page:
		get_page(page);
		pages[i] = page;
		}
		if (pages)
		pages[i] = page + pfn_offset;

		if (vmas)
		vmas[i] = vma;

		vaddr += PAGE_SIZE;
		++vpfn;
		++pfn_offset;
		--remainder;
		++i;
		if (vaddr < vma->vm_end && remainder &&
		pfn_offset < HPAGE_SIZE/PAGE_SIZE) {
		/*
		* We use pfn_offset to avoid touching the pageframes
		* of this compound page.
		*/
		goto same_page;
		}
		}
		spin_unlock(&mm->page_table_lock);
		*length = remainder;

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