Commit d992895b authored Aug 28, 2005 by Nick Piggin Committed by Linus Torvalds Aug 29, 2005

[PATCH] Lazy page table copies in fork()



Defer copying of ptes until fault time when it is possible to reconstruct
the pte from backing store. Idea from Andi Kleen and Nick Piggin.

Thanks to input from Rik van Riel and Linus and to Hugh for correcting
my blundering.

Ray Fucillo <fucillo@intersystems.com> reports:

  "I applied this latest patch to a 2.6.12 kernel and found that it does
   resolve the problem.  Prior to the patch on this machine, I was
   seeing about 23ms spent in fork for ever 100MB of shared memory
   segment.

   After applying the patch, fork is taking about 1ms regardless of the
   shared memory size."

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

parent 40193713

mm/memory.c

+11 −0

Original line number	Diff line number	Diff line
		@@ -498,6 +498,17 @@ int copy_page_range(struct mm_struct dst_mm, struct mm_struct src_mm,
		unsigned long addr = vma->vm_start;
		unsigned long end = vma->vm_end;

		/*
		* Don't copy ptes where a page fault will fill them correctly.
		* Fork becomes much lighter when there are big shared or private
		* readonly mappings. The tradeoff is that copy_page_range is more
		* efficient than faulting.
		*/
		if (!(vma->vm_flags & (VM_HUGETLB\|VM_NONLINEAR\|VM_RESERVED))) {
		if (!vma->anon_vma)
		return 0;
		}

		if (is_vm_hugetlb_page(vma))
		return copy_hugetlb_page_range(dst_mm, src_mm, vma);

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