[PATCH] mremap: skip page table lookup for non-faulted anonymous VMAs

[PATCH] mremap: skip page table lookup for non-faulted anonymous VMAs

[Nai Xia]

If an anonymous vma has not yet been faulted, move_page_tables() in move_vma()
is not necessary for it.

Signed-off-by: Nai Xia <nai.xia@gmail.com>
---
diff --git a/mm/mremap.c b/mm/mremap.c
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -200,6 +200,10 @@ static unsigned long move_vma(struct vm_area_struct *vma,
 	if (!new_vma)
 		return -ENOMEM;
 
+	/* An anonymous vma has not been faulted, no pagetables lookup. */
+	if (!vma->vm_file && !vma->anon_vma)
+		goto page_tables_ok;
+
 	moved_len = move_page_tables(vma, old_addr, new_vma, new_addr, old_len);
 	if (moved_len < old_len) {
 		/*
@@ -213,6 +217,7 @@ static unsigned long move_vma(struct vm_area_struct *vma,
 		old_addr = new_addr;
 		new_addr = -ENOMEM;
 	}
+page_tables_ok:
 
 	/* Conceal VM_ACCOUNT so old reservation is not undone */
 	if (vm_flags & VM_ACCOUNT) {

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Re: [PATCH] mremap: skip page table lookup for non-faulted anonymous VMAs

[Andrea Arcangeli]

On Mon, Nov 07, 2011 at 12:21:35PM +0800, Nai Xia wrote:
> If an anonymous vma has not yet been faulted, move_page_tables() in move_vma()
> is not necessary for it.

I actually thought of adding that (in fact fork has it and it's more
likely to be beneficial for fork than for mremap I suspect), but this
adds a branch to the fast path for a case that shouldn't normally
materialize. So I don't think it's worth adding it as I expect it to
add overhead in average.

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Re: [PATCH] mremap: skip page table lookup for non-faulted anonymous VMAs

[Nai Xia]

On Mon, Nov 7, 2011 at 11:32 PM, Andrea Arcangeli <aarcange@redhat.com> wrote:
> On Mon, Nov 07, 2011 at 12:21:35PM +0800, Nai Xia wrote:
>> If an anonymous vma has not yet been faulted, move_page_tables() in move_vma()
>> is not necessary for it.
>
> I actually thought of adding that (in fact fork has it and it's more
> likely to be beneficial for fork than for mremap I suspect), but this
> adds a branch to the fast path for a case that shouldn't normally
> materialize. So I don't think it's worth adding it as I expect it to
> add overhead in average.
>

Well, it seems like I forgot to embrace it with unlikely(), with instr
prefetching  this line seems a trivial even for the average cases.

But in case it really materializes, it may avoid tedious page table
locking and pmd allocation and furthermore, a semantically
confusing operation of move_page_tables() on a same VMA.

Anyway, it's not a big deal, indeed. You are the maintainers,
it's left for you to decide.

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