Re: [PATCH -mm] change anon_vma linking to fix multi-process serverscalability issue

From: Rik van Riel
Date: Thu Jan 28 2010 - 12:25:48 EST

On 01/28/2010 11:37 AM, Minchan Kim wrote:
On Thu, 2010-01-28 at 00:20 -0500, Rik van Riel wrote:

This patch changes the way anon_vmas and VMAs are linked, which
allows us to associate multiple anon_vmas with a VMA. At fork
time, each child process gets its own anon_vmas, in which its
COWed pages will be instantiated. The parents' anon_vma is also
linked to the VMA, because non-COWed pages could be present in
any of the children.

any of the children?

IMHO, "parent" is right. :)
Do I miss something? Could you elaborate it?

I am talking about an anonymous page that is shared by parent
and children processes and has not been COW copied yet.

-void vma_adjust(struct vm_area_struct *vma, unsigned long start,
+int vma_adjust(struct vm_area_struct *vma, unsigned long start,
unsigned long end, pgoff_t pgoff, struct vm_area_struct *insert)
struct mm_struct *mm = vma->vm_mm;
@@ -542,6 +541,29 @@ again: remove_next = 1 + (end> next->vm_end);

+ /*
+ * When changing only vma->vm_end, we don't really need
+ * anon_vma lock.
+ */
+ if (vma->anon_vma&& (insert || importer || start != vma->vm_start))
+ anon_vma = vma->anon_vma;
+ if (anon_vma) {
+ /*
+ * Easily overlooked: when mprotect shifts the boundary,
+ * make sure the expanding vma has anon_vma set if the
+ * shrinking vma had, to cover any anon pages imported.
+ */
+ if (importer&& !importer->anon_vma) {
+ /* Block reverse map lookups until things are set up. */
+ importer->vm_flags |= VM_LOCK_RMAP;
+ if (anon_vma_clone(importer, vma)) {
+ importer->vm_flags&= ~VM_LOCK_RMAP;
+ return -ENOMEM;

If we fail in here during progressing on next vmas in case of mprotect case 6,
the previous vmas would become inconsistent state.

I've re-read the code, but I don't see what you are referring
to. If vma_adjust bails out early, no VMAs will be adjusted
and all the VMAs will stay the way they were before mprotect
was called.

What am I overlooking?

@@ -2260,6 +2306,12 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
return new_vma;
+ out_free_mempol:
+ mpol_put(pol);
+ out_free_vma:
+ kmem_cache_free(vm_area_cachep, new_vma);
+ return NULL;

As I said previously, I have a concern about memory footprint.
It adds anon_vma_chain and increases anon_vma's size for KSM.

I think it will increase 3 times more than only anon_vma.

Although you think it's not big in normal machine,
it's not good in embedded system which is no anon_vma scalability issue
and even no-swap. so I wanted you to make it configurable.

That is a fair point. With CONFIG_SWAP=n we do not need the
anon_vma structs or anon_vma_chain structs at all.

I would be happy to integrate a patch into my series that
stubs out all of that code for CONFIG_SWAP=n, but I am going
to work on something else myself right now :)

All rights reversed.
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