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SubjectRe: process creation time increases linearly with shmem
On Fri, 26 Aug 2005, Nick Piggin wrote:

> > Skipping MAP_SHARED in fork() sounds like a good idea to me...
> Indeed. Linus, can you remember why we haven't done this before?

Where "this" looks something like the patch below, shamelessly
merging Nick's and Andy's patches and adding the initialization
of retval.

I suspect this may be a measurable win on database servers with
a web frontend, where the connections to the database server are
set up basically for each individual query, and don't stick around
for a long time.

No, I haven't actually tested this patch - but feel free to go
wild while I sign off for the night.

Signed-off-by: Rik van Riel <>

--- linux-2.6.12/kernel/fork.c.mapshared 2005-08-25 18:40:44.000000000 -0400
+++ linux-2.6.12/kernel/fork.c 2005-08-25 18:47:16.000000000 -0400
@@ -184,7 +184,7 @@
struct vm_area_struct * mpnt, *tmp, **pprev;
struct rb_node **rb_link, *rb_parent;
- int retval;
+ int retval = 0;
unsigned long charge;
struct mempolicy *pol;

@@ -265,7 +265,10 @@
rb_parent = &tmp->vm_rb;

- retval = copy_page_range(mm, current->mm, tmp);
+ /* Skip pte copying if page faults can take care of things. */
+ if (!file || !(tmp->vm_flags & VM_SHARED) ||
+ is_vm_hugetlb_page(vma))
+ retval = copy_page_range(mm, current->mm, tmp);

if (tmp->vm_ops && tmp->vm_ops->open)
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