avoiding duplicate icache flushing of shared maps on nommu

[Mike Frysinger <vapier.adi@gmail.com>]

when working with FDPIC, there are many shared maps of read only text
regions (the C library, applet packages like busybox, ...) between
applications.  but the current mm/nommu.c:do_mmap_pgoff() function
will issue an icache flush whenever a vma is added to a mm instead of
only doing it when the map is initially created.  am i missing
something obvious here, or would a change like below be OK ?  this
easily cuts the number of icache flushes during boot by 50% if not
more.

(yes, this now does the icache flush while holding the
nommu_region_sem, but i'm interested if the _idea_ is OK)

--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -1409,14 +1409,14 @@ unsigned long do_mmap_pgoff(struct file *file,

    current->mm->total_vm += len >> PAGE_SHIFT;

+   if (prot & PROT_EXEC)
+       flush_icache_range(result, result + len);
+
 share:
    add_vma_to_mm(current->mm, vma);

    up_write(&nommu_region_sem);

-   if (prot & PROT_EXEC)
-       flush_icache_range(result, result + len);
-
    kleave(" = %lx", result);
    return result;

-mike