Re: avoiding duplicate icache flushing of shared maps on nommu

[Jie Zhang <jie.zhang@analog.com>]

On 11/28/2009 11:40 PM, Mike Frysinger wrote:
> 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)
>
I don't see any problem. But I'm not a kernel expert. I tried to take a 
look at the source code history. But I cannot find code older than 
2.6.12. In the CVS repository of uClinux, there is no similar code in 
kernel sub-directory. :-(


Jie


> --- 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