Re: Vanilla Linux and has_foreign_mapping

[Jeremy Fitzhardinge <jeremy@goop.org>]

Michael Abd-El-Malek wrote:
> I'm trying to add support to Linux 2.6.25 for the 
> "has_foreign_mappings" MMU context flag.  Xen's Linux 2.6.18 tree uses 
> this flag, so that page tables are properly disposed of when an 
> application exits when it has foreign mappings.

I was hoping to avoid having to introduce that flag, but I have to admit 
I haven't given it much analysis.  How are you using it?

>   See:
> http://lists.xensource.com/archives/html/xen-devel/2006-08/msg00038.html
>
> Here is my attempt:
> diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
> index 2a054ef..3e51897 100644
> --- a/arch/x86/xen/mmu.c
> +++ b/arch/x86/xen/mmu.c
> @@ -633,8 +633,13 @@ void xen_exit_mmap(struct mm_struct *mm)
>      spin_lock(&mm->page_table_lock);
>
>      /* pgd may not be pinned in the error exit path of execve */
> -    if (PagePinned(virt_to_page(mm->pgd)))
> -        xen_pgd_unpin(mm->pgd);
> +    if (PagePinned(virt_to_page(mm->pgd))) {
> +        if (mm->context.has_foreign_mappings) {
> +            printk("%s: because of has_foreign_mappings, delaying 
> unpinning\n", __FUNCTION__);
> +        } else {
> +            xen_pgd_unpin(mm->pgd);
> +        }
> +    }
>
>      spin_unlock(&mm->page_table_lock);
>  }
> diff --git a/include/asm-x86/mmu.h b/include/asm-x86/mmu.h
> index efa962c..7194698 100644
> --- a/include/asm-x86/mmu.h
> +++ b/include/asm-x86/mmu.h
> @@ -18,6 +18,9 @@ typedef struct {
>      int size;
>      struct mutex lock;
>      void *vdso;
> +#ifdef CONFIG_XEN
> +    int has_foreign_mappings;
> +#endif
>  } mm_context_t;
>
>  #ifdef CONFIG_SMP
>
> Unfortunately, I got the following kernel crash on process exit:
>
> BUG: unable to handle kernel paging request at ebdae008
> IP: [<c01157f9>] pgd_mop_up_pmds+0x6a/0xd8

Which line is that?

    J