Re: [PATCH] x86: don't use destroy_xen_mappings() for vunmap()

[Keir Fraser <keir.xen@gmail.com>]

On 17/07/2013 08:17, "Jan Beulich" <JBeulich@suse.com> wrote:

> Its attempt to tear down intermediate page table levels may race with
> map_pages_to_xen() establishing them, and now that
> map_domain_page_global() is backed by vmap() this teardown is also
> wasteful (as it's very likely to need the same address space populated
> again within foreseeable time).
> 
> As the race between vmap() and vunmap(), according to the latest stage
> tester logs, doesn't appear to be the only one still left, the patch
> also adds logging for vmap() and vunmap() uses (there shouldn't be too
> many of them, so logs shouldn't get flooded). These are supposed to
> get removed (and are made stand out clearly) as soon as we're certain
> that there's no issue left.
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Acked-by: Keir Fraser <keir@xen.org>

> --- a/xen/common/vmap.c
> +++ b/xen/common/vmap.c
> @@ -177,6 +177,7 @@ void *__vmap(const unsigned long *mfn, u
>      void *va = vm_alloc(nr * granularity, align);
>      unsigned long cur = (unsigned long)va;
>  
> +printk("vmap(%p:%#x)\n", va, nr * granularity);//temp
>      for ( ; va && nr--; ++mfn, cur += PAGE_SIZE * granularity )
>      {
>          if ( map_pages_to_xen(cur, *mfn, granularity, flags) )
> @@ -196,9 +197,14 @@ void *vmap(const unsigned long *mfn, uns
>  
>  void vunmap(const void *va)
>  {
> +#ifndef _PAGE_NONE
>      unsigned long addr = (unsigned long)va;
>  
>      destroy_xen_mappings(addr, addr + PAGE_SIZE * vm_size(va));
> +#else /* Avoid tearing down intermediate page tables. */
> +printk("vunmap(%p:%#x)\n", va, vm_size(va));//temp
> +    map_pages_to_xen((unsigned long)va, 0, vm_size(va), _PAGE_NONE);
> +#endif
>      vm_free(va);
>  }
>  #endif
> --- a/xen/include/asm-x86/page.h
> +++ b/xen/include/asm-x86/page.h
> @@ -288,6 +288,7 @@ extern l1_pgentry_t l1_identmap[L1_PAGET
>  void paging_init(void);
>  #endif /* !defined(__ASSEMBLY__) */
>  
> +#define _PAGE_NONE     _AC(0x000,U)
>  #define _PAGE_PRESENT  _AC(0x001,U)
>  #define _PAGE_RW       _AC(0x002,U)
>  #define _PAGE_USER     _AC(0x004,U)
> 
> 
>