[Xen-devel] Re: [PATCH RFC] vmalloc: eagerly clear ptes on vunmap

[Nick Piggin <npiggin@xxxxxxxxx>]

On Mon, Nov 29, 2010 at 12:32:11PM -0800, Jeremy Fitzhardinge wrote:
> When unmapping a region in the vmalloc space, clear the ptes immediately.
> There's no point in deferring this because there's no amortization
> benefit.
> 
> The TLBs are left dirty, and they are flushed lazily to amortize the
> cost of the IPIs.
> 
> This specific motivation for this patch is a regression since 2.6.36 when
> using NFS under Xen, triggered by the NFS client's use of vm_map_ram()
> introduced in 56e4ebf877b6043c289bda32a5a7385b80c17dee.  XFS also uses
> vm_map_ram() and could cause similar problems.

I do wonder whether there are cache benefits from batching page table
updates, especially the batched per cpu maps (and in your version they
get double-cleared as well).  I think this patch is good, but I think
perhaps making it configurable would be nice.

So... main question, does it allow Xen to use lazy flushing and avoid
vm_unmap_aliases() calls?


> 
> Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
> Cc: Nick Piggin <npiggin@xxxxxxxxx>
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index a3d66b3..9960644 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -566,7 +566,6 @@ static void __purge_vmap_area_lazy(unsigned long *start, 
> unsigned long *end,
>                       if (va->va_end > *end)
>                               *end = va->va_end;
>                       nr += (va->va_end - va->va_start) >> PAGE_SHIFT;
> -                     unmap_vmap_area(va);
>                       list_add_tail(&va->purge_list, &valist);
>                       va->flags |= VM_LAZY_FREEING;
>                       va->flags &= ~VM_LAZY_FREE;
> @@ -616,6 +615,8 @@ static void purge_vmap_area_lazy(void)
>   */
>  static void free_unmap_vmap_area_noflush(struct vmap_area *va)
>  {
> +     unmap_vmap_area(va);
> +
>       va->flags |= VM_LAZY_FREE;
>       atomic_add((va->va_end - va->va_start) >> PAGE_SHIFT, &vmap_lazy_nr);
>       if (unlikely(atomic_read(&vmap_lazy_nr) > lazy_max_pages()))
> @@ -944,8 +945,10 @@ static void vb_free(const void *addr, unsigned long size)
>               BUG_ON(vb->free);
>               spin_unlock(&vb->lock);
>               free_vmap_block(vb);
> -     } else
> +     } else {
>               spin_unlock(&vb->lock);
> +             vunmap_page_range((unsigned long)addr, (unsigned long)addr + 
> size);
> +     }
>  }
>  
>  /**
> @@ -988,7 +991,6 @@ void vm_unmap_aliases(void)
>  
>                               s = vb->va->va_start + (i << PAGE_SHIFT);
>                               e = vb->va->va_start + (j << PAGE_SHIFT);
> -                             vunmap_page_range(s, e);
>                               flush = 1;
>  
>                               if (s < start)
> 

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