Re: [Xen-devel] [PATCH] xen-gnttab: do not add m2p override entries for blkback mappings

[Matt Wilson <msw@xxxxxxxxx>]

On Tue, Nov 12, 2013 at 05:48:56PM -0800, Anthony Liguori wrote:
> From: Anthony Liguori <aliguori@xxxxxxxxxx>
> 
> Commit 5dc03639 switched blkback to also add m2p override entries
> when mapping grant pages but history seems to have forgotten why
> this is useful if it ever was.
> 
> The blkback driver does not need m2p override entries to exist
> and there is significant overhead due to the locking in the m2p
> override table.  We see about a 10% improvement in IOP rate with
> this patch applied running FIO in the guest.
> 
> See http://article.gmane.org/gmane.linux.kernel/1590932 for a full
> analysis of current users.
> 
> Signed-off-by: Anthony Liguori <aliguori@xxxxxxxxxx>

Reviewed-by: Matt Wilson <msw@xxxxxxxxxx>

One comment for discussion below.

> ---
> A backported version of this has been heavily tested but the testing
> against the latest Linux tree is light so far.

[...]

> diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
> index c4d2298..081be8d 100644
> --- a/drivers/xen/grant-table.c
> +++ b/drivers/xen/grant-table.c
> @@ -881,7 +881,8 @@ EXPORT_SYMBOL_GPL(gnttab_batch_copy);
>  
>  int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
>                   struct gnttab_map_grant_ref *kmap_ops,
> -                 struct page **pages, unsigned int count)
> +                 struct page **pages, unsigned int count,
> +                 int override)
>  {
>       int i, ret;
>       bool lazy = false;
> @@ -918,10 +919,29 @@ int gnttab_map_refs(struct gnttab_map_grant_ref 
> *map_ops,
>               } else {
>                       mfn = PFN_DOWN(map_ops[i].dev_bus_addr);
>               }
> -             ret = m2p_add_override(mfn, pages[i], kmap_ops ?
> -                                    &kmap_ops[i] : NULL);
> -             if (ret)
> -                     return ret;
> +
> +             if (override) {
> +                     ret = m2p_add_override(mfn, pages[i], kmap_ops ?
> +                                            &kmap_ops[i] : NULL);
> +                     if (ret)
> +                             return ret;

The original code, which remains unmodified by this patch, is
returning without calling arch_leave_lazy_mmu_mode() at the bottom of
this function.

> +             } else {
> +                     unsigned long pfn, old_mfn;
> +
> +                     pfn = page_to_pfn(pages[i]);
> +                     old_mfn = pfn_to_mfn(pfn);
> +
> +                     /* Save previous MFN in page private*/
> +                     WARN_ON(PagePrivate(pages[i]));
> +                     SetPagePrivate(pages[i]);
> +                     set_page_private(pages[i], old_mfn);
> +
> +                     if (!set_phys_to_machine(pfn, FOREIGN_FRAME(mfn))) {
> +                             ret = -ENOMEM;
> +                             break;

The new path doesn't repeat this error. I think that the override path
needs to be changed to do the same in a separate patch.

--msw

> +                     }
> +             }
> +                     
>       }
>  
>       if (lazy)
                arch_leave_lazy_mmu_mode();

        return ret;
}

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