Re: [Xen-devel] [PATCH v3] xen/grant-table: Avoid m2p_override during mapping

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Mon, 20 Jan 2014, Zoltan Kiss wrote:
> The grant mapping API does m2p_override unnecessarily: only gntdev needs it,
> for blkback and future netback patches it just cause a lock contention, as
> those pages never go to userspace. Therefore this series does the following:
> - the original functions were renamed to __gnttab_[un]map_refs, with a new
>   parameter m2p_override
> - based on m2p_override either they follow the original behaviour, or just set
>   the private flag and call set_phys_to_machine
> - gnttab_[un]map_refs are now a wrapper to call __gnttab_[un]map_refs with
>   m2p_override false
> - a new function gnttab_[un]map_refs_userspace provides the old behaviour
> 
> v2:
> - move the storing of the old mfn in page->index to gnttab_map_refs
> - move the function header update to a separate patch
> 
> v3:
> - a new approach to retain old behaviour where it needed
> - squash the patches into one
> 
> Signed-off-by: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>
> Suggested-by: David Vrabel <david.vrabel@xxxxxxxxxx>
> ---
>  drivers/block/xen-blkback/blkback.c |   15 +++----
>  drivers/xen/gntdev.c                |   13 +++---
>  drivers/xen/grant-table.c           |   81 
> +++++++++++++++++++++++++++++------
>  include/xen/grant_table.h           |    8 +++-
>  4 files changed, 87 insertions(+), 30 deletions(-)
> 
> diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
> index aa846a4..87ded60 100644
> --- a/drivers/xen/grant-table.c
> +++ b/drivers/xen/grant-table.c
> @@ -880,15 +880,17 @@ void gnttab_batch_copy(struct gnttab_copy *batch, 
> unsigned count)
>  }
>  EXPORT_SYMBOL_GPL(gnttab_batch_copy);
>  
> -int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
> +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,
> +                 bool m2p_override)
>  {
>       int i, ret;
>       bool lazy = false;
>       pte_t *pte;
>       unsigned long mfn;
>  
> +     BUG_ON(kmap_ops && !m2p_override);
>       ret = HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref, map_ops, count);
>       if (ret)
>               return ret;
> @@ -907,10 +909,12 @@ int gnttab_map_refs(struct gnttab_map_grant_ref 
> *map_ops,
>                       set_phys_to_machine(map_ops[i].host_addr >> PAGE_SHIFT,
>                                       map_ops[i].dev_bus_addr >> PAGE_SHIFT);
>               }
> -             return ret;
> +             return 0;
>       }
>  
> -     if (!in_interrupt() && paravirt_get_lazy_mode() == PARAVIRT_LAZY_NONE) {
> +     if (m2p_override &&
> +         !in_interrupt() &&
> +         paravirt_get_lazy_mode() == PARAVIRT_LAZY_NONE) {
>               arch_enter_lazy_mmu_mode();
>               lazy = true;
>       }
> @@ -927,8 +931,18 @@ 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 (m2p_override)
> +                     ret = m2p_add_override(mfn, pages[i], kmap_ops ?
> +                                            &kmap_ops[i] : NULL);
> +             else {
> +                     unsigned long pfn = page_to_pfn(pages[i]);
> +                     WARN_ON(PagePrivate(pages[i]));
> +                     SetPagePrivate(pages[i]);
> +                     set_page_private(pages[i], mfn);
> +                     pages[i]->index = pfn_to_mfn(pfn);
> +                     if (unlikely(!set_phys_to_machine(pfn, 
> FOREIGN_FRAME(mfn))))
> +                             return -ENOMEM;

What happens if the page is PageHighMem?

This looks like a subset of m2p_add_override, but it is missing some
relevant bits, like the PageHighMem check, or the p2m(m2p(mfn)) == mfn
check.  Maybe we can find a way to avoid duplicating the code.
We could split m2p_add_override in two functions or add yet another
parameter to it.


> +             }
>               if (ret)
>                       goto out;
>       }
> @@ -937,17 +951,33 @@ int gnttab_map_refs(struct gnttab_map_grant_ref 
> *map_ops,
>       if (lazy)
>               arch_leave_lazy_mmu_mode();
>  
> -     return ret;
> +     return 0;
> +}
> +
> +int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
> +                 struct page **pages, unsigned int count)
> +{
> +     return __gnttab_map_refs(map_ops, NULL, pages, count, false);
>  }
>  EXPORT_SYMBOL_GPL(gnttab_map_refs);
>  
> -int gnttab_unmap_refs(struct gnttab_unmap_grant_ref *unmap_ops,
> +int gnttab_map_refs_userspace(struct gnttab_map_grant_ref *map_ops,
> +                           struct gnttab_map_grant_ref *kmap_ops,
> +                           struct page **pages, unsigned int count)
> +{
> +     return __gnttab_map_refs(map_ops, kmap_ops, pages, count, true);
> +}
> +EXPORT_SYMBOL_GPL(gnttab_map_refs_userspace);
> +
> +int __gnttab_unmap_refs(struct gnttab_unmap_grant_ref *unmap_ops,
>                     struct gnttab_map_grant_ref *kmap_ops,
> -                   struct page **pages, unsigned int count)
> +                   struct page **pages, unsigned int count,
> +                   bool m2p_override)
>  {
>       int i, ret;
>       bool lazy = false;
>  
> +     BUG_ON(kmap_ops && !m2p_override);
>       ret = HYPERVISOR_grant_table_op(GNTTABOP_unmap_grant_ref, unmap_ops, 
> count);
>       if (ret)
>               return ret;
> @@ -958,17 +988,26 @@ int gnttab_unmap_refs(struct gnttab_unmap_grant_ref 
> *unmap_ops,
>                       set_phys_to_machine(unmap_ops[i].host_addr >> 
> PAGE_SHIFT,
>                                       INVALID_P2M_ENTRY);
>               }
> -             return ret;
> +             return 0;
>       }
>  
> -     if (!in_interrupt() && paravirt_get_lazy_mode() == PARAVIRT_LAZY_NONE) {
> +     if (m2p_override &&
> +         !in_interrupt() &&
> +         paravirt_get_lazy_mode() == PARAVIRT_LAZY_NONE) {
>               arch_enter_lazy_mmu_mode();
>               lazy = true;
>       }
>  
>       for (i = 0; i < count; i++) {
> -             ret = m2p_remove_override(pages[i], kmap_ops ?
> -                                    &kmap_ops[i] : NULL);
> +             if (m2p_override)
> +                     ret = m2p_remove_override(pages[i], kmap_ops ?
> +                                               &kmap_ops[i] : NULL);
> +             else {
> +                     unsigned long pfn = page_to_pfn(pages[i]);
> +                     WARN_ON(!PagePrivate(pages[i]));
> +                     ClearPagePrivate(pages[i]);
> +                     set_phys_to_machine(pfn, pages[i]->index);

same here: let's try to avoid code duplication


> +             }
>               if (ret)
>                       goto out;
>       }
> @@ -977,10 +1016,24 @@ int gnttab_unmap_refs(struct gnttab_unmap_grant_ref 
> *unmap_ops,
>       if (lazy)
>               arch_leave_lazy_mmu_mode();
>  
> -     return ret;
> +     return 0;
> +}
> +
> +int gnttab_unmap_refs(struct gnttab_unmap_grant_ref *map_ops,
> +                 struct page **pages, unsigned int count)
> +{
> +     return __gnttab_unmap_refs(map_ops, NULL, pages, count, false);
>  }
>  EXPORT_SYMBOL_GPL(gnttab_unmap_refs);
>  
> +int gnttab_unmap_refs_userspace(struct gnttab_unmap_grant_ref *map_ops,
> +                             struct gnttab_map_grant_ref *kmap_ops,
> +                             struct page **pages, unsigned int count)
> +{
> +     return __gnttab_unmap_refs(map_ops, kmap_ops, pages, count, true);
> +}
> +EXPORT_SYMBOL_GPL(gnttab_unmap_refs_userspace);
> +
>  static unsigned nr_status_frames(unsigned nr_grant_frames)
>  {
>       BUG_ON(grefs_per_grant_frame == 0);

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