Re: [Xen-devel] [PATCH v8] xen/grant-table: Avoid m2p_override during mapping

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 6 Feb 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
> - it cuts out common parts from m2p_*_override functions to
>   *_foreign_p2m_mapping functions
> 
> It also removes a stray space from page.h and change ret to 0 if
> XENFEAT_auto_translated_physmap, as that is the only possible return value
> there.
> 
> 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
> 
> v4:
> - move out the common bits from m2p* functions, and pass pfn/mfn as parameter
> - clear page->private before doing anything with the page, so 
> m2p_find_override
>   won't race with this
> 
> v5:
> - change return value handling in __gnttab_[un]map_refs
> - remove a stray space in page.h
> - add detail why ret = 0 now at some places
> 
> v6:
> - don't pass pfn to m2p* functions, just get it locally
> 
> v7:
> - the previous version broke build on ARM, as there is no need for those p2m
>   changes. I've put them into arch specific functions, which are stubs on arm
> 
> v8:
> - give credit to Anthony Liguori who submitted a very similar patch 
> originally:
> http://marc.info/?i=1384307336-5328-1-git-send-email-anthony%40codemonkey.ws
> - create ARM stub for get_phys_to_machine
> - move definition of mfn in __gnttab_unmap_refs to the right place
> 
> Signed-off-by: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>
> Original-by: Anthony Liguori <aliguori@xxxxxxxxxx>
> ---
>  arch/arm/include/asm/xen/page.h     |   20 +++++++++-
>  arch/x86/include/asm/xen/page.h     |    7 +++-
>  arch/x86/xen/p2m.c                  |   49 ++++++++++++++++-------
>  drivers/block/xen-blkback/blkback.c |   15 +++----
>  drivers/xen/gntdev.c                |   13 +++---
>  drivers/xen/grant-table.c           |   74 
> +++++++++++++++++++++++++++++------
>  include/xen/grant_table.h           |    8 +++-
>  7 files changed, 140 insertions(+), 46 deletions(-)
> 
> diff --git a/arch/arm/include/asm/xen/page.h b/arch/arm/include/asm/xen/page.h
> index e0965ab..d26c3d7 100644
> --- a/arch/arm/include/asm/xen/page.h
> +++ b/arch/arm/include/asm/xen/page.h
> @@ -97,13 +97,31 @@ static inline pte_t *lookup_address(unsigned long 
> address, unsigned int *level)
>       return NULL;
>  }
>  
> +static inline unsigned long get_phys_to_machine(unsigned long pfn)
> +{
> +     return 0;
> +}

We can probably just rename phys_to_machine to get_phys_to_machine.
Be careful about the other callers in page.h.


> +static inline int set_foreign_p2m_mapping(struct page *page, unsigned long 
> mfn)
> +{
> +     return 0;
> +}
> +
>  static inline int m2p_add_override(unsigned long mfn, struct page *page,
>               struct gnttab_map_grant_ref *kmap_op)
>  {
>       return 0;
>  }
>  
> -static inline int m2p_remove_override(struct page *page, bool clear_pte)
> +static inline int restore_foreign_p2m_mapping(struct page *page,
> +                                           unsigned long mfn)
> +{
> +     return 0;
> +}
> +
> +static inline int m2p_remove_override(struct page *page,
> +                                   struct gnttab_map_grant_ref *kmap_op,
> +                                   unsigned long mfn)
>  {
>       return 0;
>  }

I like set_foreign_p2m_mapping and restore_foreign_p2m_mapping but we
need to use them coherently.
I think that on ARM it makes sense to rename the current implementation of
set_phys_to_machine to set_foreign_p2m_mapping.
I also makes sense to change the call to set_phys_to_machine with
INVALID_P2M_ENTRY as a parameter to restore_foreign_p2m_mapping.

Then you can implement set_phys_to_machine as

BUG_ON(pfn != mfn):
    
and get_phys_to_machine as

return pfn;


> diff --git a/arch/x86/include/asm/xen/page.h b/arch/x86/include/asm/xen/page.h
> index 3e276eb..0340954 100644
> --- a/arch/x86/include/asm/xen/page.h
> +++ b/arch/x86/include/asm/xen/page.h
> @@ -49,10 +49,13 @@ extern bool __set_phys_to_machine(unsigned long pfn, 
> unsigned long mfn);
>  extern unsigned long set_phys_range_identity(unsigned long pfn_s,
>                                            unsigned long pfn_e);
>  
> +extern int set_foreign_p2m_mapping(struct page *page, unsigned long mfn);
>  extern int m2p_add_override(unsigned long mfn, struct page *page,
>                           struct gnttab_map_grant_ref *kmap_op);
> +extern int restore_foreign_p2m_mapping(struct page *page, unsigned long mfn);
>  extern int m2p_remove_override(struct page *page,
> -                             struct gnttab_map_grant_ref *kmap_op);
> +                            struct gnttab_map_grant_ref *kmap_op,
> +                            unsigned long mfn);
>  extern struct page *m2p_find_override(unsigned long mfn);
>  extern unsigned long m2p_find_override_pfn(unsigned long mfn, unsigned long 
> pfn);
>  
> @@ -121,7 +124,7 @@ static inline unsigned long mfn_to_pfn(unsigned long mfn)
>               pfn = m2p_find_override_pfn(mfn, ~0);
>       }
>  
> -     /* 
> +     /*
>        * pfn is ~0 if there are no entries in the m2p for mfn or if the
>        * entry doesn't map back to the mfn and m2p_override doesn't have a
>        * valid entry for it.
> diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
> index 696c694..3e7cfa9 100644
> --- a/arch/x86/xen/p2m.c
> +++ b/arch/x86/xen/p2m.c
> @@ -881,6 +881,22 @@ static unsigned long mfn_hash(unsigned long mfn)
>       return hash_long(mfn, M2P_OVERRIDE_HASH_SHIFT);
>  }
>  
> +int set_foreign_p2m_mapping(struct page *page, unsigned long mfn)
> +{
> +     unsigned long pfn = page_to_pfn(page);
> +
> +     WARN_ON(PagePrivate(page));
> +     SetPagePrivate(page);
> +     set_page_private(page, mfn);
> +
> +     page->index = pfn_to_mfn(pfn);
> +     if (unlikely(!set_phys_to_machine(pfn, FOREIGN_FRAME(mfn))))
> +             return -ENOMEM;
> +
> +     return 0;
> +}
> +EXPORT_SYMBOL_GPL(set_foreign_p2m_mapping);
> +
>  /* Add an MFN override for a particular page */
>  int m2p_add_override(unsigned long mfn, struct page *page,
>               struct gnttab_map_grant_ref *kmap_op)
> @@ -899,13 +915,6 @@ int m2p_add_override(unsigned long mfn, struct page 
> *page,
>                                       "m2p_add_override: pfn %lx not mapped", 
> pfn))
>                       return -EINVAL;
>       }
> -     WARN_ON(PagePrivate(page));
> -     SetPagePrivate(page);
> -     set_page_private(page, mfn);
> -     page->index = pfn_to_mfn(pfn);
> -
> -     if (unlikely(!set_phys_to_machine(pfn, FOREIGN_FRAME(mfn))))
> -             return -ENOMEM;
>  
>       if (kmap_op != NULL) {
>               if (!PageHighMem(page)) {
> @@ -943,20 +952,33 @@ int m2p_add_override(unsigned long mfn, struct page 
> *page,
>       return 0;
>  }
>  EXPORT_SYMBOL_GPL(m2p_add_override);
> +
> +int restore_foreign_p2m_mapping(struct page *page, unsigned long mfn)
> +{
> +     unsigned long pfn = page_to_pfn(page);
> +
> +     if (mfn == INVALID_P2M_ENTRY || !(mfn & FOREIGN_FRAME_BIT))
> +             return -EINVAL;
> +
> +     set_page_private(page, INVALID_P2M_ENTRY);
> +     WARN_ON(!PagePrivate(page));
> +     ClearPagePrivate(page);
> +     set_phys_to_machine(pfn, page->index);
> +     return 0;
> +}
> +EXPORT_SYMBOL_GPL(restore_foreign_p2m_mapping);
> +
>  int m2p_remove_override(struct page *page,
> -             struct gnttab_map_grant_ref *kmap_op)
> +                     struct gnttab_map_grant_ref *kmap_op,
> +                     unsigned long mfn)
>  {
>       unsigned long flags;
> -     unsigned long mfn;
>       unsigned long pfn;
>       unsigned long uninitialized_var(address);
>       unsigned level;
>       pte_t *ptep = NULL;
>  
>       pfn = page_to_pfn(page);
> -     mfn = get_phys_to_machine(pfn);
> -     if (mfn == INVALID_P2M_ENTRY || !(mfn & FOREIGN_FRAME_BIT))
> -             return -EINVAL;
>  
>       if (!PageHighMem(page)) {
>               address = (unsigned long)__va(pfn << PAGE_SHIFT);
> @@ -970,10 +992,7 @@ int m2p_remove_override(struct page *page,
>       spin_lock_irqsave(&m2p_override_lock, flags);
>       list_del(&page->lru);
>       spin_unlock_irqrestore(&m2p_override_lock, flags);
> -     WARN_ON(!PagePrivate(page));
> -     ClearPagePrivate(page);
>  
> -     set_phys_to_machine(pfn, page->index);
>       if (kmap_op != NULL) {
>               if (!PageHighMem(page)) {
>                       struct multicall_space mcs;
> diff --git a/drivers/block/xen-blkback/blkback.c 
> b/drivers/block/xen-blkback/blkback.c
> index 4b97b86..da18046 100644
> --- a/drivers/block/xen-blkback/blkback.c
> +++ b/drivers/block/xen-blkback/blkback.c
> @@ -285,8 +285,7 @@ static void free_persistent_gnts(struct xen_blkif *blkif, 
> struct rb_root *root,
>  
>               if (++segs_to_unmap == BLKIF_MAX_SEGMENTS_PER_REQUEST ||
>                       !rb_next(&persistent_gnt->node)) {
> -                     ret = gnttab_unmap_refs(unmap, NULL, pages,
> -                             segs_to_unmap);
> +                     ret = gnttab_unmap_refs(unmap, pages, segs_to_unmap);
>                       BUG_ON(ret);
>                       put_free_pages(blkif, pages, segs_to_unmap);
>                       segs_to_unmap = 0;
> @@ -321,8 +320,7 @@ static void unmap_purged_grants(struct work_struct *work)
>               pages[segs_to_unmap] = persistent_gnt->page;
>  
>               if (++segs_to_unmap == BLKIF_MAX_SEGMENTS_PER_REQUEST) {
> -                     ret = gnttab_unmap_refs(unmap, NULL, pages,
> -                             segs_to_unmap);
> +                     ret = gnttab_unmap_refs(unmap, pages, segs_to_unmap);
>                       BUG_ON(ret);
>                       put_free_pages(blkif, pages, segs_to_unmap);
>                       segs_to_unmap = 0;
> @@ -330,7 +328,7 @@ static void unmap_purged_grants(struct work_struct *work)
>               kfree(persistent_gnt);
>       }
>       if (segs_to_unmap > 0) {
> -             ret = gnttab_unmap_refs(unmap, NULL, pages, segs_to_unmap);
> +             ret = gnttab_unmap_refs(unmap, pages, segs_to_unmap);
>               BUG_ON(ret);
>               put_free_pages(blkif, pages, segs_to_unmap);
>       }
> @@ -670,15 +668,14 @@ static void xen_blkbk_unmap(struct xen_blkif *blkif,
>                                   GNTMAP_host_map, pages[i]->handle);
>               pages[i]->handle = BLKBACK_INVALID_HANDLE;
>               if (++invcount == BLKIF_MAX_SEGMENTS_PER_REQUEST) {
> -                     ret = gnttab_unmap_refs(unmap, NULL, unmap_pages,
> -                                             invcount);
> +                     ret = gnttab_unmap_refs(unmap, unmap_pages, invcount);
>                       BUG_ON(ret);
>                       put_free_pages(blkif, unmap_pages, invcount);
>                       invcount = 0;
>               }
>       }
>       if (invcount) {
> -             ret = gnttab_unmap_refs(unmap, NULL, unmap_pages, invcount);
> +             ret = gnttab_unmap_refs(unmap, unmap_pages, invcount);
>               BUG_ON(ret);
>               put_free_pages(blkif, unmap_pages, invcount);
>       }
> @@ -740,7 +737,7 @@ again:
>       }
>  
>       if (segs_to_map) {
> -             ret = gnttab_map_refs(map, NULL, pages_to_gnt, segs_to_map);
> +             ret = gnttab_map_refs(map, pages_to_gnt, segs_to_map);
>               BUG_ON(ret);
>       }
>  
> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> index 073b4a1..34a2704 100644
> --- a/drivers/xen/gntdev.c
> +++ b/drivers/xen/gntdev.c
> @@ -284,8 +284,10 @@ static int map_grant_pages(struct grant_map *map)
>       }
>  
>       pr_debug("map %d+%d\n", map->index, map->count);
> -     err = gnttab_map_refs(map->map_ops, use_ptemod ? map->kmap_ops : NULL,
> -                     map->pages, map->count);
> +     err = gnttab_map_refs_userspace(map->map_ops,
> +                                     use_ptemod ? map->kmap_ops : NULL,
> +                                     map->pages,
> +                                     map->count);
>       if (err)
>               return err;
>  
> @@ -315,9 +317,10 @@ static int __unmap_grant_pages(struct grant_map *map, 
> int offset, int pages)
>               }
>       }
>  
> -     err = gnttab_unmap_refs(map->unmap_ops + offset,
> -                     use_ptemod ? map->kmap_ops + offset : NULL, map->pages 
> + offset,
> -                     pages);
> +     err = gnttab_unmap_refs_userspace(map->unmap_ops + offset,
> +                                       use_ptemod ? map->kmap_ops + offset : 
> NULL,
> +                                       map->pages + offset,
> +                                       pages);
>       if (err)
>               return err;
>  
> diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
> index b84e3ab..c719bc8 100644
> --- a/drivers/xen/grant-table.c
> +++ b/drivers/xen/grant-table.c
> @@ -928,15 +928,17 @@ 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;
> @@ -955,10 +957,12 @@ 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);

I think that this should become a call to set_foreign_p2m_mapping


>               }
> -             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;
>       }
> @@ -975,8 +979,14 @@ 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);
> +
> +             ret = set_foreign_p2m_mapping(pages[i], mfn);
> +             if (ret)
> +                     goto out;
> +
> +             if (m2p_override)
> +                     ret = m2p_add_override(mfn, pages[i], kmap_ops ?
> +                                            &kmap_ops[i] : NULL);
>               if (ret)
>                       goto out;
>       }
> @@ -987,15 +997,31 @@ int gnttab_map_refs(struct gnttab_map_grant_ref 
> *map_ops,
>  
>       return ret;
>  }
> +
> +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;
> @@ -1006,17 +1032,27 @@ 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);

this should become a call to restore_foreign_p2m_mapping


>               }
> -             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);
> +             unsigned long mfn = get_phys_to_machine(page_to_pfn(pages[i]));
> +             ret = restore_foreign_p2m_mapping(pages[i], mfn);
> +             if (ret)
> +                     goto out;
> +
> +             if (m2p_override)
> +                     ret = m2p_remove_override(pages[i],
> +                                               kmap_ops ?
> +                                                &kmap_ops[i] : NULL,
> +                                               mfn);
>               if (ret)
>                       goto out;
>       }
> @@ -1027,8 +1063,22 @@ int gnttab_unmap_refs(struct gnttab_unmap_grant_ref 
> *unmap_ops,
>  
>       return ret;
>  }
> +
> +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);
> diff --git a/include/xen/grant_table.h b/include/xen/grant_table.h
> index a5af2a2..7ad033d 100644
> --- a/include/xen/grant_table.h
> +++ b/include/xen/grant_table.h
> @@ -191,11 +191,15 @@ void gnttab_free_auto_xlat_frames(void);
>  #define gnttab_map_vaddr(map) ((void *)(map.host_virt_addr))
>  
>  int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
> -                 struct gnttab_map_grant_ref *kmap_ops,
>                   struct page **pages, unsigned int count);
> +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);
>  int gnttab_unmap_refs(struct gnttab_unmap_grant_ref *unmap_ops,
> -                   struct gnttab_map_grant_ref *kunmap_ops,
>                     struct page **pages, unsigned int count);
> +int gnttab_unmap_refs_userspace(struct gnttab_unmap_grant_ref *unmap_ops,
> +                             struct gnttab_map_grant_ref *kunmap_ops,
> +                             struct page **pages, unsigned int count);
>  
>  /* Perform a batch of grant map/copy operations. Retry every batch slot
>   * for which the hypervisor returns GNTST_eagain. This is typically due
> 

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