Re: [PATCH v6] xen/grant-table: Avoid m2p_override during mapping

[Stefano Stabellini]

On Thu, 23 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
> 
> 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
> 
> Signed-off-by: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>
> Suggested-by: David Vrabel <david.vrabel@xxxxxxxxxx>

Reviewed-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>



>  arch/x86/include/asm/xen/page.h     |    5 +-
>  arch/x86/xen/p2m.c                  |   17 +------
>  drivers/block/xen-blkback/blkback.c |   15 +++---
>  drivers/xen/gntdev.c                |   13 +++--
>  drivers/xen/grant-table.c           |   89 ++++++++++++++++++++++++++++++-----
>  include/xen/grant_table.h           |    8 +++-
>  6 files changed, 101 insertions(+), 46 deletions(-)
> 
> diff --git a/arch/x86/include/asm/xen/page.h b/arch/x86/include/asm/xen/page.h
> index b913915..ce47243 100644
> --- a/arch/x86/include/asm/xen/page.h
> +++ b/arch/x86/include/asm/xen/page.h
> @@ -52,7 +52,8 @@ extern unsigned long set_phys_range_identity(unsigned long pfn_s,
>  extern int m2p_add_override(unsigned long mfn, struct page *page,
>  			    struct gnttab_map_grant_ref *kmap_op);
>  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 +122,7 @@ 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 2ae8699..bd4724b 100644
> --- a/arch/x86/xen/p2m.c
> +++ b/arch/x86/xen/p2m.c
> @@ -888,13 +888,6 @@ 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)) {
> @@ -933,19 +926,16 @@ int m2p_add_override(unsigned long mfn, struct page *page,
>  }
>  EXPORT_SYMBOL_GPL(m2p_add_override);
>  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);
> @@ -959,10 +949,7 @@ 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 6620b73..875025f 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 e41c79c..e652c0e 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 aa846a4..e4ddfeb 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;
> +	unsigned long mfn, pfn;
>  
> +	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 +931,20 @@ 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);
> +		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)))) {
> +			ret = -ENOMEM;
> +			goto out;
> +		}
> +		if (m2p_override)
> +			ret = m2p_add_override(mfn, pages[i], kmap_ops ?
> +					       &kmap_ops[i] : NULL);
>  		if (ret)
>  			goto out;
>  	}
> @@ -939,15 +955,32 @@ 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;
> +	unsigned long pfn, mfn;
>  
> +	BUG_ON(kmap_ops && !m2p_override);
>  	ret = HYPERVISOR_grant_table_op(GNTTABOP_unmap_grant_ref, unmap_ops, count);
>  	if (ret)
>  		return ret;
> @@ -958,17 +991,33 @@ 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);
> +		pfn = page_to_pfn(pages[i]);
> +		mfn = get_phys_to_machine(pfn);
> +		if (mfn == INVALID_P2M_ENTRY || !(mfn & FOREIGN_FRAME_BIT)) {
> +			ret = -EINVAL;
> +			goto out;
> +		}
> +
> +		set_page_private(pages[i], INVALID_P2M_ENTRY);
> +		WARN_ON(!PagePrivate(pages[i]));
> +		ClearPagePrivate(pages[i]);
> +		set_phys_to_machine(pfn, pages[i]->index);
> +		if (m2p_override)
> +			ret = m2p_remove_override(pages[i],
> +						  kmap_ops ?
> +						   &kmap_ops[i] : NULL,
> +						  mfn);
>  		if (ret)
>  			goto out;
>  	}
> @@ -979,8 +1028,22 @@ 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 694dcaf..9a919b1 100644
> --- a/include/xen/grant_table.h
> +++ b/include/xen/grant_table.h
> @@ -184,11 +184,15 @@ unsigned int gnttab_max_grant_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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