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Date:   Wed, 16 Sep 2020 08:09:21 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Ralph Campbell <rcampbell@nvidia.com>
Cc:     linux-mm@kvack.org, kvm-ppc@vger.kernel.org,
        nouveau@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        Dan Williams <dan.j.williams@intel.com>,
        Ira Weiny <ira.weiny@intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Jerome Glisse <jglisse@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Alistair Popple <apopple@nvidia.com>,
        Christoph Hellwig <hch@lst.de>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Bharata B Rao <bharata@linux.ibm.com>,
        Zi Yan <ziy@nvidia.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        Paul Mackerras <paulus@ozlabs.org>,
        Ben Skeggs <bskeggs@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] mm: remove extra ZONE_DEVICE struct page refcount
Message-ID: <20200916060921.GB7321@lst.de>
References: <20200914224509.17699-1-rcampbell@nvidia.com>
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> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 517751310dd2..5a82037a4b26 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1093,34 +1093,6 @@ static inline bool is_zone_device_page(const struct page *page)
>  #ifdef CONFIG_DEV_PAGEMAP_OPS
>  void free_devmap_managed_page(struct page *page);
>  DECLARE_STATIC_KEY_FALSE(devmap_managed_key);

The export for devmap_managed_key can be dropped now.  In fact I think
we can remove devmap_managed_key entirely now - it is only checked in
the actual page free path instead of for each refcount manipulation,
so a good old unlikely is probably enough.

Also free_devmap_managed_page can move to mm/internal.h.

> +#ifdef CONFIG_DEV_PAGEMAP_OPS
> +static void __put_devmap_managed_page(struct page *page)
> +{
> +	if (!static_branch_unlikely(&devmap_managed_key))
> +		return;
> +
> +	switch (page->pgmap->type) {
> +	case MEMORY_DEVICE_PRIVATE:
> +	case MEMORY_DEVICE_FS_DAX:
> +		free_devmap_managed_page(page);
> +		break;
> +	default:
> +		break;
> +	}
> +}
> +#else
> +static inline void __put_devmap_managed_page(struct page *page)
> +{
> +}
> +#endif

I think this should be moved to mm/memremap.c or even better
actually be folded into free_devmap_managed_page, which would need
a new name like free_zone_device_page().

Something like this incremental patch:


diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7bb9e93cf86cde..29350dc27cd0cd 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1090,11 +1090,6 @@ static inline bool is_zone_device_page(const struct page *page)
 }
 #endif
 
-#ifdef CONFIG_DEV_PAGEMAP_OPS
-void free_devmap_managed_page(struct page *page);
-DECLARE_STATIC_KEY_FALSE(devmap_managed_key);
-#endif /* CONFIG_DEV_PAGEMAP_OPS */
-
 static inline bool is_device_private_page(const struct page *page)
 {
 	return IS_ENABLED(CONFIG_DEV_PAGEMAP_OPS) &&
diff --git a/mm/internal.h b/mm/internal.h
index 6345b08ce86ccf..629959a6f26d7c 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -618,4 +618,12 @@ struct migration_target_control {
 	gfp_t gfp_mask;
 };
 
+#ifdef CONFIG_DEV_PAGEMAP_OPS
+void free_zone_device_page(struct page *page);
+#else
+static inline void free_zone_device_page(struct page *page)
+{
+}
+#endif
+
 #endif	/* __MM_INTERNAL_H */
diff --git a/mm/memremap.c b/mm/memremap.c
index d549e3733f4098..b15ad2264a4f1c 100644
--- a/mm/memremap.c
+++ b/mm/memremap.c
@@ -12,6 +12,7 @@
 #include <linux/types.h>
 #include <linux/wait_bit.h>
 #include <linux/xarray.h>
+#include "internal.h"
 
 static DEFINE_XARRAY(pgmap_array);
 
@@ -37,36 +38,6 @@ unsigned long memremap_compat_align(void)
 EXPORT_SYMBOL_GPL(memremap_compat_align);
 #endif
 
-#ifdef CONFIG_DEV_PAGEMAP_OPS
-DEFINE_STATIC_KEY_FALSE(devmap_managed_key);
-EXPORT_SYMBOL(devmap_managed_key);
-
-static void devmap_managed_enable_put(void)
-{
-	static_branch_dec(&devmap_managed_key);
-}
-
-static int devmap_managed_enable_get(struct dev_pagemap *pgmap)
-{
-	if (pgmap->type == MEMORY_DEVICE_PRIVATE &&
-	    (!pgmap->ops || !pgmap->ops->page_free)) {
-		WARN(1, "Missing page_free method\n");
-		return -EINVAL;
-	}
-
-	static_branch_inc(&devmap_managed_key);
-	return 0;
-}
-#else
-static int devmap_managed_enable_get(struct dev_pagemap *pgmap)
-{
-	return -EINVAL;
-}
-static void devmap_managed_enable_put(void)
-{
-}
-#endif /* CONFIG_DEV_PAGEMAP_OPS */
-
 static void pgmap_array_delete(struct range *range)
 {
 	xa_store_range(&pgmap_array, PHYS_PFN(range->start), PHYS_PFN(range->end),
@@ -181,7 +152,6 @@ void memunmap_pages(struct dev_pagemap *pgmap)
 		pageunmap_range(pgmap, i);
 
 	WARN_ONCE(pgmap->altmap.alloc, "failed to free all reserved pages\n");
-	devmap_managed_enable_put();
 }
 EXPORT_SYMBOL_GPL(memunmap_pages);
 
@@ -319,7 +289,6 @@ void *memremap_pages(struct dev_pagemap *pgmap, int nid)
 		.pgprot = PAGE_KERNEL,
 	};
 	const int nr_range = pgmap->nr_range;
-	bool need_devmap_managed = true;
 	int error, i;
 
 	if (WARN_ONCE(!nr_range, "nr_range must be specified\n"))
@@ -331,8 +300,9 @@ void *memremap_pages(struct dev_pagemap *pgmap, int nid)
 			WARN(1, "Device private memory not supported\n");
 			return ERR_PTR(-EINVAL);
 		}
-		if (!pgmap->ops || !pgmap->ops->migrate_to_ram) {
-			WARN(1, "Missing migrate_to_ram method\n");
+		if (!pgmap->ops ||
+		    !pgmap->ops->migrate_to_ram || !pgmap->ops->page_free) {
+			WARN(1, "Missing ops\n");
 			return ERR_PTR(-EINVAL);
 		}
 		if (!pgmap->owner) {
@@ -348,11 +318,9 @@ void *memremap_pages(struct dev_pagemap *pgmap, int nid)
 		}
 		break;
 	case MEMORY_DEVICE_GENERIC:
-		need_devmap_managed = false;
 		break;
 	case MEMORY_DEVICE_PCI_P2PDMA:
 		params.pgprot = pgprot_noncached(params.pgprot);
-		need_devmap_managed = false;
 		break;
 	default:
 		WARN(1, "Invalid pgmap type %d\n", pgmap->type);
@@ -376,12 +344,6 @@ void *memremap_pages(struct dev_pagemap *pgmap, int nid)
 		}
 	}
 
-	if (need_devmap_managed) {
-		error = devmap_managed_enable_get(pgmap);
-		if (error)
-			return ERR_PTR(error);
-	}
-
 	/*
 	 * Clear the pgmap nr_range as it will be incremented for each
 	 * successfully processed range. This communicates how many
@@ -496,16 +458,9 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
 EXPORT_SYMBOL_GPL(get_dev_pagemap);
 
 #ifdef CONFIG_DEV_PAGEMAP_OPS
-void free_devmap_managed_page(struct page *page)
+static void free_device_private_page(struct page *page)
 {
-	/* notify page idle for dax */
-	if (!is_device_private_page(page)) {
-		wake_up_var(&page->_refcount);
-		return;
-	}
-
 	__ClearPageWaiters(page);
-
 	mem_cgroup_uncharge(page);
 
 	/*
@@ -540,4 +495,19 @@ void free_devmap_managed_page(struct page *page)
 	page->mapping = NULL;
 	page->pgmap->ops->page_free(page);
 }
+
+void free_zone_device_page(struct page *page)
+{
+	switch (page->pgmap->type) {
+	case MEMORY_DEVICE_FS_DAX:
+		/* notify page idle */
+		wake_up_var(&page->_refcount);
+		return;
+	case MEMORY_DEVICE_PRIVATE:
+		free_device_private_page(page);
+		return;
+	default:
+		return;
+	}
+}
 #endif /* CONFIG_DEV_PAGEMAP_OPS */
diff --git a/mm/swap.c b/mm/swap.c
index bcab5db351184a..83451ac70d0f05 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -113,36 +113,14 @@ static void __put_compound_page(struct page *page)
 	destroy_compound_page(page);
 }
 
-#ifdef CONFIG_DEV_PAGEMAP_OPS
-static void __put_devmap_managed_page(struct page *page)
-{
-	if (!static_branch_unlikely(&devmap_managed_key))
-		return;
-
-	switch (page->pgmap->type) {
-	case MEMORY_DEVICE_PRIVATE:
-	case MEMORY_DEVICE_FS_DAX:
-		free_devmap_managed_page(page);
-		break;
-	default:
-		break;
-	}
-}
-#else
-static inline void __put_devmap_managed_page(struct page *page)
-{
-}
-#endif
-
 void __put_page(struct page *page)
 {
 	if (is_zone_device_page(page)) {
-		__put_devmap_managed_page(page);
-
 		/*
 		 * The page belongs to the device that created pgmap. Do
 		 * not return it to page allocator.
 		 */
+		free_zone_device_page(page);
 		return;
 	}
 
@@ -923,7 +901,7 @@ void release_pages(struct page **pages, int nr)
 						       flags);
 				locked_pgdat = NULL;
 			}
-			__put_devmap_managed_page(page);
+			free_zone_device_page(page);
 			return;
 		}