[PATCH] mm: remove extra ZONE_DEVICE struct page refcount

[Ralph Campbell <rcampbell@nvidia.com>]

ZONE_DEVICE struct pages have an extra reference count that complicates the
code for put_page() and several places in the kernel that need to check the
reference count to see that a page is not being used (gup, compaction,
migration, etc.). Clean up the code so the reference count doesn't need to
be treated specially for ZONE_DEVICE.

Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
---

Matthew Wilcox, Ira Weiny, and others have complained that ZONE_DEVICE
struct page reference counting is ugly/broken. This is my attempt to
fix it and it works for the HMM migration self tests.

I'm only sending this out as a RFC since I'm not that familiar with the
DAX, PMEM, XEN, and other uses of ZONE_DEVICE struct pages allocated
with devm_memremap_pages() or memremap_pages() but my best reading of
the code looks like it might be OK. I could use help testing these
configurations.

I have a modified THP migration patch series that applies on top of
this one and is cleaner since I don't have to add code to handle the
+1 reference count. The link below is for the earlier v2:
("mm/hmm/nouveau: add THP migration to migrate_vma_*")
https://lore.kernel.org/linux-mm/20200902165830.5367-1-rcampbell@nvidia.com


 arch/powerpc/kvm/book3s_hv_uvmem.c     |  1 -
 drivers/gpu/drm/nouveau/nouveau_dmem.c |  1 -
 include/linux/memremap.h               |  6 +--
 include/linux/mm.h                     | 39 ---------------
 lib/test_hmm.c                         |  1 -
 mm/gup.c                               | 44 -----------------
 mm/memremap.c                          | 20 ++++----
 mm/migrate.c                           |  5 --
 mm/swap.c                              | 66 +++++++++++---------------
 9 files changed, 41 insertions(+), 142 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c b/arch/powerpc/kvm/book3s_hv_uvmem.c
index 84e5a2dc8be5..00d97050d7ff 100644
--- a/arch/powerpc/kvm/book3s_hv_uvmem.c
+++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
@@ -711,7 +711,6 @@ static struct page *kvmppc_uvmem_get_page(unsigned long gpa, struct kvm *kvm)
 
 	dpage = pfn_to_page(uvmem_pfn);
 	dpage->zone_device_data = pvt;
-	get_page(dpage);
 	lock_page(dpage);
 	return dpage;
 out_clear:
diff --git a/drivers/gpu/drm/nouveau/nouveau_dmem.c b/drivers/gpu/drm/nouveau/nouveau_dmem.c
index a13c6215bba8..2a4bbe01a455 100644
--- a/drivers/gpu/drm/nouveau/nouveau_dmem.c
+++ b/drivers/gpu/drm/nouveau/nouveau_dmem.c
@@ -324,7 +324,6 @@ nouveau_dmem_page_alloc_locked(struct nouveau_drm *drm)
 			return NULL;
 	}
 
-	get_page(page);
 	lock_page(page);
 	return page;
 }
diff --git a/include/linux/memremap.h b/include/linux/memremap.h
index 4e9c738f4b31..7dd9802d2612 100644
--- a/include/linux/memremap.h
+++ b/include/linux/memremap.h
@@ -67,9 +67,9 @@ enum memory_type {
 
 struct dev_pagemap_ops {
 	/*
-	 * Called once the page refcount reaches 1.  (ZONE_DEVICE pages never
-	 * reach 0 refcount unless there is a refcount bug. This allows the
-	 * device driver to implement its own memory management.)
+	 * Called once the page refcount reaches 0. The reference count is
+	 * reset to 1 before calling page_free(). This allows the
+	 * device driver to implement its own memory management.
 	 */
 	void (*page_free)(struct page *page);
 
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);
-
-static inline bool page_is_devmap_managed(struct page *page)
-{
-	if (!static_branch_unlikely(&devmap_managed_key))
-		return false;
-	if (!is_zone_device_page(page))
-		return false;
-	switch (page->pgmap->type) {
-	case MEMORY_DEVICE_PRIVATE:
-	case MEMORY_DEVICE_FS_DAX:
-		return true;
-	default:
-		break;
-	}
-	return false;
-}
-
-void put_devmap_managed_page(struct page *page);
-
-#else /* CONFIG_DEV_PAGEMAP_OPS */
-static inline bool page_is_devmap_managed(struct page *page)
-{
-	return false;
-}
-
-static inline void put_devmap_managed_page(struct page *page)
-{
-}
 #endif /* CONFIG_DEV_PAGEMAP_OPS */
 
 static inline bool is_device_private_page(const struct page *page)
@@ -1169,17 +1141,6 @@ static inline void put_page(struct page *page)
 {
 	page = compound_head(page);
 
-	/*
-	 * For devmap managed pages we need to catch refcount transition from
-	 * 2 to 1, when refcount reach one it means the page is free and we
-	 * need to inform the device driver through callback. See
-	 * include/linux/memremap.h and HMM for details.
-	 */
-	if (page_is_devmap_managed(page)) {
-		put_devmap_managed_page(page);
-		return;
-	}
-
 	if (put_page_testzero(page))
 		__put_page(page);
 }
diff --git a/lib/test_hmm.c b/lib/test_hmm.c
index c8133f50160b..5410f7328577 100644
--- a/lib/test_hmm.c
+++ b/lib/test_hmm.c
@@ -561,7 +561,6 @@ static struct page *dmirror_devmem_alloc_page(struct dmirror_device *mdevice)
 	}
 
 	dpage->zone_device_data = rpage;
-	get_page(dpage);
 	lock_page(dpage);
 	return dpage;
 
diff --git a/mm/gup.c b/mm/gup.c
index b9b1cd8bcd6a..3c54f0c190a3 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -177,41 +177,6 @@ bool __must_check try_grab_page(struct page *page, unsigned int flags)
 	return true;
 }
 
-#ifdef CONFIG_DEV_PAGEMAP_OPS
-static bool __unpin_devmap_managed_user_page(struct page *page)
-{
-	int count, refs = 1;
-
-	if (!page_is_devmap_managed(page))
-		return false;
-
-	if (hpage_pincount_available(page))
-		hpage_pincount_sub(page, 1);
-	else
-		refs = GUP_PIN_COUNTING_BIAS;
-
-	count = page_ref_sub_return(page, refs);
-
-	mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED, 1);
-	/*
-	 * devmap page refcounts are 1-based, rather than 0-based: if
-	 * refcount is 1, then the page is free and the refcount is
-	 * stable because nobody holds a reference on the page.
-	 */
-	if (count == 1)
-		free_devmap_managed_page(page);
-	else if (!count)
-		__put_page(page);
-
-	return true;
-}
-#else
-static bool __unpin_devmap_managed_user_page(struct page *page)
-{
-	return false;
-}
-#endif /* CONFIG_DEV_PAGEMAP_OPS */
-
 /**
  * unpin_user_page() - release a dma-pinned page
  * @page:            pointer to page to be released
@@ -227,15 +192,6 @@ void unpin_user_page(struct page *page)
 
 	page = compound_head(page);
 
-	/*
-	 * For devmap managed pages we need to catch refcount transition from
-	 * GUP_PIN_COUNTING_BIAS to 1, when refcount reach one it means the
-	 * page is free and we need to inform the device driver through
-	 * callback. See include/linux/memremap.h and HMM for details.
-	 */
-	if (__unpin_devmap_managed_user_page(page))
-		return;
-
 	if (hpage_pincount_available(page))
 		hpage_pincount_sub(page, 1);
 	else
diff --git a/mm/memremap.c b/mm/memremap.c
index 9c951bbfee91..f09562789079 100644
--- a/mm/memremap.c
+++ b/mm/memremap.c
@@ -91,13 +91,6 @@ static unsigned long pfn_end(struct dev_pagemap *pgmap, int range_id)
 	return (range->start + range_len(range)) >> PAGE_SHIFT;
 }
 
-static unsigned long pfn_next(unsigned long pfn)
-{
-	if (pfn % 1024 == 0)
-		cond_resched();
-	return pfn + 1;
-}
-
 /*
  * This returns true if the page is reserved by ZONE_DEVICE driver.
  */
@@ -176,13 +169,12 @@ static void pageunmap_range(struct dev_pagemap *pgmap, int range_id)
 
 void memunmap_pages(struct dev_pagemap *pgmap)
 {
-	unsigned long pfn;
 	int i;
 
 	dev_pagemap_kill(pgmap);
 	for (i = 0; i < pgmap->nr_range; i++)
-		for_each_device_pfn(pfn, pgmap, i)
-			put_page(pfn_to_page(pfn));
+		percpu_ref_put_many(pgmap->ref, pfn_end(pgmap, i) -
+						pfn_first(pgmap, i));
 	dev_pagemap_cleanup(pgmap);
 
 	for (i = 0; i < pgmap->nr_range; i++)
@@ -516,6 +508,14 @@ void free_devmap_managed_page(struct page *page)
 
 	mem_cgroup_uncharge(page);
 
+	/*
+	 * ZONE_DEVICE drivers keep a reference to the page while it is on
+	 * the driver's free list so we reset the reference count here.
+	 * This matches the initial reference when the struct pages are
+	 * created by memremap_pages().
+	 */
+	set_page_refcounted(page);
+
 	/*
 	 * When a device_private page is freed, the page->mapping field
 	 * may still contain a (stale) mapping value. For example, the
diff --git a/mm/migrate.c b/mm/migrate.c
index 4f89360d9e77..be1586582b52 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -380,11 +380,6 @@ static int expected_page_refs(struct address_space *mapping, struct page *page)
 {
 	int expected_count = 1;
 
-	/*
-	 * Device private pages have an extra refcount as they are
-	 * ZONE_DEVICE pages.
-	 */
-	expected_count += is_device_private_page(page);
 	if (mapping)
 		expected_count += thp_nr_pages(page) + page_has_private(page);
 
diff --git a/mm/swap.c b/mm/swap.c
index feff680d3de9..8468e72e397f 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -113,10 +113,31 @@ 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_dev_pagemap(page->pgmap);
+		__put_devmap_managed_page(page);
 
 		/*
 		 * The page belongs to the device that created pgmap. Do
@@ -851,27 +872,19 @@ void release_pages(struct page **pages, int nr)
 		if (is_huge_zero_page(page))
 			continue;
 
+		page = compound_head(page);
+		if (!put_page_testzero(page))
+			continue;
+
 		if (is_zone_device_page(page)) {
 			if (locked_pgdat) {
 				spin_unlock_irqrestore(&locked_pgdat->lru_lock,
 						       flags);
 				locked_pgdat = NULL;
 			}
-			/*
-			 * ZONE_DEVICE pages that return 'false' from
-			 * put_devmap_managed_page() do not require special
-			 * processing, and instead, expect a call to
-			 * put_page_testzero().
-			 */
-			if (page_is_devmap_managed(page)) {
-				put_devmap_managed_page(page);
-				continue;
-			}
-		}
-
-		page = compound_head(page);
-		if (!put_page_testzero(page))
+			__put_devmap_managed_page(page);
 			continue;
+		}
 
 		if (PageCompound(page)) {
 			if (locked_pgdat) {
@@ -1149,26 +1162,3 @@ void __init swap_setup(void)
 	 * _really_ don't want to cluster much more
 	 */
 }
-
-#ifdef CONFIG_DEV_PAGEMAP_OPS
-void put_devmap_managed_page(struct page *page)
-{
-	int count;
-
-	if (WARN_ON_ONCE(!page_is_devmap_managed(page)))
-		return;
-
-	count = page_ref_dec_return(page);
-
-	/*
-	 * devmap page refcounts are 1-based, rather than 0-based: if
-	 * refcount is 1, then the page is free and the refcount is
-	 * stable because nobody holds a reference on the page.
-	 */
-	if (count == 1)
-		free_devmap_managed_page(page);
-	else if (!count)
-		__put_page(page);
-}
-EXPORT_SYMBOL(put_devmap_managed_page);
-#endif
-- 
2.20.1