[PATCH v2] ext4/xfs: add page refcount helper

[Ralph Campbell]

There are several places where ZONE_DEVICE struct pages assume a reference
count == 1 means the page is idle and free. Instead of open coding this,
add helper functions to hide this detail.

Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Acked-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
Acked-by: Theodore Ts'o <tytso@xxxxxxx> # for fs/ext4/inode.c
---

Changes in v2:
I strongly resisted the idea of extending this patch but after Jan
Kara's comment about there being more places that could be cleaned
up, I felt compelled to make this one tensy wensy change to add
a dax_wakeup_page() to match the dax_wait_page().
I kept the Reviewed/Acked-bys since I don't think this substantially
changes the patch.

 fs/dax.c            |  4 ++--
 fs/ext4/inode.c     |  5 +----
 fs/xfs/xfs_file.c   |  4 +---
 include/linux/dax.h | 15 +++++++++++++++
 mm/memremap.c       |  3 ++-
 5 files changed, 21 insertions(+), 10 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 5b47834f2e1b..85c63f735909 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -358,7 +358,7 @@ static void dax_disassociate_entry(void *entry, struct address_space *mapping,
 	for_each_mapped_pfn(entry, pfn) {
 		struct page *page = pfn_to_page(pfn);
 
-		WARN_ON_ONCE(trunc && page_ref_count(page) > 1);
+		WARN_ON_ONCE(trunc && !dax_layout_is_idle_page(page));
 		WARN_ON_ONCE(page->mapping && page->mapping != mapping);
 		page->mapping = NULL;
 		page->index = 0;
@@ -372,7 +372,7 @@ static struct page *dax_busy_page(void *entry)
 	for_each_mapped_pfn(entry, pfn) {
 		struct page *page = pfn_to_page(pfn);
 
-		if (page_ref_count(page) > 1)
+		if (!dax_layout_is_idle_page(page))
 			return page;
 	}
 	return NULL;
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 771ed8b1fadb..132620cbfa13 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3937,10 +3937,7 @@ int ext4_break_layouts(struct inode *inode)
 		if (!page)
 			return 0;
 
-		error = ___wait_var_event(&page->_refcount,
-				atomic_read(&page->_refcount) == 1,
-				TASK_INTERRUPTIBLE, 0, 0,
-				ext4_wait_dax_page(ei));
+		error = dax_wait_page(ei, page, ext4_wait_dax_page);
 	} while (error == 0);
 
 	return error;
diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index 3d1b95124744..a5304aaeaa3a 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -749,9 +749,7 @@ xfs_break_dax_layouts(
 		return 0;
 
 	*retry = true;
-	return ___wait_var_event(&page->_refcount,
-			atomic_read(&page->_refcount) == 1, TASK_INTERRUPTIBLE,
-			0, 0, xfs_wait_dax_page(inode));
+	return dax_wait_page(inode, page, xfs_wait_dax_page);
 }
 
 int
diff --git a/include/linux/dax.h b/include/linux/dax.h
index b52f084aa643..e2da78e87338 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -243,6 +243,21 @@ static inline bool dax_mapping(struct address_space *mapping)
 	return mapping->host && IS_DAX(mapping->host);
 }
 
+static inline bool dax_layout_is_idle_page(struct page *page)
+{
+	return page_ref_count(page) == 1;
+}
+
+static inline void dax_wakeup_page(struct page *page)
+{
+	wake_up_var(&page->_refcount);
+}
+
+#define dax_wait_page(_inode, _page, _wait_cb)				\
+	___wait_var_event(&(_page)->_refcount,				\
+		dax_layout_is_idle_page(_page),				\
+		TASK_INTERRUPTIBLE, 0, 0, _wait_cb(_inode))
+
 #ifdef CONFIG_DEV_DAX_HMEM_DEVICES
 void hmem_register_device(int target_nid, struct resource *r);
 #else
diff --git a/mm/memremap.c b/mm/memremap.c
index 2bb276680837..504a10ff2edf 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 <linux/dax.h>
 
 static DEFINE_XARRAY(pgmap_array);
 
@@ -508,7 +509,7 @@ void free_devmap_managed_page(struct page *page)
 {
 	/* notify page idle for dax */
 	if (!is_device_private_page(page)) {
-		wake_up_var(&page->_refcount);
+		dax_wakeup_page(page);
 		return;
 	}
 
-- 
2.20.1