[PATCH] ext4/xfs: add page refcount helper

[Ralph Campbell <rcampbell@nvidia.com>]

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 a helper function to hide this detail.

Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
---

I'm resending this as a separate patch since I think it is ready to
merge. Originally, this was part of an RFC and is unchanged from v3:
https://lore.kernel.org/linux-mm/20201001181715.17416-1-rcampbell@nvidia.com

It applies cleanly to linux-5.9.0-rc7-mm1 but doesn't really
depend on anything, just simple merge conflicts when applied to
other trees.
I'll let the various maintainers decide which tree and when to merge.
It isn't urgent since it is a clean up patch.

 fs/dax.c            |  4 ++--
 fs/ext4/inode.c     |  5 +----
 fs/xfs/xfs_file.c   |  4 +---
 include/linux/dax.h | 10 ++++++++++
 4 files changed, 14 insertions(+), 9 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..8909a91cd381 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -243,6 +243,16 @@ 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;
+}
+
+#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
-- 
2.20.1