[PATCH v10 5/9] fsdax: Introduce dax_load_page()

[Shiyang Ruan]

The current dax_lock_page() locks dax entry by obtaining mapping and
index in page.  To support 1-to-N RMAP in NVDIMM, we need a new function
to lock a specific dax entry corresponding to this file's mapping,index.
And output the page corresponding to the specific dax entry for caller
use.

Signed-off-by: Shiyang Ruan <ruansy.fnst@xxxxxxxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
---
 fs/dax.c            | 44 ++++++++++++++++++++++++++++++++++++++++++++
 include/linux/dax.h |  8 ++++++++
 2 files changed, 52 insertions(+)

diff --git a/fs/dax.c b/fs/dax.c
index c8d57080c1aa..964512107c23 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -455,6 +455,50 @@ void dax_unlock_page(struct page *page, dax_entry_t cookie)
 	dax_unlock_entry(&xas, (void *)cookie);
 }
 
+/*
+ * dax_load_page - Load the page corresponding to a (mapping,offset)
+ * @mapping: the file's mapping whose entry we want to load
+ * @index:   the offset within this file
+ * @page:    output the dax page corresponding to this dax entry
+ *
+ * Return: error if it isn't a dax mapping, otherwise 0.
+ */
+int dax_load_page(struct address_space *mapping, pgoff_t index,
+		struct page **page)
+{
+	XA_STATE(xas, &mapping->i_pages, 0);
+	void *entry;
+
+	if (!dax_mapping(mapping))
+		return -EBUSY;
+
+	rcu_read_lock();
+	for (;;) {
+		entry = NULL;
+		xas_lock_irq(&xas);
+		xas_set(&xas, index);
+		entry = xas_load(&xas);
+		if (dax_is_locked(entry)) {
+			rcu_read_unlock();
+			wait_entry_unlocked(&xas, entry);
+			rcu_read_lock();
+			continue;
+		}
+		if (entry &&
+		    !dax_is_zero_entry(entry) && !dax_is_empty_entry(entry)) {
+			/*
+			 * Output the page if the dax entry exists and isn't
+			 * a zero or empty entry.
+			 */
+			*page = pfn_to_page(dax_to_pfn(entry));
+		}
+		xas_unlock_irq(&xas);
+		break;
+	}
+	rcu_read_unlock();
+	return 0;
+}
+
 /*
  * Find page cache entry at given index. If it is a DAX entry, return it
  * with the entry locked. If the page cache doesn't contain an entry at
diff --git a/include/linux/dax.h b/include/linux/dax.h
index 96cfc63b12fd..530ff9733dd9 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -155,6 +155,8 @@ struct page *dax_layout_busy_page(struct address_space *mapping);
 struct page *dax_layout_busy_page_range(struct address_space *mapping, loff_t start, loff_t end);
 dax_entry_t dax_lock_page(struct page *page);
 void dax_unlock_page(struct page *page, dax_entry_t cookie);
+int dax_load_page(struct address_space *mapping,
+		unsigned long index, struct page **page);
 #else
 static inline struct page *dax_layout_busy_page(struct address_space *mapping)
 {
@@ -182,6 +184,12 @@ static inline dax_entry_t dax_lock_page(struct page *page)
 static inline void dax_unlock_page(struct page *page, dax_entry_t cookie)
 {
 }
+
+static inline int dax_load_page(struct address_space *mapping,
+		unsigned long index, struct page **page)
+{
+	return 0;
+}
 #endif
 
 int dax_zero_range(struct inode *inode, loff_t pos, loff_t len, bool *did_zero,
-- 
2.34.1