[PATCH v8 8/8] fsdax: add exception for reflinked files

[Shiyang Ruan]

For reflinked files, one dax page may be associated more than once with
different fime mapping and index.  It will report warning.  Now, since
we have introduced dax-RMAP for this case and also have to keep its
functionality for other filesystems who are not support rmap, we add a
is cow flag, which is set true when it is CoW operation, for reflink
case.  Normal cases work as before.

We will get the cow flag in the dax-reflink(v10) patchset.  So, always
set false here for now.

Signed-off-by: Shiyang Ruan <ruansy.fnst@xxxxxxxxxxx>
---
 fs/dax.c | 39 +++++++++++++++++++++++++++------------
 1 file changed, 27 insertions(+), 12 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 7e46569c6129..bc269852d91a 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -340,7 +340,7 @@ static unsigned long dax_end_pfn(void *entry)
  * offsets.
  */
 static void dax_associate_entry(void *entry, struct address_space *mapping,
-		struct vm_area_struct *vma, unsigned long address)
+		struct vm_area_struct *vma, unsigned long address, bool cow)
 {
 	unsigned long size = dax_entry_size(entry), pfn, index;
 	int i = 0;
@@ -352,14 +352,21 @@ static void dax_associate_entry(void *entry, struct address_space *mapping,
 	for_each_mapped_pfn(entry, pfn) {
 		struct page *page = pfn_to_page(pfn);
 
-		WARN_ON_ONCE(page->mapping);
-		page->mapping = mapping;
-		page->index = index + i++;
+		if (cow) {
+			if (!page->mapping) {
+				page->mapping = mapping;
+				page->index = index + i++;
+			}
+		} else {
+			WARN_ON_ONCE(page->mapping);
+			page->mapping = mapping;
+			page->index = index + i++;
+		}
 	}
 }
 
 static void dax_disassociate_entry(void *entry, struct address_space *mapping,
-		bool trunc)
+		bool trunc, bool cow)
 {
 	unsigned long pfn;
 
@@ -370,9 +377,16 @@ static void dax_disassociate_entry(void *entry, struct address_space *mapping,
 		struct page *page = pfn_to_page(pfn);
 
 		WARN_ON_ONCE(trunc && page_ref_count(page) > 1);
-		WARN_ON_ONCE(page->mapping && page->mapping != mapping);
-		page->mapping = NULL;
-		page->index = 0;
+		if (cow) {
+			if (page->mapping == mapping) {
+				page->mapping = NULL;
+				page->index = 0;
+			}
+		} else {
+			WARN_ON_ONCE(page->mapping && page->mapping != mapping);
+			page->mapping = NULL;
+			page->index = 0;
+		}
 	}
 }
 
@@ -597,7 +611,7 @@ static void *grab_mapping_entry(struct xa_state *xas,
 			xas_lock_irq(xas);
 		}
 
-		dax_disassociate_entry(entry, mapping, false);
+		dax_disassociate_entry(entry, mapping, false, false);
 		xas_store(xas, NULL);	/* undo the PMD join */
 		dax_wake_entry(xas, entry, WAKE_ALL);
 		mapping->nrpages -= PG_PMD_NR;
@@ -734,7 +748,7 @@ static int __dax_invalidate_entry(struct address_space *mapping,
 	    (xas_get_mark(&xas, PAGECACHE_TAG_DIRTY) ||
 	     xas_get_mark(&xas, PAGECACHE_TAG_TOWRITE)))
 		goto out;
-	dax_disassociate_entry(entry, mapping, trunc);
+	dax_disassociate_entry(entry, mapping, trunc, false);
 	xas_store(&xas, NULL);
 	mapping->nrpages -= 1UL << dax_entry_order(entry);
 	ret = 1;
@@ -827,8 +841,9 @@ static void *dax_insert_entry(struct xa_state *xas,
 	if (dax_is_zero_entry(entry) || dax_is_empty_entry(entry)) {
 		void *old;
 
-		dax_disassociate_entry(entry, mapping, false);
-		dax_associate_entry(new_entry, mapping, vmf->vma, vmf->address);
+		dax_disassociate_entry(entry, mapping, false, false);
+		dax_associate_entry(new_entry, mapping, vmf->vma, vmf->address,
+				false);
 		/*
 		 * Only swap our new entry into the page cache if the current
 		 * entry is a zero page or an empty entry.  If a normal PTE or
-- 
2.33.0