[PATCH v2 07/28] gup: Remove an assumption of a contiguous memmap

[Matthew Wilcox (Oracle)]

This assumption needs the inverse of nth_page(), which I've temporarily
named page_nth() until someone comes up with a better name.

Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
---
 include/linux/mm.h | 2 ++
 mm/gup.c           | 4 ++--
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index d8b7d7ed14dd..f2f3400665a4 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -216,8 +216,10 @@ int overcommit_policy_handler(struct ctl_table *, int, void *, size_t *,
 
 #if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
 #define nth_page(page,n) pfn_to_page(page_to_pfn((page)) + (n))
+#define page_nth(head, tail)	(page_to_pfn(tail) - page_to_pfn(head))
 #else
 #define nth_page(page,n) ((page) + (n))
+#define page_nth(head, tail)	((tail) - (head))
 #endif
 
 /* to align the pointer to the (next) page boundary */
diff --git a/mm/gup.c b/mm/gup.c
index 9c0a702a4e03..afb638a30e44 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -238,8 +238,8 @@ static inline struct page *compound_range_next(unsigned long i,
 	next = nth_page(start, i);
 	page = compound_head(next);
 	if (PageHead(page))
-		nr = min_t(unsigned int,
-			   page + compound_nr(page) - next, npages - i);
+		nr = min_t(unsigned int, npages - i,
+			   compound_nr(page) - page_nth(page, next));
 
 	*ntails = nr;
 	return page;
-- 
2.33.0