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Subject[RFC][PATCH] introduce ptr_diff()
Date
When I compiled allyesconfig'ed kernel with C=1, I got 1519 lines of
following message:

include/linux/mm.h:599:16: warning: potentially expensive pointer subtraction

which is around 10% of total warnings. this was caused by page_to_pfn() macro
so I think it's worth to remove it by calculating pointer subtraction
manually.

ptr_diff() does subtraction of two pointers as if they were integer values
and divides it by the size of their base type. Currently it does not deal
with a difference alignment requirement than the size of base type, it
would be a trivial job.

Impact: remove a _lot_ of sparse warnings
Signed-off-by: Namhyung Kim <namhyung@gmail.com>
---
I have no idea where the right place is.
Please kindly point me to the place if here is not the one.
And any comments are greatly welcomed also.

Thanks.

include/asm-generic/memory_model.h | 23 +++++++++++++++++++----
1 files changed, 19 insertions(+), 4 deletions(-)
diff --git a/include/asm-generic/memory_model.h b/include/asm-generic/memory_model.h
index fb2d63f..ffec625 100644
--- a/include/asm-generic/memory_model.h
+++ b/include/asm-generic/memory_model.h
@@ -22,13 +22,28 @@

#endif /* CONFIG_DISCONTIGMEM */

+#include <linux/log2.h>
+
+#define ptr_diff(p1, p2) \
+({ long __diff; \
+ unsigned long __addr1 = (unsigned long) (p1); \
+ unsigned long __addr2 = (unsigned long) (p2); \
+ unsigned __size = sizeof(typeof(*p1)); \
+ \
+ if (is_power_of_2(__size)) \
+ __diff = (__addr1 - __addr2) >> ilog2(__size); \
+ else \
+ __diff = (__addr1 - __addr2) / __size; \
+ __diff; \
+})
+
/*
* supports 3 memory models.
*/
#if defined(CONFIG_FLATMEM)

#define __pfn_to_page(pfn) (mem_map + ((pfn) - ARCH_PFN_OFFSET))
-#define __page_to_pfn(page) ((unsigned long)((page) - mem_map) + \
+#define __page_to_pfn(page) ((unsigned long)(ptr_diff((page), mem_map)) + \
ARCH_PFN_OFFSET)
#elif defined(CONFIG_DISCONTIGMEM)

@@ -41,7 +56,7 @@
#define __page_to_pfn(pg) \
({ struct page *__pg = (pg); \
struct pglist_data *__pgdat = NODE_DATA(page_to_nid(__pg)); \
- (unsigned long)(__pg - __pgdat->node_mem_map) + \
+ (unsigned long)(ptr_diff(__pg, __pgdat->node_mem_map)) + \
__pgdat->node_start_pfn; \
})

@@ -49,7 +64,7 @@

/* memmap is virtually contiguous. */
#define __pfn_to_page(pfn) (vmemmap + (pfn))
-#define __page_to_pfn(page) (unsigned long)((page) - vmemmap)
+#define __page_to_pfn(page) (unsigned long)(ptr_diff((page), vmemmap))

#elif defined(CONFIG_SPARSEMEM)
/*
@@ -59,7 +74,7 @@
#define __page_to_pfn(pg) \
({ struct page *__pg = (pg); \
int __sec = page_to_section(__pg); \
- (unsigned long)(__pg - __section_mem_map_addr(__nr_to_section(__sec))); \
+ (unsigned long)(ptr_diff(__pg, __section_mem_map_addr(__nr_to_section(__sec)))); \
})

#define __pfn_to_page(pfn) \
--
1.7.0.4


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