mm/page_alloc.c: simplify pageblock bitmap access

Due to commit e58469b ("mm: page_alloc: use word-based accesses for
get/set pageblock bitmaps"), pageblock bitmap is accessed with word-based
access.  This operation could be simplified a little.

Intuitively, if we want to get a bit range [start_idx, end_idx] in a word,
we can do like this:

    mask = (1 << (end_bitidx - start_bitidx + 1)) - 1;
    ret = (word >> start_idx) & mask;

And also if we want to set a bit range [start_idx, end_idx] with flags, we
can do the same by just shift start_bitidx.

By doing so we reduce some instructions for these two helper functions:

                                Before   Patched
    set_pfnblock_flags_mask     209      198(-5%)
    get_pfnblock_flags_mask     101      87(-13%)

Since the syntax is changed a little, we need to check the whole 4-bit
migrate_type instead of part of it.

Signed-off-by: Wei Yang <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Cc: Mel Gorman <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

Author: weiyang-linux

include/linux/pageblock-flags.h

@@ -66,25 +66,17 @@ void set_pfnblock_flags_mask(struct page *page,
 				unsigned long mask);
 
 /* Declarations for getting and setting flags. See mm/page_alloc.c */
-#define get_pageblock_flags_group(page, start_bitidx, end_bitidx) \
-	get_pfnblock_flags_mask(page, page_to_pfn(page),		\
-			end_bitidx,					\
-			(1 << (end_bitidx - start_bitidx + 1)) - 1)
-#define set_pageblock_flags_group(page, flags, start_bitidx, end_bitidx) \
-	set_pfnblock_flags_mask(page, flags, page_to_pfn(page),		\
-			end_bitidx,					\
-			(1 << (end_bitidx - start_bitidx + 1)) - 1)
-
 #ifdef CONFIG_COMPACTION
 #define get_pageblock_skip(page) \
-			get_pageblock_flags_group(page, PB_migrate_skip,     \
-							PB_migrate_skip)
+	get_pfnblock_flags_mask(page, page_to_pfn(page),	\
+			PB_migrate_skip, (1 << (PB_migrate_skip)))
 #define clear_pageblock_skip(page) \
-			set_pageblock_flags_group(page, 0, PB_migrate_skip,  \
-							PB_migrate_skip)
+	set_pfnblock_flags_mask(page, 0, page_to_pfn(page),	\
+			PB_migrate_skip, (1 << PB_migrate_skip))
 #define set_pageblock_skip(page) \
-			set_pageblock_flags_group(page, 1, PB_migrate_skip,  \
-							PB_migrate_skip)
+	set_pfnblock_flags_mask(page, (1 << PB_migrate_skip),	\
+			page_to_pfn(page),			\
+			PB_migrate_skip, (1 << PB_migrate_skip))
 #else
 static inline bool get_pageblock_skip(struct page *page)
 {

mm/page_alloc.c

@@ -489,8 +489,7 @@ static __always_inline unsigned long __get_pfnblock_flags_mask(struct page *page
 	bitidx &= (BITS_PER_LONG-1);
 
 	word = bitmap[word_bitidx];
-	bitidx += end_bitidx;
-	return (word >> (BITS_PER_LONG - bitidx - 1)) & mask;
+	return (word >> bitidx) & mask;
 }
 
 unsigned long get_pfnblock_flags_mask(struct page *page, unsigned long pfn,
@@ -532,9 +531,8 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
 
 	VM_BUG_ON_PAGE(!zone_spans_pfn(page_zone(page), pfn), page);
 
-	bitidx += end_bitidx;
-	mask <<= (BITS_PER_LONG - bitidx - 1);
-	flags <<= (BITS_PER_LONG - bitidx - 1);
+	mask <<= bitidx;
+	flags <<= bitidx;
 
 	word = READ_ONCE(bitmap[word_bitidx]);
 	for (;;) {
@@ -551,8 +549,9 @@ void set_pageblock_migratetype(struct page *page, int migratetype)
 		     migratetype < MIGRATE_PCPTYPES))
 		migratetype = MIGRATE_UNMOVABLE;
 
-	set_pageblock_flags_group(page, (unsigned long)migratetype,
-					PB_migrate, PB_migrate_end);
+	set_pfnblock_flags_mask(page, (unsigned long)migratetype,
+				page_to_pfn(page), PB_migrate_end,
+				MIGRATETYPE_MASK);
 }
 
 #ifdef CONFIG_DEBUG_VM