mm/page_alloc: pageblock flags functions clean up

Patch series "Make MIGRATE_ISOLATE a standalone bit", v10.

This patchset moves MIGRATE_ISOLATE to a standalone bit to avoid being
overwritten during pageblock isolation process.  Currently,
MIGRATE_ISOLATE is part of enum migratetype (in include/linux/mmzone.h),
thus, setting a pageblock to MIGRATE_ISOLATE overwrites its original
migratetype.  This causes pageblock migratetype loss during
alloc_contig_range() and memory offline, especially when the process fails
due to a failed pageblock isolation and the code tries to undo the
finished pageblock isolations.

In terms of performance for changing pageblock types, no performance
change is observed:

1. I used perf to collect stats of offlining and onlining all memory
   of a 40GB VM 10 times and see that get_pfnblock_flags_mask() and
   set_pfnblock_flags_mask() take about 0.12% and 0.02% of the whole
   process respectively with and without this patchset across 3 runs.

2. I used perf to collect stats of dd from /dev/random to a 40GB tmpfs
   file and find get_pfnblock_flags_mask() takes about 0.05% of the
   process with and without this patchset across 3 runs.


This patch (of 6):

No functional change is intended.

1. Add __NR_PAGEBLOCK_BITS for the number of pageblock flag bits and use
   roundup_pow_of_two(__NR_PAGEBLOCK_BITS) as NR_PAGEBLOCK_BITS to take
   right amount of bits for pageblock flags.
2. Rename PB_migrate_skip to PB_compact_skip.
3. Add {get,set,clear}_pfnblock_bit() to operate one a standalone bit,
   like PB_compact_skip.
3. Make {get,set}_pfnblock_flags_mask() internal functions and use
   {get,set}_pfnblock_migratetype() for pageblock migratetype operations.
4. Move pageblock flags common code to get_pfnblock_bitmap_bitidx().
3. Use MIGRATETYPE_MASK to get the migratetype of a pageblock from its
   flags.
4. Use PB_migrate_end in the definition of MIGRATETYPE_MASK instead of
   PB_migrate_bits.
5. Add a comment on is_migrate_cma_folio() to prevent one from changing it
   to use get_pageblock_migratetype() and causing issues.

Link: https://lkml.kernel.org/r/20250617021115.2331563-1-ziy@nvidia.com
Link: https://lkml.kernel.org/r/20250617021115.2331563-2-ziy@nvidia.com
Signed-off-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kirill A. Shuemov <kirill.shutemov@linux.intel.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Richard Chang <richardycc@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/mm/physical_memory.rst

@@ -584,7 +584,7 @@ Compaction control
 
 ``compact_blockskip_flush``
   Set to true when compaction migration scanner and free scanner meet, which
-  means the ``PB_migrate_skip`` bits should be cleared.
+  means the ``PB_compact_skip`` bits should be cleared.
 
 ``contiguous``
   Set to true when the zone is contiguous (in other words, no hole).

include/linux/mmzone.h

@@ -92,8 +92,12 @@ extern const char * const migratetype_names[MIGRATE_TYPES];
 #ifdef CONFIG_CMA
 #  define is_migrate_cma(migratetype) unlikely((migratetype) == MIGRATE_CMA)
 #  define is_migrate_cma_page(_page) (get_pageblock_migratetype(_page) == MIGRATE_CMA)
-#  define is_migrate_cma_folio(folio, pfn)	(MIGRATE_CMA ==		\
-	get_pfnblock_flags_mask(&folio->page, pfn, MIGRATETYPE_MASK))
+/*
+ * __dump_folio() in mm/debug.c passes a folio pointer to on-stack struct folio,
+ * so folio_pfn() cannot be used and pfn is needed.
+ */
+#  define is_migrate_cma_folio(folio, pfn) \
+	(get_pfnblock_migratetype(&folio->page, pfn) == MIGRATE_CMA)
 #else
 #  define is_migrate_cma(migratetype) false
 #  define is_migrate_cma_page(_page) false
@@ -122,14 +126,12 @@ static inline bool migratetype_is_mergeable(int mt)
 
 extern int page_group_by_mobility_disabled;
 
-#define MIGRATETYPE_MASK ((1UL << PB_migratetype_bits) - 1)
-
 #define get_pageblock_migratetype(page) \
-	get_pfnblock_flags_mask(page, page_to_pfn(page), MIGRATETYPE_MASK)
+	get_pfnblock_migratetype(page, page_to_pfn(page))
 
 #define folio_migratetype(folio) \
-	get_pfnblock_flags_mask(&folio->page, folio_pfn(folio),		\
-			MIGRATETYPE_MASK)
+	get_pageblock_migratetype(&folio->page)
+
 struct free_area {
 	struct list_head	free_list[MIGRATE_TYPES];
 	unsigned long		nr_free;

include/linux/page-isolation.h

@@ -25,7 +25,7 @@ static inline bool is_migrate_isolate(int migratetype)
 #define MEMORY_OFFLINE	0x1
 #define REPORT_FAILURE	0x2
 
-void set_pageblock_migratetype(struct page *page, int migratetype);
+void set_pageblock_migratetype(struct page *page, enum migratetype migratetype);
 
 bool move_freepages_block_isolate(struct zone *zone, struct page *page,
 				  int migratetype);

include/linux/pageblock-flags.h

@@ -19,15 +19,19 @@ enum pageblock_bits {
 	PB_migrate,
 	PB_migrate_end = PB_migrate + PB_migratetype_bits - 1,
 			/* 3 bits required for migrate types */
-	PB_migrate_skip,/* If set the block is skipped by compaction */
+	PB_compact_skip,/* If set the block is skipped by compaction */
 
 	/*
 	 * Assume the bits will always align on a word. If this assumption
 	 * changes then get/set pageblock needs updating.
 	 */
-	NR_PAGEBLOCK_BITS
+	__NR_PAGEBLOCK_BITS
 };
 
+#define NR_PAGEBLOCK_BITS (roundup_pow_of_two(__NR_PAGEBLOCK_BITS))
+
+#define MIGRATETYPE_MASK ((1UL << (PB_migrate_end + 1)) - 1)
+
 #if defined(CONFIG_HUGETLB_PAGE)
 
 #ifdef CONFIG_HUGETLB_PAGE_SIZE_VARIABLE
@@ -65,27 +69,23 @@ extern unsigned int pageblock_order;
 /* Forward declaration */
 struct page;
 
-unsigned long get_pfnblock_flags_mask(const struct page *page,
-				unsigned long pfn,
-				unsigned long mask);
-
-void set_pfnblock_flags_mask(struct page *page,
-				unsigned long flags,
-				unsigned long pfn,
-				unsigned long mask);
+enum migratetype get_pfnblock_migratetype(const struct page *page,
+					  unsigned long pfn);
+bool get_pfnblock_bit(const struct page *page, unsigned long pfn,
+		      enum pageblock_bits pb_bit);
+void set_pfnblock_bit(const struct page *page, unsigned long pfn,
+		      enum pageblock_bits pb_bit);
+void clear_pfnblock_bit(const struct page *page, unsigned long pfn,
+			enum pageblock_bits pb_bit);
 
 /* Declarations for getting and setting flags. See mm/page_alloc.c */
 #ifdef CONFIG_COMPACTION
 #define get_pageblock_skip(page) \
-	get_pfnblock_flags_mask(page, page_to_pfn(page),	\
-			(1 << (PB_migrate_skip)))
+	get_pfnblock_bit(page, page_to_pfn(page), PB_compact_skip)
 #define clear_pageblock_skip(page) \
-	set_pfnblock_flags_mask(page, 0, page_to_pfn(page),	\
-			(1 << PB_migrate_skip))
+	clear_pfnblock_bit(page, page_to_pfn(page), PB_compact_skip)
 #define set_pageblock_skip(page) \
-	set_pfnblock_flags_mask(page, (1 << PB_migrate_skip),	\
-			page_to_pfn(page),			\
-			(1 << PB_migrate_skip))
+	set_pfnblock_bit(page, page_to_pfn(page), PB_compact_skip)
 #else
 static inline bool get_pageblock_skip(struct page *page)
 {

mm/memory_hotplug.c

@@ -774,7 +774,7 @@ void move_pfn_range_to_zone(struct zone *zone, unsigned long start_pfn,
 
 	/*
 	 * TODO now we have a visible range of pages which are not associated
-	 * with their zone properly. Not nice but set_pfnblock_flags_mask
+	 * with their zone properly. Not nice but set_pfnblock_migratetype()
 	 * expects the zone spans the pfn range. All the pages in the range
 	 * are reserved so nobody should be touching them so we should be safe
 	 */

mm/page_alloc.c

@@ -353,81 +353,174 @@ static inline int pfn_to_bitidx(const struct page *page, unsigned long pfn)
 	return (pfn >> pageblock_order) * NR_PAGEBLOCK_BITS;
 }
 
+static __always_inline bool is_standalone_pb_bit(enum pageblock_bits pb_bit)
+{
+	return pb_bit > PB_migrate_end && pb_bit < __NR_PAGEBLOCK_BITS;
+}
+
+static __always_inline void
+get_pfnblock_bitmap_bitidx(const struct page *page, unsigned long pfn,
+			   unsigned long **bitmap_word, unsigned long *bitidx)
+{
+	unsigned long *bitmap;
+	unsigned long word_bitidx;
+
+	BUILD_BUG_ON(NR_PAGEBLOCK_BITS != 4);
+	BUILD_BUG_ON(MIGRATE_TYPES > (1 << PB_migratetype_bits));
+	VM_BUG_ON_PAGE(!zone_spans_pfn(page_zone(page), pfn), page);
+
+	bitmap = get_pageblock_bitmap(page, pfn);
+	*bitidx = pfn_to_bitidx(page, pfn);
+	word_bitidx = *bitidx / BITS_PER_LONG;
+	*bitidx &= (BITS_PER_LONG - 1);
+	*bitmap_word = &bitmap[word_bitidx];
+}
+
+
 /**
- * get_pfnblock_flags_mask - Return the requested group of flags for the pageblock_nr_pages block of pages
+ * __get_pfnblock_flags_mask - Return the requested group of flags for
+ * a pageblock_nr_pages block of pages
  * @page: The page within the block of interest
  * @pfn: The target page frame number
  * @mask: mask of bits that the caller is interested in
  *
  * Return: pageblock_bits flags
  */
-unsigned long get_pfnblock_flags_mask(const struct page *page,
-					unsigned long pfn, unsigned long mask)
-{
-	unsigned long *bitmap;
-	unsigned long bitidx, word_bitidx;
-	unsigned long word;
-
-	bitmap = get_pageblock_bitmap(page, pfn);
-	bitidx = pfn_to_bitidx(page, pfn);
-	word_bitidx = bitidx / BITS_PER_LONG;
-	bitidx &= (BITS_PER_LONG-1);
-	/*
-	 * This races, without locks, with set_pfnblock_flags_mask(). Ensure
-	 * a consistent read of the memory array, so that results, even though
-	 * racy, are not corrupted.
-	 */
-	word = READ_ONCE(bitmap[word_bitidx]);
-	return (word >> bitidx) & mask;
-}
-
-static __always_inline int get_pfnblock_migratetype(const struct page *page,
-					unsigned long pfn)
-{
-	return get_pfnblock_flags_mask(page, pfn, MIGRATETYPE_MASK);
-}
-
-/**
- * set_pfnblock_flags_mask - Set the requested group of flags for a pageblock_nr_pages block of pages
- * @page: The page within the block of interest
- * @flags: The flags to set
- * @pfn: The target page frame number
- * @mask: mask of bits that the caller is interested in
- */
-void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
+static unsigned long __get_pfnblock_flags_mask(const struct page *page,
 					       unsigned long pfn,
 					       unsigned long mask)
 {
-	unsigned long *bitmap;
-	unsigned long bitidx, word_bitidx;
+	unsigned long *bitmap_word;
+	unsigned long bitidx;
 	unsigned long word;
 
-	BUILD_BUG_ON(NR_PAGEBLOCK_BITS != 4);
-	BUILD_BUG_ON(MIGRATE_TYPES > (1 << PB_migratetype_bits));
+	get_pfnblock_bitmap_bitidx(page, pfn, &bitmap_word, &bitidx);
+	/*
+	 * This races, without locks, with set_pfnblock_migratetype(). Ensure
+	 * a consistent read of the memory array, so that results, even though
+	 * racy, are not corrupted.
+	 */
+	word = READ_ONCE(*bitmap_word);
+	return (word >> bitidx) & mask;
+}
 
-	bitmap = get_pageblock_bitmap(page, pfn);
-	bitidx = pfn_to_bitidx(page, pfn);
-	word_bitidx = bitidx / BITS_PER_LONG;
-	bitidx &= (BITS_PER_LONG-1);
+/**
+ * get_pfnblock_bit - Check if a standalone bit of a pageblock is set
+ * @page: The page within the block of interest
+ * @pfn: The target page frame number
+ * @pb_bit: pageblock bit to check
+ *
+ * Return: true if the bit is set, otherwise false
+ */
+bool get_pfnblock_bit(const struct page *page, unsigned long pfn,
+		      enum pageblock_bits pb_bit)
+{
+	unsigned long *bitmap_word;
+	unsigned long bitidx;
 
-	VM_BUG_ON_PAGE(!zone_spans_pfn(page_zone(page), pfn), page);
+	if (WARN_ON_ONCE(!is_standalone_pb_bit(pb_bit)))
+		return false;
+
+	get_pfnblock_bitmap_bitidx(page, pfn, &bitmap_word, &bitidx);
+
+	return test_bit(bitidx + pb_bit, bitmap_word);
+}
+
+/**
+ * get_pfnblock_migratetype - Return the migratetype of a pageblock
+ * @page: The page within the block of interest
+ * @pfn: The target page frame number
+ *
+ * Return: The migratetype of the pageblock
+ *
+ * Use get_pfnblock_migratetype() if caller already has both @page and @pfn
+ * to save a call to page_to_pfn().
+ */
+__always_inline enum migratetype
+get_pfnblock_migratetype(const struct page *page, unsigned long pfn)
+{
+	return __get_pfnblock_flags_mask(page, pfn, MIGRATETYPE_MASK);
+}
+
+/**
+ * __set_pfnblock_flags_mask - Set the requested group of flags for
+ * a pageblock_nr_pages block of pages
+ * @page: The page within the block of interest
+ * @pfn: The target page frame number
+ * @flags: The flags to set
+ * @mask: mask of bits that the caller is interested in
+ */
+static void __set_pfnblock_flags_mask(struct page *page, unsigned long pfn,
+				      unsigned long flags, unsigned long mask)
+{
+	unsigned long *bitmap_word;
+	unsigned long bitidx;
+	unsigned long word;
+
+	get_pfnblock_bitmap_bitidx(page, pfn, &bitmap_word, &bitidx);
 
 	mask <<= bitidx;
 	flags <<= bitidx;
 
-	word = READ_ONCE(bitmap[word_bitidx]);
+	word = READ_ONCE(*bitmap_word);
 	do {
-	} while (!try_cmpxchg(&bitmap[word_bitidx], &word, (word & ~mask) | flags));
+	} while (!try_cmpxchg(bitmap_word, &word, (word & ~mask) | flags));
 }
 
-void set_pageblock_migratetype(struct page *page, int migratetype)
+/**
+ * set_pfnblock_bit - Set a standalone bit of a pageblock
+ * @page: The page within the block of interest
+ * @pfn: The target page frame number
+ * @pb_bit: pageblock bit to set
+ */
+void set_pfnblock_bit(const struct page *page, unsigned long pfn,
+		      enum pageblock_bits pb_bit)
+{
+	unsigned long *bitmap_word;
+	unsigned long bitidx;
+
+	if (WARN_ON_ONCE(!is_standalone_pb_bit(pb_bit)))
+		return;
+
+	get_pfnblock_bitmap_bitidx(page, pfn, &bitmap_word, &bitidx);
+
+	set_bit(bitidx + pb_bit, bitmap_word);
+}
+
+/**
+ * clear_pfnblock_bit - Clear a standalone bit of a pageblock
+ * @page: The page within the block of interest
+ * @pfn: The target page frame number
+ * @pb_bit: pageblock bit to clear
+ */
+void clear_pfnblock_bit(const struct page *page, unsigned long pfn,
+			enum pageblock_bits pb_bit)
+{
+	unsigned long *bitmap_word;
+	unsigned long bitidx;
+
+	if (WARN_ON_ONCE(!is_standalone_pb_bit(pb_bit)))
+		return;
+
+	get_pfnblock_bitmap_bitidx(page, pfn, &bitmap_word, &bitidx);
+
+	clear_bit(bitidx + pb_bit, bitmap_word);
+}
+
+/**
+ * set_pageblock_migratetype - Set the migratetype of a pageblock
+ * @page: The page within the block of interest
+ * @migratetype: migratetype to set
+ */
+__always_inline void set_pageblock_migratetype(struct page *page,
+					       enum migratetype migratetype)
 {
 	if (unlikely(page_group_by_mobility_disabled &&
 		     migratetype < MIGRATE_PCPTYPES))
 		migratetype = MIGRATE_UNMOVABLE;
 
-	set_pfnblock_flags_mask(page, (unsigned long)migratetype,
-				page_to_pfn(page), MIGRATETYPE_MASK);
+	__set_pfnblock_flags_mask(page, page_to_pfn(page),
+				  (unsigned long)migratetype, MIGRATETYPE_MASK);
 }
 
 #ifdef CONFIG_DEBUG_VM
@@ -667,7 +760,7 @@ static inline void __add_to_free_list(struct page *page, struct zone *zone,
 	int nr_pages = 1 << order;
 
 	VM_WARN_ONCE(get_pageblock_migratetype(page) != migratetype,
-		     "page type is %lu, passed migratetype is %d (nr=%d)\n",
+		     "page type is %d, passed migratetype is %d (nr=%d)\n",
 		     get_pageblock_migratetype(page), migratetype, nr_pages);
 
 	if (tail)
@@ -693,7 +786,7 @@ static inline void move_to_free_list(struct page *page, struct zone *zone,
 
 	/* Free page moving can fail, so it happens before the type update */
 	VM_WARN_ONCE(get_pageblock_migratetype(page) != old_mt,
-		     "page type is %lu, passed migratetype is %d (nr=%d)\n",
+		     "page type is %d, passed migratetype is %d (nr=%d)\n",
 		     get_pageblock_migratetype(page), old_mt, nr_pages);
 
 	list_move_tail(&page->buddy_list, &area->free_list[new_mt]);
@@ -715,7 +808,7 @@ static inline void __del_page_from_free_list(struct page *page, struct zone *zon
 	int nr_pages = 1 << order;
 
         VM_WARN_ONCE(get_pageblock_migratetype(page) != migratetype,
-		     "page type is %lu, passed migratetype is %d (nr=%d)\n",
+		     "page type is %d, passed migratetype is %d (nr=%d)\n",
 		     get_pageblock_migratetype(page), migratetype, nr_pages);
 
 	/* clear reported state and update reported page count */
@@ -3123,7 +3216,7 @@ static struct page *rmqueue_pcplist(struct zone *preferred_zone,
 
 /*
  * Do not instrument rmqueue() with KMSAN. This function may call
- * __msan_poison_alloca() through a call to set_pfnblock_flags_mask().
+ * __msan_poison_alloca() through a call to set_pfnblock_migratetype().
  * If __msan_poison_alloca() attempts to allocate pages for the stack depot, it
  * may call rmqueue() again, which will result in a deadlock.
  */