[RFC PATCH v1 06/57] mm: Remove PAGE_SIZE compile-time constant assumption

[Ryan Roberts <ryan.roberts@arm.com>]

To prepare for supporting boot-time page size selection, refactor code
to remove assumptions about PAGE_SIZE being compile-time constant. Code
intended to be equivalent when compile-time page size is active.

Refactor "struct vmap_block" to use a flexible array for used_mmap since
VMAP_BBMAP_BITS is not a compile time constant for the boot-time page
size case.

Update various BUILD_BUG_ON() instances to check against appropriate
page size limit.

Re-define "union swap_header" so that it's no longer exactly page-sized.
Instead define a flexible "magic" array with a define which tells the
offset to where the magic signature begins.

Consider page size limit in some CPP condditionals.

Wrap global variables that are initialized with PAGE_SIZE derived values
using DEFINE_GLOBAL_PAGE_SIZE_VAR() so their initialization can be
deferred for boot-time page size builds.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---

***NOTE***
Any confused maintainers may want to read the cover note here for context:
https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@arm.com/

 drivers/mtd/mtdswap.c         |  4 ++--
 include/linux/mm.h            |  2 +-
 include/linux/mm_types_task.h |  2 +-
 include/linux/mmzone.h        |  3 ++-
 include/linux/slab.h          |  7 ++++---
 include/linux/swap.h          | 17 ++++++++++++-----
 include/linux/swapops.h       |  6 +++++-
 mm/memcontrol.c               |  2 +-
 mm/memory.c                   |  4 ++--
 mm/mmap.c                     |  2 +-
 mm/page-writeback.c           |  2 +-
 mm/slub.c                     |  2 +-
 mm/sparse.c                   |  2 +-
 mm/swapfile.c                 |  2 +-
 mm/vmalloc.c                  |  7 ++++---
 15 files changed, 39 insertions(+), 25 deletions(-)

diff --git a/drivers/mtd/mtdswap.c b/drivers/mtd/mtdswap.c
index 680366616da24..7412a32708114 100644
--- a/drivers/mtd/mtdswap.c
+++ b/drivers/mtd/mtdswap.c
@@ -1062,13 +1062,13 @@ static int mtdswap_auto_header(struct mtdswap_dev *d, char *buf)
 {
 	union swap_header *hd = (union swap_header *)(buf);
 
-	memset(buf, 0, PAGE_SIZE - 10);
+	memset(buf, 0, SWAP_HEADER_MAGIC);
 
 	hd->info.version = 1;
 	hd->info.last_page = d->mbd_dev->size - 1;
 	hd->info.nr_badpages = 0;
 
-	memcpy(buf + PAGE_SIZE - 10, "SWAPSPACE2", 10);
+	memcpy(buf + SWAP_HEADER_MAGIC, "SWAPSPACE2", 10);
 
 	return 0;
 }
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 09a840517c23a..49c2078354e6e 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2927,7 +2927,7 @@ static inline spinlock_t *pte_lockptr(struct mm_struct *mm, pmd_t *pmd)
 static inline spinlock_t *ptep_lockptr(struct mm_struct *mm, pte_t *pte)
 {
 	BUILD_BUG_ON(IS_ENABLED(CONFIG_HIGHPTE));
-	BUILD_BUG_ON(MAX_PTRS_PER_PTE * sizeof(pte_t) > PAGE_SIZE);
+	BUILD_BUG_ON(MAX_PTRS_PER_PTE * sizeof(pte_t) > PAGE_SIZE_MAX);
 	return ptlock_ptr(virt_to_ptdesc(pte));
 }
 
diff --git a/include/linux/mm_types_task.h b/include/linux/mm_types_task.h
index a2f6179b672b8..c356897d5f41c 100644
--- a/include/linux/mm_types_task.h
+++ b/include/linux/mm_types_task.h
@@ -37,7 +37,7 @@ struct page;
 
 struct page_frag {
 	struct page *page;
-#if (BITS_PER_LONG > 32) || (PAGE_SIZE >= 65536)
+#if (BITS_PER_LONG > 32) || (PAGE_SIZE_MAX >= 65536)
 	__u32 offset;
 	__u32 size;
 #else
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 1dc6248feb832..cd58034b82c81 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1744,6 +1744,7 @@ static inline bool movable_only_nodes(nodemask_t *nodes)
  */
 #define PA_SECTION_SHIFT	(SECTION_SIZE_BITS)
 #define PFN_SECTION_SHIFT	(SECTION_SIZE_BITS - PAGE_SHIFT)
+#define PFN_SECTION_SHIFT_MIN	(SECTION_SIZE_BITS - PAGE_SHIFT_MAX)
 
 #define NR_MEM_SECTIONS		(1UL << SECTIONS_SHIFT)
 
@@ -1753,7 +1754,7 @@ static inline bool movable_only_nodes(nodemask_t *nodes)
 #define SECTION_BLOCKFLAGS_BITS \
 	((1UL << (PFN_SECTION_SHIFT - pageblock_order)) * NR_PAGEBLOCK_BITS)
 
-#if (MAX_PAGE_ORDER + PAGE_SHIFT) > SECTION_SIZE_BITS
+#if (MAX_PAGE_ORDER + PAGE_SHIFT_MAX) > SECTION_SIZE_BITS
 #error Allocator MAX_PAGE_ORDER exceeds SECTION_SIZE
 #endif
 
diff --git a/include/linux/slab.h b/include/linux/slab.h
index eb2bf46291576..11c6ff3a12579 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -347,7 +347,7 @@ static inline unsigned int arch_slab_minalign(void)
  */
 #define __assume_kmalloc_alignment __assume_aligned(ARCH_KMALLOC_MINALIGN)
 #define __assume_slab_alignment __assume_aligned(ARCH_SLAB_MINALIGN)
-#define __assume_page_alignment __assume_aligned(PAGE_SIZE)
+#define __assume_page_alignment __assume_aligned(PAGE_SIZE_MIN)
 
 /*
  * Kmalloc array related definitions
@@ -358,6 +358,7 @@ static inline unsigned int arch_slab_minalign(void)
  * (PAGE_SIZE*2).  Larger requests are passed to the page allocator.
  */
 #define KMALLOC_SHIFT_HIGH	(PAGE_SHIFT + 1)
+#define KMALLOC_SHIFT_HIGH_MAX	(PAGE_SHIFT_MAX + 1)
 #define KMALLOC_SHIFT_MAX	(MAX_PAGE_ORDER + PAGE_SHIFT)
 #ifndef KMALLOC_SHIFT_LOW
 #define KMALLOC_SHIFT_LOW	3
@@ -426,7 +427,7 @@ enum kmalloc_cache_type {
 	NR_KMALLOC_TYPES
 };
 
-typedef struct kmem_cache * kmem_buckets[KMALLOC_SHIFT_HIGH + 1];
+typedef struct kmem_cache * kmem_buckets[KMALLOC_SHIFT_HIGH_MAX + 1];
 
 extern kmem_buckets kmalloc_caches[NR_KMALLOC_TYPES];
 
@@ -524,7 +525,7 @@ static __always_inline unsigned int __kmalloc_index(size_t size,
 	/* Will never be reached. Needed because the compiler may complain */
 	return -1;
 }
-static_assert(PAGE_SHIFT <= 20);
+static_assert(PAGE_SHIFT_MAX <= 20);
 #define kmalloc_index(s) __kmalloc_index(s, true)
 
 #include <linux/alloc_tag.h>
diff --git a/include/linux/swap.h b/include/linux/swap.h
index ba7ea95d1c57a..e85df0332979f 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -132,10 +132,17 @@ static inline int current_is_kswapd(void)
  * bootbits...
  */
 union swap_header {
-	struct {
-		char reserved[PAGE_SIZE - 10];
-		char magic[10];			/* SWAP-SPACE or SWAPSPACE2 */
-	} magic;
+	/*
+	 * Exists conceptually, but since PAGE_SIZE may not be known at compile
+	 * time, we must access through pointer arithmetic at run time.
+	 *
+	 * struct {
+	 * 	char reserved[PAGE_SIZE - 10];
+	 * 	char magic[10];			   SWAP-SPACE or SWAPSPACE2
+	 * } magic;
+	 */
+#define SWAP_HEADER_MAGIC	(PAGE_SIZE - 10)
+	char magic[1];
 	struct {
 		char		bootbits[1024];	/* Space for disklabel etc. */
 		__u32		version;
@@ -201,7 +208,7 @@ struct swap_extent {
  * Max bad pages in the new format..
  */
 #define MAX_SWAP_BADPAGES \
-	((offsetof(union swap_header, magic.magic) - \
+	((SWAP_HEADER_MAGIC - \
 	  offsetof(union swap_header, info.badpages)) / sizeof(int))
 
 enum {
diff --git a/include/linux/swapops.h b/include/linux/swapops.h
index cb468e418ea11..890fe6a3e6702 100644
--- a/include/linux/swapops.h
+++ b/include/linux/swapops.h
@@ -34,10 +34,14 @@
  */
 #ifdef MAX_PHYSMEM_BITS
 #define SWP_PFN_BITS		(MAX_PHYSMEM_BITS - PAGE_SHIFT)
+#define SWP_PFN_BITS_MAX	(MAX_PHYSMEM_BITS - PAGE_SHIFT_MIN)
 #else  /* MAX_PHYSMEM_BITS */
 #define SWP_PFN_BITS		min_t(int, \
 				      sizeof(phys_addr_t) * 8 - PAGE_SHIFT, \
 				      SWP_TYPE_SHIFT)
+#define SWP_PFN_BITS_MAX	min_t(int, \
+				      sizeof(phys_addr_t) * 8 - PAGE_SHIFT_MIN, \
+				      SWP_TYPE_SHIFT)
 #endif	/* MAX_PHYSMEM_BITS */
 #define SWP_PFN_MASK		(BIT(SWP_PFN_BITS) - 1)
 
@@ -519,7 +523,7 @@ static inline struct folio *pfn_swap_entry_folio(swp_entry_t entry)
 static inline bool is_pfn_swap_entry(swp_entry_t entry)
 {
 	/* Make sure the swp offset can always store the needed fields */
-	BUILD_BUG_ON(SWP_TYPE_SHIFT < SWP_PFN_BITS);
+	BUILD_BUG_ON(SWP_TYPE_SHIFT < SWP_PFN_BITS_MAX);
 
 	return is_migration_entry(entry) || is_device_private_entry(entry) ||
 	       is_device_exclusive_entry(entry) || is_hwpoison_entry(entry);
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c5f9195f76c65..4b17bec566fbd 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4881,7 +4881,7 @@ static int __init mem_cgroup_init(void)
 	 * to work fine, we should make sure that the overfill threshold can't
 	 * exceed S32_MAX / PAGE_SIZE.
 	 */
-	BUILD_BUG_ON(MEMCG_CHARGE_BATCH > S32_MAX / PAGE_SIZE);
+	BUILD_BUG_ON(MEMCG_CHARGE_BATCH > S32_MAX / PAGE_SIZE_MIN);
 
 	cpuhp_setup_state_nocalls(CPUHP_MM_MEMCQ_DEAD, "mm/memctrl:dead", NULL,
 				  memcg_hotplug_cpu_dead);
diff --git a/mm/memory.c b/mm/memory.c
index ebfc9768f801a..14b5ef6870486 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4949,8 +4949,8 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
 	return ret;
 }
 
-static unsigned long fault_around_pages __read_mostly =
-	65536 >> PAGE_SHIFT;
+static __DEFINE_GLOBAL_PAGE_SIZE_VAR(unsigned long, fault_around_pages,
+				     __read_mostly, 65536 >> PAGE_SHIFT);
 
 #ifdef CONFIG_DEBUG_FS
 static int fault_around_bytes_get(void *data, u64 *val)
diff --git a/mm/mmap.c b/mm/mmap.c
index d0dfc85b209bb..d9642aba07ac4 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2279,7 +2279,7 @@ int expand_downwards(struct vm_area_struct *vma, unsigned long address)
 }
 
 /* enforced gap between the expanding stack and other mappings. */
-unsigned long stack_guard_gap = 256UL<<PAGE_SHIFT;
+DEFINE_GLOBAL_PAGE_SIZE_VAR(unsigned long, stack_guard_gap, 256UL<<PAGE_SHIFT);
 
 static int __init cmdline_parse_stack_guard_gap(char *p)
 {
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 4430ac68e4c41..8fc9ac50749bd 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2292,7 +2292,7 @@ static int page_writeback_cpu_online(unsigned int cpu)
 #ifdef CONFIG_SYSCTL
 
 /* this is needed for the proc_doulongvec_minmax of vm_dirty_bytes */
-static const unsigned long dirty_bytes_min = 2 * PAGE_SIZE;
+static DEFINE_GLOBAL_PAGE_SIZE_VAR_CONST(unsigned long, dirty_bytes_min, 2 * PAGE_SIZE);
 
 static struct ctl_table vm_page_writeback_sysctls[] = {
 	{
diff --git a/mm/slub.c b/mm/slub.c
index a77f354f83251..82f6e98cf25bb 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -5001,7 +5001,7 @@ init_kmem_cache_node(struct kmem_cache_node *n)
 static inline int alloc_kmem_cache_cpus(struct kmem_cache *s)
 {
 	BUILD_BUG_ON(PERCPU_DYNAMIC_EARLY_SIZE <
-			NR_KMALLOC_TYPES * KMALLOC_SHIFT_HIGH *
+			NR_KMALLOC_TYPES * KMALLOC_SHIFT_HIGH_MAX *
 			sizeof(struct kmem_cache_cpu));
 
 	/*
diff --git a/mm/sparse.c b/mm/sparse.c
index dc38539f85603..2491425930c4d 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -277,7 +277,7 @@ static unsigned long sparse_encode_mem_map(struct page *mem_map, unsigned long p
 {
 	unsigned long coded_mem_map =
 		(unsigned long)(mem_map - (section_nr_to_pfn(pnum)));
-	BUILD_BUG_ON(SECTION_MAP_LAST_BIT > PFN_SECTION_SHIFT);
+	BUILD_BUG_ON(SECTION_MAP_LAST_BIT > PFN_SECTION_SHIFT_MIN);
 	BUG_ON(coded_mem_map & ~SECTION_MAP_MASK);
 	return coded_mem_map;
 }
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 38bdc439651ac..6311a1cc7e46b 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -2931,7 +2931,7 @@ static unsigned long read_swap_header(struct swap_info_struct *p,
 	unsigned long swapfilepages;
 	unsigned long last_page;
 
-	if (memcmp("SWAPSPACE2", swap_header->magic.magic, 10)) {
+	if (memcmp("SWAPSPACE2", &swap_header->magic[SWAP_HEADER_MAGIC], 10)) {
 		pr_err("Unable to find swap-space signature\n");
 		return 0;
 	}
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index a0df1e2e155a8..b4fbba204603c 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2497,12 +2497,12 @@ struct vmap_block {
 	spinlock_t lock;
 	struct vmap_area *va;
 	unsigned long free, dirty;
-	DECLARE_BITMAP(used_map, VMAP_BBMAP_BITS);
 	unsigned long dirty_min, dirty_max; /*< dirty range */
 	struct list_head free_list;
 	struct rcu_head rcu_head;
 	struct list_head purge;
 	unsigned int cpu;
+	unsigned long used_map[];
 };
 
 /* Queue of free and dirty vmap blocks, for allocation and flushing purposes */
@@ -2600,11 +2600,12 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask)
 	unsigned long vb_idx;
 	int node, err;
 	void *vaddr;
+	size_t size;
 
 	node = numa_node_id();
 
-	vb = kmalloc_node(sizeof(struct vmap_block),
-			gfp_mask & GFP_RECLAIM_MASK, node);
+	size = struct_size(vb, used_map, BITS_TO_LONGS(VMAP_BBMAP_BITS));
+	vb = kmalloc_node(size, gfp_mask & GFP_RECLAIM_MASK, node);
 	if (unlikely(!vb))
 		return ERR_PTR(-ENOMEM);
 
-- 
2.43.0