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

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

[Ryan Roberts]

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

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

[Ryan Roberts]

+ Matthew Wilcox

This was a rather tricky series to get the recipients correct for and my script
did not realize that "supporter" was a pseudonym for "maintainer" so you were
missed off the original post. Appologies!

More context in cover letter:
https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@arm.com/


On 14/10/2024 11:58, Ryan Roberts wrote:
> 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);
>  

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

[Vlastimil Babka]

On 10/14/24 12:58, Ryan Roberts wrote:
> 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(-)
> 

> --- 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];

I wonder if it makes sense to even keep this magic field anymore.

>  	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 {

<snip>

> --- 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)) {

I'd expect static checkers to scream here because we overflow the magic[1]
both due to copying 10 bytes into 1 byte array and also with the insane
offset. Hence my suggestion to drop the field and use purely pointer arithmetic.

>  		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

Hm I'm actually looking at yourwip branch which also has:

--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -969,7 +969,7 @@ static inline int get_order_from_str(const char *size_str)
        return -EINVAL;
 }

-static char str_dup[PAGE_SIZE] __initdata;
+static char str_dup[PAGE_SIZE_MAX] __initdata;
 static int __init setup_thp_anon(char *str)
 {
        char *token, *range, *policy, *subtoken;

Why PAGE_SIZE_MAX? Isn't this the same case as "mm/memcontrol: Fix seq_buf
size to save memory when PAGE_SIZE is large"

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

[Ryan Roberts]

Hi Vlastimil,

Sorry about the slow response to your review of this series - I'm just getting
to it now. Comment's below...

On 14/11/2024 10:17, Vlastimil Babka wrote:
> On 10/14/24 12:58, Ryan Roberts wrote:
>> 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(-)
>>
> 
>> --- 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];
> 
> I wonder if it makes sense to even keep this magic field anymore.
> 
>>  	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 {
> 
> <snip>
> 
>> --- 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)) {
> 
> I'd expect static checkers to scream here because we overflow the magic[1]
> both due to copying 10 bytes into 1 byte array and also with the insane
> offset. Hence my suggestion to drop the field and use purely pointer arithmetic.

Yeah, good point. I'll remove magic[] and use pointer arithmetic.

> 
>>  		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
> 
> Hm I'm actually looking at yourwip branch which also has:
> 
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -969,7 +969,7 @@ static inline int get_order_from_str(const char *size_str)
>         return -EINVAL;
>  }
> 
> -static char str_dup[PAGE_SIZE] __initdata;
> +static char str_dup[PAGE_SIZE_MAX] __initdata;
>  static int __init setup_thp_anon(char *str)
>  {
>         char *token, *range, *policy, *subtoken;
> 
> Why PAGE_SIZE_MAX? Isn't this the same case as "mm/memcontrol: Fix seq_buf
> size to save memory when PAGE_SIZE is large"

Hmm, you're probably right. I had a vague notion that "str", as passed into the
function, was guarranteed to be no bigger than PAGE_SIZE (perhaps I'm wrong). So
assumed that's where the original definition of str_dup[PAGE_SIZE] was coming from.

But I think your real question is "should the max size of str be a function of
PAGE_SIZE?". I think it could; there are more page orders that can legitimately
be described when the page size is bigger (at least for arm64). But in practice,
I'd expect any sane string for any page size to be easily within 4K.

So on that basis, I'll take your advice; changing this buffer to be 4K always.

Thanks,
Ryan

[RFC PATCH v1 07/57] fs: Introduce MAX_BUF_PER_PAGE_SIZE_MAX for array sizing

[Ryan Roberts]

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.

Code that previously defined arrays with MAX_BUF_PER_PAGE will no longer
work with boot-time page selection because PAGE_SIZE is not known at
compile-time. Introduce MAX_BUF_PER_PAGE_SIZE_MAX for this purpose,
which is the requirement in the limit when PAGE_SIZE_MAX is the selected
page size.

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/

 fs/buffer.c                 | 2 +-
 fs/ext4/move_extent.c       | 2 +-
 fs/ext4/readpage.c          | 2 +-
 fs/fat/dir.c                | 4 ++--
 fs/fat/fatent.c             | 4 ++--
 include/linux/buffer_head.h | 1 +
 6 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index e55ad471c5306..f00542ad43a5c 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2371,7 +2371,7 @@ int block_read_full_folio(struct folio *folio, get_block_t *get_block)
 {
 	struct inode *inode = folio->mapping->host;
 	sector_t iblock, lblock;
-	struct buffer_head *bh, *head, *arr[MAX_BUF_PER_PAGE];
+	struct buffer_head *bh, *head, *arr[MAX_BUF_PER_PAGE_SIZE_MAX];
 	size_t blocksize;
 	int nr, i;
 	int fully_mapped = 1;
diff --git a/fs/ext4/move_extent.c b/fs/ext4/move_extent.c
index 204f53b236229..68304426c6f45 100644
--- a/fs/ext4/move_extent.c
+++ b/fs/ext4/move_extent.c
@@ -172,7 +172,7 @@ mext_page_mkuptodate(struct folio *folio, unsigned from, unsigned to)
 {
 	struct inode *inode = folio->mapping->host;
 	sector_t block;
-	struct buffer_head *bh, *head, *arr[MAX_BUF_PER_PAGE];
+	struct buffer_head *bh, *head, *arr[MAX_BUF_PER_PAGE_SIZE_MAX];
 	unsigned int blocksize, block_start, block_end;
 	int i, err,  nr = 0, partial = 0;
 	BUG_ON(!folio_test_locked(folio));
diff --git a/fs/ext4/readpage.c b/fs/ext4/readpage.c
index 8494492582abe..5808d85096aeb 100644
--- a/fs/ext4/readpage.c
+++ b/fs/ext4/readpage.c
@@ -221,7 +221,7 @@ int ext4_mpage_readpages(struct inode *inode,
 	sector_t block_in_file;
 	sector_t last_block;
 	sector_t last_block_in_file;
-	sector_t blocks[MAX_BUF_PER_PAGE];
+	sector_t blocks[MAX_BUF_PER_PAGE_SIZE_MAX];
 	unsigned page_block;
 	struct block_device *bdev = inode->i_sb->s_bdev;
 	int length;
diff --git a/fs/fat/dir.c b/fs/fat/dir.c
index acbec5bdd5210..f3e96ecf21c92 100644
--- a/fs/fat/dir.c
+++ b/fs/fat/dir.c
@@ -1146,7 +1146,7 @@ int fat_alloc_new_dir(struct inode *dir, struct timespec64 *ts)
 {
 	struct super_block *sb = dir->i_sb;
 	struct msdos_sb_info *sbi = MSDOS_SB(sb);
-	struct buffer_head *bhs[MAX_BUF_PER_PAGE];
+	struct buffer_head *bhs[MAX_BUF_PER_PAGE_SIZE_MAX];
 	struct msdos_dir_entry *de;
 	sector_t blknr;
 	__le16 date, time;
@@ -1213,7 +1213,7 @@ static int fat_add_new_entries(struct inode *dir, void *slots, int nr_slots,
 {
 	struct super_block *sb = dir->i_sb;
 	struct msdos_sb_info *sbi = MSDOS_SB(sb);
-	struct buffer_head *bhs[MAX_BUF_PER_PAGE];
+	struct buffer_head *bhs[MAX_BUF_PER_PAGE_SIZE_MAX];
 	sector_t blknr, start_blknr, last_blknr;
 	unsigned long size, copy;
 	int err, i, n, offset, cluster[2];
diff --git a/fs/fat/fatent.c b/fs/fat/fatent.c
index 1db348f8f887a..322cf5b8e5590 100644
--- a/fs/fat/fatent.c
+++ b/fs/fat/fatent.c
@@ -469,7 +469,7 @@ int fat_alloc_clusters(struct inode *inode, int *cluster, int nr_cluster)
 	struct msdos_sb_info *sbi = MSDOS_SB(sb);
 	const struct fatent_operations *ops = sbi->fatent_ops;
 	struct fat_entry fatent, prev_ent;
-	struct buffer_head *bhs[MAX_BUF_PER_PAGE];
+	struct buffer_head *bhs[MAX_BUF_PER_PAGE_SIZE_MAX];
 	int i, count, err, nr_bhs, idx_clus;
 
 	BUG_ON(nr_cluster > (MAX_BUF_PER_PAGE / 2));	/* fixed limit */
@@ -557,7 +557,7 @@ int fat_free_clusters(struct inode *inode, int cluster)
 	struct msdos_sb_info *sbi = MSDOS_SB(sb);
 	const struct fatent_operations *ops = sbi->fatent_ops;
 	struct fat_entry fatent;
-	struct buffer_head *bhs[MAX_BUF_PER_PAGE];
+	struct buffer_head *bhs[MAX_BUF_PER_PAGE_SIZE_MAX];
 	int i, err, nr_bhs;
 	int first_cl = cluster, dirty_fsinfo = 0;
 
diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
index 14acf1bbe0ce6..5dff4837b76cd 100644
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -41,6 +41,7 @@ enum bh_state_bits {
 };
 
 #define MAX_BUF_PER_PAGE (PAGE_SIZE / 512)
+#define MAX_BUF_PER_PAGE_SIZE_MAX (PAGE_SIZE_MAX / 512)
 
 struct page;
 struct buffer_head;
-- 
2.43.0

[RFC PATCH v1 08/57] fs: Remove PAGE_SIZE compile-time constant assumption

[Ryan Roberts]

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.

In binfmt_elf, convert CPP conditional to C ternary operator; this will
be folded to the same code by the compiler when in compile-time page
size mode, but will also work for runtime evaluation in boot-time page
size mode.

In coredump, modify __dump_skip() to emit zeros in blocks of
PAGE_SIZE_MIN. This resolves to the previous PAGE_SIZE for compile-time
page size, but that doesn't work for boot-time page size. PAGE_SIZE_MIN
is preferred here over PAGE_SIZE_MAX to save memory.

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/

 fs/binfmt_elf.c | 11 ++++-------
 fs/coredump.c   |  8 ++++----
 2 files changed, 8 insertions(+), 11 deletions(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 19fa49cd9907f..e439d36c43c7e 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -84,11 +84,8 @@ static int elf_core_dump(struct coredump_params *cprm);
 #define elf_core_dump	NULL
 #endif
 
-#if ELF_EXEC_PAGESIZE > PAGE_SIZE
-#define ELF_MIN_ALIGN	ELF_EXEC_PAGESIZE
-#else
-#define ELF_MIN_ALIGN	PAGE_SIZE
-#endif
+#define ELF_MIN_ALIGN							\
+	(ELF_EXEC_PAGESIZE > PAGE_SIZE ? ELF_EXEC_PAGESIZE : PAGE_SIZE)
 
 #ifndef ELF_CORE_EFLAGS
 #define ELF_CORE_EFLAGS	0
@@ -98,7 +95,7 @@ static int elf_core_dump(struct coredump_params *cprm);
 #define ELF_PAGEOFFSET(_v) ((_v) & (ELF_MIN_ALIGN-1))
 #define ELF_PAGEALIGN(_v) (((_v) + ELF_MIN_ALIGN - 1) & ~(ELF_MIN_ALIGN - 1))
 
-static struct linux_binfmt elf_format = {
+static DEFINE_GLOBAL_PAGE_SIZE_VAR(struct linux_binfmt, elf_format, {
 	.module		= THIS_MODULE,
 	.load_binary	= load_elf_binary,
 	.load_shlib	= load_elf_library,
@@ -106,7 +103,7 @@ static struct linux_binfmt elf_format = {
 	.core_dump	= elf_core_dump,
 	.min_coredump	= ELF_EXEC_PAGESIZE,
 #endif
-};
+});
 
 #define BAD_ADDR(x) (unlikely((unsigned long)(x) >= TASK_SIZE))
 
diff --git a/fs/coredump.c b/fs/coredump.c
index 7f12ff6ad1d3e..203f2a158246e 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -825,7 +825,7 @@ static int __dump_emit(struct coredump_params *cprm, const void *addr, int nr)
 
 static int __dump_skip(struct coredump_params *cprm, size_t nr)
 {
-	static char zeroes[PAGE_SIZE];
+	static char zeroes[PAGE_SIZE_MIN];
 	struct file *file = cprm->file;
 	if (file->f_mode & FMODE_LSEEK) {
 		if (dump_interrupted() ||
@@ -834,10 +834,10 @@ static int __dump_skip(struct coredump_params *cprm, size_t nr)
 		cprm->pos += nr;
 		return 1;
 	} else {
-		while (nr > PAGE_SIZE) {
-			if (!__dump_emit(cprm, zeroes, PAGE_SIZE))
+		while (nr > PAGE_SIZE_MIN) {
+			if (!__dump_emit(cprm, zeroes, PAGE_SIZE_MIN))
 				return 0;
-			nr -= PAGE_SIZE;
+			nr -= PAGE_SIZE_MIN;
 		}
 		return __dump_emit(cprm, zeroes, nr);
 	}
-- 
2.43.0