[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@xxxxxxx>
---

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

 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