[RFC 4/8] shmem: add helpers to get block size

[Luis Chamberlain <mcgrof@kernel.org>]

Stuff the block size as a struct shmem_sb_info member when CONFIG_TMPFS
is enabled, but keep the current static value for now, and use helpers
to get the blocksize. This will make the subsequent change easier to read.

The static value for block size of PAGE_SIZE is used currently.

The struct super_block s_blocksize_bits represents the blocksize in
power of two, since the block size is always PAGE_SIZE this is PAGE_SHIFT
today, but to help make this a bit more apt to scale we can use __ffs()
for it instead.

This commit introduces no functional changes other than __ffs() for the
s_blocksize_bits and extending the struct shmem_sb_info with the blocksize.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
---
 include/linux/shmem_fs.h |  3 +++
 mm/shmem.c               | 24 +++++++++++++++++++++---
 2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index 9029abd29b1c..89e471fcde1d 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -36,6 +36,9 @@ struct shmem_inode_info {
 #define SHMEM_FL_INHERITED		(FS_NODUMP_FL | FS_NOATIME_FL)
 
 struct shmem_sb_info {
+#ifdef CONFIG_TMPFS
+	u64 blocksize;
+#endif
 	unsigned long max_blocks;   /* How many blocks are allowed */
 	struct percpu_counter used_blocks;  /* How many are allocated */
 	unsigned long max_inodes;   /* How many inodes are allowed */
diff --git a/mm/shmem.c b/mm/shmem.c
index d76e86ff356e..162384b58a5c 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -125,7 +125,17 @@ struct shmem_options {
 #define SHMEM_SEEN_NOSWAP 16
 };
 
+static u64 shmem_default_bsize(void)
+{
+	return PAGE_SIZE;
+}
+
 #ifdef CONFIG_TMPFS
+static u64 shmem_sb_blocksize(struct shmem_sb_info *sbinfo)
+{
+	return sbinfo->blocksize;
+}
+
 static unsigned long shmem_default_max_blocks(void)
 {
 	return totalram_pages() / 2;
@@ -137,6 +147,12 @@ static unsigned long shmem_default_max_inodes(void)
 
 	return min(nr_pages - totalhigh_pages(), nr_pages / 2);
 }
+#else
+static u64 shmem_sb_blocksize(struct shmem_sb_info *sbinfo)
+{
+	return shmem_default_bsize();
+}
+
 #endif
 
 static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
@@ -3190,7 +3206,7 @@ static int shmem_statfs(struct dentry *dentry, struct kstatfs *buf)
 	struct shmem_sb_info *sbinfo = SHMEM_SB(dentry->d_sb);
 
 	buf->f_type = TMPFS_MAGIC;
-	buf->f_bsize = PAGE_SIZE;
+	buf->f_bsize = shmem_sb_blocksize(sbinfo);
 	buf->f_namelen = NAME_MAX;
 	if (sbinfo->max_blocks) {
 		buf->f_blocks = sbinfo->max_blocks;
@@ -4100,6 +4116,7 @@ static int shmem_fill_super(struct super_block *sb, struct fs_context *fc)
 	}
 	sb->s_export_op = &shmem_export_ops;
 	sb->s_flags |= SB_NOSEC | SB_I_VERSION;
+	sbinfo->blocksize = shmem_default_bsize();
 #else
 	sb->s_flags |= SB_NOUSER;
 #endif
@@ -4125,8 +4142,9 @@ static int shmem_fill_super(struct super_block *sb, struct fs_context *fc)
 	INIT_LIST_HEAD(&sbinfo->shrinklist);
 
 	sb->s_maxbytes = MAX_LFS_FILESIZE;
-	sb->s_blocksize = PAGE_SIZE;
-	sb->s_blocksize_bits = PAGE_SHIFT;
+	sb->s_blocksize = shmem_sb_blocksize(sbinfo);
+	sb->s_blocksize_bits = __ffs(sb->s_blocksize);
+	WARN_ON_ONCE(sb->s_blocksize_bits != PAGE_SHIFT);
 	sb->s_magic = TMPFS_MAGIC;
 	sb->s_op = &shmem_ops;
 	sb->s_time_gran = 1;
-- 
2.39.2