[PATCH] btrfs: fix build warning due to u64 devided by u32 for 32bit arch

[Qu Wenruo <wqu@suse.com>]

[BUG]
When building the kernel with subpage preparation patches, 32bit arches
will complain about the following linking error:

   ld: fs/btrfs/extent_io.o: in function `release_extent_buffer':
   fs/btrfs/extent_io.c:5340: undefined reference to `__udivdi3'

[CAUSE]
For 32bits, dividing u64 with u32 need to call div_u64(), not directly
call u64 / u32.

[FIX]
Instead of calling the div_u64() macros, here we introduce a helper,
btrfs_sector_shift(), to calculate the sector shift, and we just do bit
shift to avoid executing the expensive division instruction.

The sector_shift may be better cached in btrfs_fs_info, but so far there
are only very limited callers for that, thus the fs_info::sector_shift
can be there for further cleanup.

David, can this patch be folded into the offending commit?
The patch is small enough, and doesn't change btrfs_fs_info.
Thus should be OK to fold.

Fixes: ef57afc454fb ("btrfs: extent_io: make btrfs_fs_info::buffer_radix to take sector size devided values")
Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/ctree.h     |  5 +++++
 fs/btrfs/extent_io.c | 14 +++++++++-----
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 8a83bce3225c..eb282af985f5 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -3489,6 +3489,11 @@ static inline int __btrfs_fs_compat_ro(struct btrfs_fs_info *fs_info, u64 flag)
 	return !!(btrfs_super_compat_ro_flags(disk_super) & flag);
 }
 
+static inline u8 btrfs_sector_shift(struct btrfs_fs_info *fs_info)
+{
+	return ilog2(fs_info->sectorsize);
+}
+
 /* acl.c */
 #ifdef CONFIG_BTRFS_FS_POSIX_ACL
 struct posix_acl *btrfs_get_acl(struct inode *inode, int type);
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 80b35885004a..3452019aef79 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -5129,10 +5129,10 @@ struct extent_buffer *find_extent_buffer(struct btrfs_fs_info *fs_info,
 					 u64 start)
 {
 	struct extent_buffer *eb;
+	u8 sector_shift = btrfs_sector_shift(fs_info);
 
 	rcu_read_lock();
-	eb = radix_tree_lookup(&fs_info->buffer_radix,
-			       start / fs_info->sectorsize);
+	eb = radix_tree_lookup(&fs_info->buffer_radix, start >> sector_shift);
 	if (eb && atomic_inc_not_zero(&eb->refs)) {
 		rcu_read_unlock();
 		/*
@@ -5167,6 +5167,7 @@ struct extent_buffer *alloc_test_extent_buffer(struct btrfs_fs_info *fs_info,
 					u64 start)
 {
 	struct extent_buffer *eb, *exists = NULL;
+	u8 sector_shift = btrfs_sector_shift(fs_info);
 	int ret;
 
 	eb = find_extent_buffer(fs_info, start);
@@ -5184,7 +5185,7 @@ struct extent_buffer *alloc_test_extent_buffer(struct btrfs_fs_info *fs_info,
 	}
 	spin_lock(&fs_info->buffer_lock);
 	ret = radix_tree_insert(&fs_info->buffer_radix,
-				start / fs_info->sectorsize, eb);
+				start >> sector_shift, eb);
 	spin_unlock(&fs_info->buffer_lock);
 	radix_tree_preload_end();
 	if (ret == -EEXIST) {
@@ -5215,6 +5216,7 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
 	struct extent_buffer *exists = NULL;
 	struct page *p;
 	struct address_space *mapping = fs_info->btree_inode->i_mapping;
+	u8 sector_shift = btrfs_sector_shift(fs_info);
 	int uptodate = 1;
 	int ret;
 
@@ -5292,7 +5294,7 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
 
 	spin_lock(&fs_info->buffer_lock);
 	ret = radix_tree_insert(&fs_info->buffer_radix,
-				start / fs_info->sectorsize, eb);
+				start >> sector_shift, eb);
 	spin_unlock(&fs_info->buffer_lock);
 	radix_tree_preload_end();
 	if (ret == -EEXIST) {
@@ -5337,6 +5339,8 @@ static inline void btrfs_release_extent_buffer_rcu(struct rcu_head *head)
 static int release_extent_buffer(struct extent_buffer *eb)
 	__releases(&eb->refs_lock)
 {
+	u8 sector_shift = btrfs_sector_shift(eb->fs_info);
+
 	lockdep_assert_held(&eb->refs_lock);
 
 	WARN_ON(atomic_read(&eb->refs) == 0);
@@ -5348,7 +5352,7 @@ static int release_extent_buffer(struct extent_buffer *eb)
 
 			spin_lock(&fs_info->buffer_lock);
 			radix_tree_delete(&fs_info->buffer_radix,
-					  eb->start / fs_info->sectorsize);
+					  eb->start >> sector_shift);
 			spin_unlock(&fs_info->buffer_lock);
 		} else {
 			spin_unlock(&eb->refs_lock);
-- 
2.29.1