[PATCH 2/2] block_dev: don't take the write lock if block size doesn't change

[Mikulas Patocka <mpatocka@redhat.com>]

block_dev: don't take the write lock if block size doesn't change

Taking the write lock has a big performance impact on the whole system
(because of synchronize_sched_expedited). This patch avoids taking the
write lock if the block size doesn't change (i.e. when mounting
filesystem with block size equal to the default block size).

The logic to test if the block device is mapped was moved to a separate
function is_bdev_mapped to avoid code duplication.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

---
 fs/block_dev.c |   25 ++++++++++++++++++-------
 1 file changed, 18 insertions(+), 7 deletions(-)

Index: linux-3.7-rc7/fs/block_dev.c
===================================================================
--- linux-3.7-rc7.orig/fs/block_dev.c	2012-11-28 04:09:01.000000000 +0100
+++ linux-3.7-rc7/fs/block_dev.c	2012-11-28 04:13:53.000000000 +0100
@@ -114,10 +114,18 @@ void invalidate_bdev(struct block_device
 }
 EXPORT_SYMBOL(invalidate_bdev);
 
-int set_blocksize(struct block_device *bdev, int size)
+static int is_bdev_mapped(struct block_device *bdev)
 {
-	struct address_space *mapping;
+	int ret_val;
+	struct address_space *mapping = bdev->bd_inode->i_mapping;
+	mutex_lock(&mapping->i_mmap_mutex);
+	ret_val = mapping_mapped(mapping);
+	mutex_unlock(&mapping->i_mmap_mutex);
+	return ret_val;
+}
 
+int set_blocksize(struct block_device *bdev, int size)
+{
 	/* Size must be a power of two, and between 512 and PAGE_SIZE */
 	if (size > PAGE_SIZE || size < 512 || !is_power_of_2(size))
 		return -EINVAL;
@@ -126,18 +134,21 @@ int set_blocksize(struct block_device *b
 	if (size < bdev_logical_block_size(bdev))
 		return -EINVAL;
 
+	/*
+	 * If the block size doesn't change, don't take the write lock.
+	 * We check for is_bdev_mapped anyway, for consistent behavior.
+	 */
+	if (size == bdev->bd_block_size)
+		return is_bdev_mapped(bdev) ? -EBUSY : 0;
+
 	/* Prevent starting I/O or mapping the device */
 	percpu_down_write(&bdev->bd_block_size_semaphore);
 
 	/* Check that the block device is not memory mapped */
-	mapping = bdev->bd_inode->i_mapping;
-	mutex_lock(&mapping->i_mmap_mutex);
-	if (mapping_mapped(mapping)) {
-		mutex_unlock(&mapping->i_mmap_mutex);
+	if (is_bdev_mapped(bdev)) {
 		percpu_up_write(&bdev->bd_block_size_semaphore);
 		return -EBUSY;
 	}
-	mutex_unlock(&mapping->i_mmap_mutex);
 
 	/* Don't change the size if it is same as current */
 	if (bdev->bd_block_size != size) {