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

Jeff Chua
2012-11-28T14:24:11Z

On Wed, Nov 28, 2012 at 12:01 PM, Mikulas Patocka <mpatocka@redhat.com> wrote:
> 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) {

This patch didn't really make any difference for ext2/3/4 but for
reiserfs it does.

With the synchronize_sched_expedited() patch applied, it didn't make
any difference.

Thanks,
Jeff