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

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Message-ID: <CAAJw_Zs+fM4Tj1CJxW_60opEKHXgdsYn4xxGRewbP0AdcriyNA@mail.gmail.com>
Date:	Wed, 28 Nov 2012 22:24:08 +0800
From:	Jeff Chua <jeff.chua.linux@...il.com>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jens Axboe <axboe@...nel.dk>,
	Lai Jiangshan <laijs@...fujitsu.com>, Jan Kara <jack@...e.cz>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 2/2] block_dev: don't take the write lock if block size
 doesn't change

On Wed, Nov 28, 2012 at 12:01 PM, Mikulas Patocka <mpatocka@...hat.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@...hat.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
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