Re: [PATCH] mm: fix blkdev size calculation in generic_write_checks

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 15 Aug 2007 17:52:28 +0400 Dmitry Monakhov <dmonakhov@openvz.org> wrote:

> Currently block device size calculated regardless its
> bd_block_size. This may result attempt to write outside
> block device if i_size not aligned to bdev->bd_block_size
> and result in EIO.
> 
> #### TEST_CASE_BEGIN
> # fdisk -l /dev/sdc
> Disk /dev/sdc: 36.7 GB, 36703918080 bytes
> 255 heads, 63 sectors/track, 4462 cylinders
> Units = cylinders of 16065 * 512 = 8225280 bytes
> 
>    Device Boot      Start         End      Blocks   Id  System
> /dev/sdc1   *           1         254     2040223+  83  Ldinux
> /dev/sdc2             255         379     1004062+  83  Linux
> ####
> #### /dev/sdc2 size not aligned to 4K
> 
> #### at this time bd_block_size == 512 so generic_write_check
> #### performed correctly
> # dd if=/dev/zero of=/dev/sdc2 bs=1k count=7 seek=1004058
> dd: writing `/dev/sdc2': No space left on device
> 5+0 records in
> 4+0 records out
> 
> #### this bdev contain ext4fs with blksize = 4K
> # mount /dev/sdc2 /mnt/
> #### after we mounted this bdev bd_block_size == fsblksize == 4K
> 
> #### the same write operation failed with EIO
> # dd if=/dev/zero of=/dev/sdc2 bs=1k count=7 seek=1004058
> dd: writing `/dev/sdc2': Input/output error
> 3+0 records in
> 2+0 records out
> #### Attempt to write whole fsblock result write access outside
> #### blkdevice and cause -EIO (returned by blkdev_get_block)
> #### TEST_CASE_END
> 
> Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
> ---
>  mm/filemap.c |    4 +++-
>  1 files changed, 3 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 2c8776b..a23ee8a 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1867,9 +1867,11 @@ inline int generic_write_checks(struct file *file, loff_t *pos, size_t *count, i
>  	} else {
>  #ifdef CONFIG_BLOCK
>  		loff_t isize;
> +		unsigned int blksize;
>  		if (bdev_read_only(I_BDEV(inode)))
>  			return -EPERM;
> -		isize = i_size_read(inode);
> +		blksize = block_size(I_BDEV(inode));
> +		isize = i_size_read(inode) & ~(blksize - 1);
>  		if (*pos >= isize) {
>  			if (*count || *pos > isize)
>  				return -ENOSPC;

Can't say I really like the idea of adding additional overhead in this
hotpath for such an odd case.  Is there a faster way of doing it?  Maybe
adjust i_size, perhaps when the blocksize gets changed?