Re: [PATCH] xfs: remove use of do_div with 32-bit dividend in quota debug code

Darrick J. Wong
2017-04-17T19:00:10Z

On Sat, Apr 15, 2017 at 09:46:09AM -0500, Eric Sandeen wrote:
> The kbuild test robot caught this; in debug code we have another
> caller of do_div with a 32-bit dividend (j) which is caught now
> that we are using the kernel-supplied do_div.
>
> None of the values used here are 64-bit; just use simple division.
>
> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
> ---
>
> diff --git a/fs/xfs/xfs_qm.c b/fs/xfs/xfs_qm.c
> index b669b12..dedeb7f 100644
> --- a/fs/xfs/xfs_qm.c
> +++ b/fs/xfs/xfs_qm.c
> @@ -851,8 +851,7 @@ struct xfs_qm_isolate {
> * started afresh by xfs_qm_quotacheck.
> */
> #ifdef DEBUG
> - j = XFS_FSB_TO_B(mp, XFS_DQUOT_CLUSTER_SIZE_FSB);
> - do_div(j, sizeof(xfs_dqblk_t));
> + j = XFS_FSB_TO_B(mp, XFS_DQUOT_CLUSTER_SIZE_FSB) / sizeof(xfs_dqblk_t);

Uh...

#define XFS_FSB_TO_B(mp,fsbno) ((xfs_fsize_t)(fsbno) << (mp)->m_sb.sb_blocklog)
typedef __int64_t xfs_fsize_t; /* bytes in a file */

So the macro resolves to a 64-bit value, which is then fed directly into
an integer division, which breaks the i386 build with the usual __divdi3
complaint.

--D