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

Eric Sandeen
2017-04-19T19:29:23Z

On 4/17/17 1:59 PM, Darrick J. Wong wrote:
> 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 */

Assigned to "int j", riiiiight. hohum.

> 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.

sandeen--

> --D
>
>> ASSERT(mp->m_quotainfo->qi_dqperchunk == j);
>> #endif
>> dqb = bp->b_addr;
>>
>> --
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