Re: [PATCH] xfs: fix infinite loop by detaching the group/project hints from user dquot

[Jeff Liu <jeff.liu@oracle.com>]

On 11/19 2013 19:12 PM, Dave Chinner wrote:
> On Tue, Nov 19, 2013 at 04:42:30PM +0800, Jeff Liu wrote:
>> From: Jie Liu <jeff.liu@oracle.com>
>>
>> xfs_quota(8) will hang up if trying to turn group quota or project
>> quota before the user quota is off, this could be 100% reproduced by:
>> # mount -ouquota,gquota /dev/sda7 /xfs
>> # mkdir /xfs/test
>> # xfs_quota -xc 'off -g' /xfs
>> # echo w > /proc/sysrq-trigger
>>   SysRq : Show Blocked State
>>   task                        PC stack   pid father
>>   xfs_quota       D 0000000000000000     0 27574   2551 0x00000000
>>   [snip]
>>   Call Trace:
>>   [<ffffffff81aaa21d>] schedule+0xad/0xc0
>>   [<ffffffff81aa327e>] schedule_timeout+0x35e/0x3c0
>>   [<ffffffff8114b506>] ? mark_held_locks+0x176/0x1c0
>>   [<ffffffff810ad6c0>] ? call_timer_fn+0x2c0/0x2c0
>>   [<ffffffffa0c25380>] ? xfs_qm_shrink_count+0x30/0x30 [xfs]
>>   [<ffffffff81aa3306>] schedule_timeout_uninterruptible+0x26/0x30
>>   [<ffffffffa0c26155>] xfs_qm_dquot_walk+0x235/0x260 [xfs]
>>   [<ffffffffa0c059d8>] ? xfs_perag_get+0x1d8/0x2d0 [xfs]
>>   [<ffffffffa0c05805>] ? xfs_perag_get+0x5/0x2d0 [xfs]
>>   [<ffffffffa0b7707e>] ? xfs_inode_ag_iterator+0xae/0xf0 [xfs]
>>   [<ffffffffa0c22280>] ? xfs_trans_free_dqinfo+0x50/0x50 [xfs]
>>   [<ffffffffa0b7709f>] ? xfs_inode_ag_iterator+0xcf/0xf0 [xfs]
>>   [<ffffffffa0c261e6>] xfs_qm_dqpurge_all+0x66/0xb0 [xfs]
>>   [<ffffffffa0c2497a>] xfs_qm_scall_quotaoff+0x20a/0x5f0 [xfs]
>>   [<ffffffffa0c2b8f6>] xfs_fs_set_xstate+0x136/0x180 [xfs]
>>   [<ffffffff8136cf7a>] do_quotactl+0x53a/0x6b0
>>   [<ffffffff812fba4b>] ? iput+0x5b/0x90
>>   [<ffffffff8136d257>] SyS_quotactl+0x167/0x1d0
>>   [<ffffffff814cf2ee>] ? trace_hardirqs_on_thunk+0x3a/0x3f
>>   [<ffffffff81abcd19>] system_call_fastpath+0x16/0x1b
>>
>> It's fine if we turn user quota off at first, then turn off other
>> kind of quotas if they are enabled since the group/project dquot
>> refcount is decreased to zero once the user quota if off.  Otherwse,
>> those dquots refcount is non-zero due to the user dquot maybe refer
>> to them as hint(s).  Hence, above operation hit an infinite loop at
>> xfs_qm_dquot_walk() to purge dquot cache.
>>
>> This problem has been around since Linux 3.4, it was introduced by:
>> 	b84a3a96751f93071c1863f2962273973c8b8f5e
>> 	xfs: remove the per-filesystem list of dquots
>>
>> Originally we will release the group dquot pointers because the user
>> dquots maybe carrying around as a hint via xfs_qm_detach_gdquots().
>> However, with this change, there is no such work to be done before
>> purge group/project dquot cache.
>>
>> This fix introduce a similar routine to the old xfs_qm_detach_gdquots(),
>> it will detach the group/project hints by searching the user dquot radix
>> tree and release those hints if they are there.
> 
> Ok, we do get stuck in a loop in this case. We need an xfstest to
> exercise this - removing group and project quotas while user quotas
> are left on and fsstress is active - should be pretty easy to do.
> 
> More comments below.
> 
>> Signed-off-by: Jie Liu <jeff.liu@oracle.com>
>> ---
>>  fs/xfs/xfs_qm.c |   79 +++++++++++++++++++++++++++++++++++++++++++++++++++++--
>>  1 file changed, 77 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/xfs/xfs_qm.c b/fs/xfs/xfs_qm.c
>> index 14a4996..410adf4 100644
>> --- a/fs/xfs/xfs_qm.c
>> +++ b/fs/xfs/xfs_qm.c
>> @@ -60,6 +60,77 @@ STATIC void	xfs_qm_dqfree_one(struct xfs_dquot *dqp);
>>   */
>>  #define XFS_DQ_LOOKUP_BATCH	32
>>  
>> +/*
>> + * Release the group or project dquot pointers the user dquots may be
>> + * carrying around as a hint.
>> + */
>> +STATIC void
>> +xfs_qm_dqdetach_hint(
>> +	struct xfs_mount	*mp,
>> +	int			type)
>> +{
>> +	struct xfs_quotainfo	*qi = mp->m_quotainfo;
>> +	struct radix_tree_root	*tree = xfs_dquot_tree(qi, XFS_DQ_USER);
>> +	uint32_t		next_index;
>> +	int			skipped;
>> +	int			nr_found;
>> +
>> +	ASSERT(type == XFS_DQ_GROUP || type == XFS_DQ_PROJ);
>> +
>> +restart:
>> +	next_index = 0;
>> +	skipped = 0;
>> +	nr_found = 0;
>> +
>> +	while (1) {
>> +		struct xfs_dquot *batch[XFS_DQ_LOOKUP_BATCH];
>> +		int		i;
>> +
>> +		mutex_lock(&qi->qi_tree_lock);
>> +		nr_found = radix_tree_gang_lookup(tree, (void **)batch,
>> +					next_index, XFS_DQ_LOOKUP_BATCH);
>> +		if (!nr_found) {
>> +			mutex_unlock(&qi->qi_tree_lock);
>> +			break;
>> +		}
> 
> Why reimplement xfs_qm_dquot_walk?
> 
> From a quick scan of the code, this could be done with
> xfs_qm_dquot_walk(XFS_DQ_USER) and an execute function that looks
> like the hint handling at the start of xfs_qm_dqpurge()....
> 
>>  STATIC int
>>  xfs_qm_dquot_walk(
>>  	struct xfs_mount	*mp,
>> @@ -224,10 +295,14 @@ xfs_qm_dqpurge_all(
>>  {
>>  	if (flags & XFS_QMOPT_UQUOTA)
>>  		xfs_qm_dquot_walk(mp, XFS_DQ_USER, xfs_qm_dqpurge, NULL);
>> -	if (flags & XFS_QMOPT_GQUOTA)
>> +	if (flags & XFS_QMOPT_GQUOTA) {
>> +		xfs_qm_dqdetach_hint(mp, XFS_DQ_GROUP);
>>  		xfs_qm_dquot_walk(mp, XFS_DQ_GROUP, xfs_qm_dqpurge, NULL);
> 
> and called here like:
> 
> 	if (flags & XFS_QMOPT_GQUOTA) {
> 		xfs_qm_dquot_walk(mp, XFS_DQ_USER, xfs_qm_detach_group_hint, NULL);
> 		xfs_qm_dquot_walk(mp, XFS_DQ_GROUP, xfs_qm_dqpurge, NULL);
> 	}
> 
> In effect, this means we always need to walk the user quota tree,
> so we could do all the hint freeing and purging in a single pass by
> passing the flags as an argument to a special execution function for
> the user dquot tree walk that handles releasing hints and purging
> user dquots accordingly. i.e:
> 
> 	xfs_qm_dquot_walk(mp, XFS_DQ_USER, xfs_qm_dqpurge_hints, &flags);
> 	if (flags & XFS_QMOPT_GQUOTA)
> 		xfs_qm_dquot_walk(mp, XFS_DQ_GROUP, xfs_qm_dqpurge, NULL);
> 	if (flags & XFS_QMOPT_PQUOTA)
> 		xfs_qm_dquot_walk(mp, XFS_DQ_PROJ, xfs_qm_dqpurge, NULL);
> 
> But either way, we don't need to duplicate the radix tree walk code
> to release the hints...
Ok, I'll post a revised fix as well as a test after finishing another
internal task.

Thanks,
-Jeff

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