Message-Id: <5A57590B020000F9000A2F27@prv-mh.provo.novell.com>
Date:   Wed, 10 Jan 2018 21:31:07 -0700
From:   "Gang He" <ghe@...e.com>
To:     <jlbec@...lplan.org>, <ge.changwei@....com>, <mfasheh@...sity.com>
Cc:     <ocfs2-devel@....oracle.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [Ocfs2-devel] [PATCH v2 2/2] ocfs2: add trimfs lock to
 avoid duplicated trims in cluster

Hi Changwei,


>>> 
> On 2018/1/11 11:33, Gang He wrote:
>> Hi Changwei,
>> 
>> 
>>>>>
>>> On 2018/1/11 10:07, Gang He wrote:
>>>> Hi Changwei,
>>>>
>>>>
>>>>>>>
>>>>> On 2018/1/10 18:14, Gang He wrote:
>>>>>> Hi Changwei,
>>>>>>
>>>>>>
>>>>>>>>>
>>>>>>> On 2018/1/10 17:05, Gang He wrote:
>>>>>>>> Hi Changwei,
>>>>>>>>
>>>>>>>>
>>>>>>>>>>>
>>>>>>>>> Hi Gang,
>>>>>>>>>
>>>>>>>>> On 2017/12/14 13:16, Gang He wrote:
>>>>>>>>>> As you know, ocfs2 has support trim the underlying disk via
>>>>>>>>>> fstrim command. But there is a problem, ocfs2 is a shared disk
>>>>>>>>>> cluster file system, if the user configures a scheduled fstrim
>>>>>>>>>> job on each file system node, this will trigger multiple nodes
>>>>>>>>>> trim a shared disk simultaneously, it is very wasteful for CPU
>>>>>>>>>> and IO consumption, also might negatively affect the lifetime
>>>>>>>>>> of poor-quality SSD devices.
>>>>>>>>>> Then, we introduce a trimfs dlm lock to communicate with each
>>>>>>>>>> other in this case, which will make only one fstrim command to
>>>>>>>>>> do the trimming on a shared disk among the cluster, the fstrim
>>>>>>>>>> commands from the other nodes should wait for the first fstrim
>>>>>>>>>> to finish and returned success directly, to avoid running a the
>>>>>>>>>> same trim on the shared disk again.
>>>>>>>>>>
>>>>>>>>>> Compare with first version, I change the fstrim commands' returned
>>>>>>>>>> value and behavior in case which meets a fstrim command is running
>>>>>>>>>> on a shared disk.
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: Gang He <ghe@...e.com>
>>>>>>>>>> ---
>>>>>>>>>>       fs/ocfs2/alloc.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
>>>>>>>>>>       1 file changed, 44 insertions(+)
>>>>>>>>>>
>>>>>>>>>> diff --git a/fs/ocfs2/alloc.c b/fs/ocfs2/alloc.c
>>>>>>>>>> index ab5105f..5c9c3e2 100644
>>>>>>>>>> --- a/fs/ocfs2/alloc.c
>>>>>>>>>> +++ b/fs/ocfs2/alloc.c
>>>>>>>>>> @@ -7382,6 +7382,7 @@ int ocfs2_trim_fs(struct super_block *sb, struct
>>>>>>>>> fstrim_range *range)
>>>>>>>>>>       	struct buffer_head *gd_bh = NULL;
>>>>>>>>>>       	struct ocfs2_dinode *main_bm;
>>>>>>>>>>       	struct ocfs2_group_desc *gd = NULL;
>>>>>>>>>> +	struct ocfs2_trim_fs_info info, *pinfo = NULL;
>>>>>>>>>
>>>>>>>>> I think *pinfo* is not necessary.
>>>>>>>> This pointer is necessary, since it can be NULL or non-NULL depend on the
>>>>>>> code logic.
>>>>>>>
>>>>>>> This point is OK for me.
>>>>>>>
>>>>>>>>
>>>>>>>>>>       
>>>>>>>>>>       	start = range->start >> osb->s_clustersize_bits;
>>>>>>>>>>       	len = range->len >> osb->s_clustersize_bits;
>>>>>>>>>> @@ -7419,6 +7420,42 @@ int ocfs2_trim_fs(struct super_block *sb, struct
>>>>>>>>> fstrim_range *range)
>>>>>>>>>>       
>>>>>>>>>>       	trace_ocfs2_trim_fs(start, len, minlen);
>>>>>>>>>>       
>>>>>>>>>> +	ocfs2_trim_fs_lock_res_init(osb);
>>>>>>>>>> +	ret = ocfs2_trim_fs_lock(osb, NULL, 1);
>>>>>>>>>
>>>>>>>>> I don't get why try to lock here and if fails, acquire the same lock again
>>>>>>>>> later but wait until granted.
>>>>>>>> Please think about the user case, the patch is only used to handle this
>>>>>>> case.
>>>>>>>> When the administer configures a fstrim schedule task on each node, then
>>>>>>> each node will trigger a fstrim on shared disks concurrently.
>>>>>>>> In this case, we should avoid duplicated fstrim on a shared disk since this
>>>>>>> will waste CPU/IO resources and affect SSD lifetime sometimes.
>>>>>>>
>>>>>>> I'm not worrying about that trimfs will affect SSD's lifetime quite a lot,
>>>>>>> since physical-logical address converting table resides in RAM while SSD is
>>>>>>> working.
>>>>>>> And that table won't be at a big scale. My point here is not affecting this
>>>>>>> patch. Just a tip here.
>>>>>> This depend on SSD firmware implementation, but for secure-trim, it really
>>>>> possibly affect SSD lifetime.
>>>>>>
>>>>>>>> Firstly, we use try_lock to get fstrim dlm lock to identify if there is any
>>>>>>> other node which is doing fstrim on the disk.
>>>>>>>> If not, this node is the first one, this node should do fstrim operation on
>>>>>>> the disk.
>>>>>>>> If yes, this node is not the first one, this node should wait until the
>>>>>>> first node is done for fstrim operation, then return the result from DLM
>>>>>>> lock's value.
>>>>>>>>
>>>>>>>>> Can it just acquire the _trimfs_ lock as a blocking one directly here?
>>>>>>>> We can not do a blocking lock directly, since we need to identify if there
>>>>>>> is any other node has being do fstrim operation when this node start to do
>>>>>>> fstrim.
>>>>>>>
>>>>>>> Thanks for your elaboration.
>>>>>>>
>>>>>>> Well how about the third node trying to trimming fs too?
>>>>>>> It needs LVB from the second node.
>>>>>>> But it seems that the second node can't provide a valid LVB.
>>>>>>> So the third node will perform trimfs once more.
>>>>>> No, the second node does not change DLM lock's value, but the DLM lock's
>>>>> value is still valid.
>>>>>> The third node also refer to this DLM lock's value, then do the same logic
>>>>> like the second node.
>>>>>
>>>>> Hi Gang,
>>>>> I don't see any places where ocfs2_lock_res::ocfs2_lock_res_ops::set_lvb is
>>>>> set while flag LOCK_TYPE_USES_LVB is added.
>>>>>
>>>>> Are you sure below code path can work well?
>>>> Yes, have done a full testing on two and three nodes.
>>>>
>>>>> ocfs2_process_blocked_lock
>>>>>      ocfs2_unblock_lock
>>>>>          Reference to ::set_lvb since LOCK_TYPE_USES_LVB is set.
>>>>>
>>>> the set_lvb callback function is not necessary, if we update DLM lock value
>>> by ourselves before unlock.
>>>
>>> I think this may relates to *LOCK_TYPE_REQUIRES_REFRESH* flag.
>> Alright, I think *LOCK_TYPE_REQUIRES_REFRESH* flag is harmless here, 
> although this flag is probably unnecessary.
> 
> OK, I agree with adding *LOCK_TYPE_REQUIRES_REFRESH* for now.
> Can you give a explanation for my another concern about three nodes' 
> concurrent trimming fs.?
> 
> For your convenience, I paste it here:
> 
> The LVB passing path should be like below:
> 
> NODE 1 lvb (ex granted at time1) -> NODE 2 lvb(ex granted at time2) -> NODE 3 
> lvb(ex granted at time3).
> time1 < time2  < time3
> 
> So I think NODE 3 can't obtain LVB from NODE 1 but from NODE 2.
Yes, if node1 finished the fstrim successfully, node1 updated DLM lock value, then this DLM lock value is valid and unlocked/dropped.
node2 returned the result from this DLM lock value, node2 did not change DLM lock value (but value is still valid) and unlocked/dropped.
node3 returned the result from this DLM lock value (node1 updated), node3 did not change DLM lock value (but value is still valid) and unlocked/dropped.
after the last nodeN returned and unlocked/dropped this lock resource, this DLm lock value will become invalid.

Next round, the first node gets the lock and update the fstrim result to DLM lock value directly.
the same logic like before.

And more, this DLM lock value validity is OK when some node is suddenly crashed during the above case.

Thanks
Gang

> Moreover, if node 1 is the master of trimfs lock resource, node 1's LVB will 
> be updated to be the same as node 2.
>   
>> 
>>> Actually, I don't see why this flag is necessary to _orphan scan_.
>>> Why can't _orphan scan_ also set LVB during
>>> ocfs2_process_blocked_lock->ocfs2_unblock_lock?
>>>
>>> And it seems that _orphan scan_ also doesn't need to persist any stuff in
>>> LVB into disk.
>> More comments, you can look at dlmglue.c file carefully.
>> set_lvb is a callback function, which will be invoked automatically before 
> downgrade.
>> you can use this mechanism, you also do not do like that.
>> you just need to make sure to update DLM lock value before unlock/downgrade.
>> 
>> Thanks
>> Gang
>>   
>>>
>>> Thanks,
>>> Changwei
>>>
>>>> By the way, the code is transparent to the underlying DLM stack (o2cb or
>>> pcmk).
>>>
>>> True.
>>>>
>>>> Thanks
>>>> Gang
>>>>
>>>>> Thanks,
>>>>> Changwei
>>>>>
>>>>>>
>>>>>>>
>>>>>>> IOW, three nodes are trying to trimming fs concurrently. Is your patch able
>>>>>>> to handle such a scenario?
>>>>>> Yes, the patch can handle this case.
>>>>>>
>>>>>>>
>>>>>>> Even the second lock request with QUEUE set just follows
>>>>>>> ocfs2_trim_fs_lock_res_uninit() will not get rid of concurrent trimfs.
>>>>>>>
>>>>>>>>
>>>>>>>>>
>>>>>>>>>> +	if (ret < 0) {
>>>>>>>>>> +		if (ret != -EAGAIN) {
>>>>>>>>>> +			mlog_errno(ret);
>>>>>>>>>> +			ocfs2_trim_fs_lock_res_uninit(osb);
>>>>>>>>>> +			goto out_unlock;
>>>>>>>>>> +		}
>>>>>>>>>> +
>>>>>>>>>> +		mlog(ML_NOTICE, "Wait for trim on device (%s) to "
>>>>>>>>>> +		     "finish, which is running from another node.\n",
>>>>>>>>>> +		     osb->dev_str);
>>>>>>>>>> +		ret = ocfs2_trim_fs_lock(osb, &info, 0);
>>>>>>>>>> +		if (ret < 0) {
>>>>>>>>>> +			mlog_errno(ret);
>>>>>>>>>> +			ocfs2_trim_fs_lock_res_uninit(osb);
>>>>>>>>>
>>>>>>>>> In ocfs2_trim_fs_lock_res_uninit(), you drop lock. But it is never granted.
>>>>>>>>> Still need to drop lock resource?
>>>>>>>> Yes, we need to init/uninit fstrim dlm lock resource for each time.
>>>>>>>> Otherwise, trylock does not work, this is a little different from other dlm
>>>>>>> lock usage in ocfs2.
>>>>>>>
>>>>>>> This point is OK for now, too.
>>>>>>>>
>>>>>>>>>
>>>>>>>>>> +			goto out_unlock;
>>>>>>>>>> +		}
>>>>>>>>>> +
>>>>>>>>>> +		if (info.tf_valid && info.tf_success &&
>>>>>>>>>> +		    info.tf_start == start && info.tf_len == len &&
>>>>>>>>>> +		    info.tf_minlen == minlen) {
>>>>>>>>>> +			/* Avoid sending duplicated trim to a shared device */
>>>>>>>>>> +			mlog(ML_NOTICE, "The same trim on device (%s) was "
>>>>>>>>>> +			     "just done from node (%u), return.\n",
>>>>>>>>>> +			     osb->dev_str, info.tf_nodenum);
>>>>>>>>>> +			range->len = info.tf_trimlen;
>>>>>>>>>> +			goto out_trimunlock;
>>>>>>>>>> +		}
>>>>>>>>>> +	}
>>>>>>>>>> +
>>>>>>>>>> +	info.tf_nodenum = osb->node_num;
>>>>>>>>>> +	info.tf_start = start;
>>>>>>>>>> +	info.tf_len = len;
>>>>>>>>>> +	info.tf_minlen = minlen;
>>>>>>>>>
>>>>>>>>> If we faild during dong trimfs, I think we should not cache above info in
>>>>>>>>> LVB.
>>>>>>>> It is necessary, if the second node is waiting the first node, the first
>>>>>>> node fails to do fstrim,
>>>>>>>> the first node should update dlm lock's value, then the second node can get
>>>>>>> the latest dlm lock value (rather than the last time DLM lock value),
>>>>>>>> the second node will do the fstrim again, since the first node has failed.
>>>>>>>
>>>>>>> Yes, it makes scene.
>>>>>>>>
>>>>>>>>> BTW, it seems that this patch is on top of  'try lock' patches which you
>>>>>>>>> previously sent out.
>>>>>>>>> Are they related?
>>>>>>>> try lock patch is related to non-block aio support for ocfs2.
>>>>>>>>
>>>>>>>> Thanks
>>>>>>>> Gang
>>>>>>>>>
>>>>>>>>> Thanks,
>>>>>>>>> Changwei
>>>>>>>>>
>>>>>>>>>> +
>>>>>>>>>>       	/* Determine first and last group to examine based on start and len */
>>>>>>>>>>       	first_group = ocfs2_which_cluster_group(main_bm_inode, start);
>>>>>>>>>>       	if (first_group == osb->first_cluster_group_blkno)
>>>>>>>>>> @@ -7463,6 +7500,13 @@ int ocfs2_trim_fs(struct super_block *sb, struct
>>>>>>>>> fstrim_range *range)
>>>>>>>>>>       			group += ocfs2_clusters_to_blocks(sb, osb->bitmap_cpg);
>>>>>>>>>>       	}
>>>>>>>>>>       	range->len = trimmed * sb->s_blocksize;
>>>>>>>>>> +
>>>>>>>>>> +	info.tf_trimlen = range->len;
>>>>>>>>>> +	info.tf_success = (ret ? 0 : 1);
>>>>>>>>>> +	pinfo = &info;
>>>>>>>>>> +out_trimunlock:
>>>>>>>>>> +	ocfs2_trim_fs_unlock(osb, pinfo);
>>>>>>>>>> +	ocfs2_trim_fs_lock_res_uninit(osb);
>>>>>>>>>>       out_unlock:
>>>>>>>>>>       	ocfs2_inode_unlock(main_bm_inode, 0);
>>>>>>>>>>       	brelse(main_bm_bh);
>>>>>>>>>>
>>>>>>>>
>>>>>>
>>>>
>> 

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