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Subject: Re: [PATCH V3 4/5] ext4: get discard out of jbd2 commit kthread
 contex
To:     Theodore Ts'o <tytso@mit.edu>
Cc:     linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org,
        adilger.kernel@dilger.ca
References: <20210724074124.25731-1-jianchao.wan9@gmail.com>
 <20210724074124.25731-5-jianchao.wan9@gmail.com> <YRV6qqZcsNBHZzyn@mit.edu>
From:   Wang Jianchao <jianchao.wan9@gmail.com>
Message-ID: <65c6aa35-5e4c-a717-d1dc-8842e3ce0424@gmail.com>
Date:   Thu, 26 Aug 2021 15:51:35 +0800
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On 2021/8/13 3:46 AM, Theodore Ts'o wrote:
> On Sat, Jul 24, 2021 at 03:41:23PM +0800, Wang Jianchao wrote:
>> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
>> index 34be2f07449d..a496509e61b7 100644
>> --- a/fs/ext4/mballoc.c
>> +++ b/fs/ext4/mballoc.c
>> @@ -3474,6 +3530,14 @@ int ext4_mb_release(struct super_block *sb)
>>  	struct kmem_cache *cachep = get_groupinfo_cache(sb->s_blocksize_bits);
>>  	int count;
>>  
>> +	if (test_opt(sb, DISCARD)) {
>> +		/*
>> +		 * wait the discard work to drain all of ext4_free_data
>> +		 */
>> +		queue_work(ext4_discard_wq, &sbi->s_discard_work);
>> +		flush_work(&sbi->s_discard_work);
> 
> I agree with Jan --- it's not clear to me why the call to queue_work()
> is needed.  After the flush_work() call returns, if s_discard_work is
> still non-empty, there must be something terribly wrong --- are we
> missing something?

Yes，the queue_work() is redundant.
I will get rid of it in next version.

> 
>> @@ -3672,8 +3724,14 @@ int __init ext4_init_mballoc(void)
>>  	if (ext4_free_data_cachep == NULL)
>>  		goto out_ac_free;
>>  
>> +	ext4_discard_wq = alloc_workqueue("ext4discard", WQ_UNBOUND, 0);
>> +	if (!ext4_discard_wq)
>> +		goto out_free_data;
>> +
> 
> 
> Perhaps we should only allocate the workqueue when it's needed ---
> e.g., when a file system is mounted or remounted with "-o discard"?
> 
> Then in ext4_exit_malloc(), we only free it if ext4_discard_wq is
> non-NULL.
> 
> This would save a bit of memory on systems that wouldn't need the ext4
> discard work queue.

Yes, it make sense to the system with pool memory

Thanks so much
Jianchao

> 
> 					- Ted
>