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Subject: Re: [PATCH v2] fs: Kill DCACHE_DONTCACHE dentry even if
 DCACHE_REFERENCED is set
To:     Jan Kara <jack@suse.cz>, Al Viro <viro@zeniv.linux.org.uk>
CC:     <linux-kernel@vger.kernel.org>, <linux-fsdevel@vger.kernel.org>,
        <david@fromorbit.com>, <ira.weiny@intel.com>,
        <linux-xfs@vger.kernel.org>, <y-goto@fujitsu.com>
References: <20200924055958.825515-1-lihao2018.fnst@cn.fujitsu.com>
 <20200924145856.GB3361@quack2.suse.cz>
 <20201021135133.GA25702@quack2.suse.cz>
From:   "Li, Hao" <lihao2018.fnst@cn.fujitsu.com>
Message-ID: <053ff976-8d75-b249-2bba-3a8b7a98b1a4@cn.fujitsu.com>
Date:   Thu, 5 Nov 2020 18:54:49 +0800
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Ping :)

On 2020/10/21 21:51, Jan Kara wrote:
> Hum, Al, did this patch get lost?
>
> 								Honza
>
> On Thu 24-09-20 16:58:56, Jan Kara wrote:
>> On Thu 24-09-20 13:59:58, Hao Li wrote:
>>> If DCACHE_REFERENCED is set, fast_dput() will return true, and then
>>> retain_dentry() have no chance to check DCACHE_DONTCACHE. As a result,
>>> the dentry won't be killed and the corresponding inode can't be evicted.
>>> In the following example, the DAX policy can't take effects unless we
>>> do a drop_caches manually.
>>>
>>>   # DCACHE_LRU_LIST will be set
>>>   echo abcdefg > test.txt
>>>
>>>   # DCACHE_REFERENCED will be set and DCACHE_DONTCACHE can't do anything
>>>   xfs_io -c 'chattr +x' test.txt
>>>
>>>   # Drop caches to make DAX changing take effects
>>>   echo 2 > /proc/sys/vm/drop_caches
>>>
>>> What this patch does is preventing fast_dput() from returning true if
>>> DCACHE_DONTCACHE is set. Then retain_dentry() will detect the
>>> DCACHE_DONTCACHE and will return false. As a result, the dentry will be
>>> killed and the inode will be evicted. In this way, if we change per-file
>>> DAX policy, it will take effects automatically after this file is closed
>>> by all processes.
>>>
>>> I also add some comments to make the code more clear.
>>>
>>> Signed-off-by: Hao Li <lihao2018.fnst@cn.fujitsu.com>
>> The patch looks good to me. You can add:
>>
>> Reviewed-by: Jan Kara <jack@suse.cz>
>>
>> 								Honza
>>
>>> ---
>>> v1 is split into two standalone patch as discussed in [1], and the first
>>> patch has been reviewed in [2]. This is the second patch.
>>>
>>> [1]: https://lore.kernel.org/linux-fsdevel/20200831003407.GE12096@dread.disaster.area/
>>> [2]: https://lore.kernel.org/linux-fsdevel/20200906214002.GI12131@dread.disaster.area/
>>>
>>>  fs/dcache.c | 9 ++++++++-
>>>  1 file changed, 8 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/fs/dcache.c b/fs/dcache.c
>>> index ea0485861d93..97e81a844a96 100644
>>> --- a/fs/dcache.c
>>> +++ b/fs/dcache.c
>>> @@ -793,10 +793,17 @@ static inline bool fast_dput(struct dentry *dentry)
>>>  	 * a reference to the dentry and change that, but
>>>  	 * our work is done - we can leave the dentry
>>>  	 * around with a zero refcount.
>>> +	 *
>>> +	 * Nevertheless, there are two cases that we should kill
>>> +	 * the dentry anyway.
>>> +	 * 1. free disconnected dentries as soon as their refcount
>>> +	 *    reached zero.
>>> +	 * 2. free dentries if they should not be cached.
>>>  	 */
>>>  	smp_rmb();
>>>  	d_flags = READ_ONCE(dentry->d_flags);
>>> -	d_flags &= DCACHE_REFERENCED | DCACHE_LRU_LIST | DCACHE_DISCONNECTED;
>>> +	d_flags &= DCACHE_REFERENCED | DCACHE_LRU_LIST |
>>> +			DCACHE_DISCONNECTED | DCACHE_DONTCACHE;
>>>  
>>>  	/* Nothing to do? Dropping the reference was all we needed? */
>>>  	if (d_flags == (DCACHE_REFERENCED | DCACHE_LRU_LIST) && !d_unhashed(dentry))
>>> -- 
>>> 2.28.0
>>>
>>>
>>>
>> -- 
>> Jan Kara <jack@suse.com>
>> SUSE Labs, CR