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SubjectRe: [PATCH v2] fs: Kill DCACHE_DONTCACHE dentry even if DCACHE_REFERENCED is set
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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


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