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

[Li, Hao]

On 2020/8/27 14:37, Dave Chinner wrote:
> On Fri, Aug 21, 2020 at 09:59:53AM +0800, Hao Li wrote:
>> Currently, DCACHE_REFERENCED prevents the dentry with DCACHE_DONTCACHE
>> set from being killed, so the corresponding inode can't be evicted. If
>> the DAX policy of an inode is changed, we can't make policy changing
>> take effects unless dropping caches manually.
>>
>> This patch fixes this problem and flushes the inode to disk to prepare
>> for evicting it.
>>
>> Signed-off-by: Hao Li <lihao2018.fnst@xxxxxxxxxxxxxx>
>> ---
>>  fs/dcache.c | 3 ++-
>>  fs/inode.c  | 2 +-
>>  2 files changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/dcache.c b/fs/dcache.c
>> index ea0485861d93..486c7409dc82 100644
>> --- a/fs/dcache.c
>> +++ b/fs/dcache.c
>> @@ -796,7 +796,8 @@ static inline bool fast_dput(struct dentry *dentry)
>>  	 */
>>  	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;
> Seems reasonable, but you need to update the comment above as to
> how this flag fits into this code....

Yes. I will change it. Thanks.

>
>>  	/* Nothing to do? Dropping the reference was all we needed? */
>>  	if (d_flags == (DCACHE_REFERENCED | DCACHE_LRU_LIST) && !d_unhashed(dentry))
>> diff --git a/fs/inode.c b/fs/inode.c
>> index 72c4c347afb7..5218a8aebd7f 100644
>> --- a/fs/inode.c
>> +++ b/fs/inode.c
>> @@ -1632,7 +1632,7 @@ static void iput_final(struct inode *inode)
>>  	}
>>  
>>  	state = inode->i_state;
>> -	if (!drop) {
>> +	if (!drop || (drop && (inode->i_state & I_DONTCACHE))) {
>>  		WRITE_ONCE(inode->i_state, state | I_WILL_FREE);
>>  		spin_unlock(&inode->i_lock);
> What's this supposed to do? We'll only get here with drop set if the
> filesystem is mounting or unmounting.

The variable drop will also be set to True if I_DONTCACHE is set on
inode->i_state.
Although mounting/unmounting will set the drop variable, it won't set
I_DONTCACHE if I understand correctly. As a result,
drop && (inode->i_state & I_DONTCACHE) will filter out mounting/unmounting.

> In either case, why does
> having I_DONTCACHE set require the inode to be written back here
> before it is evicted from the cache?

Mounting/unmounting won't execute the code snippet which is in that if
statement, as I have explained above. However, If I_DONTCACHE is set, we
have to execute this snippet to write back inode.

I_DONTCACHE is set in d_mark_dontcache() which will be called in two
situations:
1. DAX policy is changed.
2. The inode is read through bulkstat in XFS. See commit 5132ba8f2b77
("xfs: don't cache inodes read through bulkstat") for more details.

For the first case, we have to write back the inode together with its
dirty pages before evicting.
For the second case, I think it's also necessary to write back inode before
evicting.

Thanks,
Hao Li

>
> Cheers,
>
> Dave.