linux-kernel - Re: [PATCH v2] fs: Kill DCACHE_DONTCACHE dentry even if DCACHE

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Date:   Thu, 24 Sep 2020 16:58:56 +0200
From:   Jan Kara <jack@...e.cz>
To:     Hao Li <lihao2018.fnst@...fujitsu.com>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        david@...morbit.com, ira.weiny@...el.com,
        linux-xfs@...r.kernel.org, viro@...iv.linux.org.uk,
        y-goto@...itsu.com
Subject: Re: [PATCH v2] fs: Kill DCACHE_DONTCACHE dentry even if
 DCACHE_REFERENCED is set

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@...fujitsu.com>

The patch looks good to me. You can add:

Reviewed-by: Jan Kara <jack@...e.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@...e.com>
SUSE Labs, CR