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

[Dave Chinner]

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....

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

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx