linux-kernel - Re: [PATCH 13/13] dcache: convert to use new lru list infrastructure

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Date:	Wed, 24 Aug 2011 02:32:58 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, khlebnikov@...nvz.org
Subject: Re: [PATCH 13/13] dcache: convert to use new lru list infrastructure

> +	struct list_head *freeable = arg;
> +	struct dentry	*dentry = container_of(item, struct dentry, d_lru);
> +
> +
> +	/*

double empty line.

> +	 * we are inverting the lru lock/dentry->d_lock here,
> +	 * so use a trylock. If we fail to get the lock, just skip
> +	 * it
> +	 */
> +	if (!spin_trylock(&dentry->d_lock))
> +		return 2;
> +
> +	/*
> +	 * Referenced dentries are still in use. If they have active
> +	 * counts, just remove them from the LRU. Otherwise give them
> +	 * another pass through the LRU.
> +	 */
> +	if (dentry->d_count) {
> +		list_del_init(&dentry->d_lru);
> +		spin_unlock(&dentry->d_lock);
> +		return 0;
> +	}
> +
> +	if (dentry->d_flags & DCACHE_REFERENCED) {

The comment aove seems odd, given that it doesn't match the code.
I'd rather have something like:

	/*
	 * Used dentry, remove it from the LRU.
	 */

in its place, and a second one above the DCACHE_REFERENCED check:

	/*
	 * Referenced dentry, give it another pass through the LRU.
	 */

> +		dentry->d_flags &= ~DCACHE_REFERENCED;
> +		spin_unlock(&dentry->d_lock);
> +
> +		/*
> +		 * XXX: this list move should be be done under d_lock. Need to
> +		 * determine if it is safe just to do it under the lru lock.
> +		 */
> +		return 1;
> +	}
> +
> +	list_move_tail(&dentry->d_lru, freeable);

Another odd comment.  It talks about doing a list_move in the branch
that doesn't do the list_move, and the list_move outside the branch
actually has the d_lock, thus disagreeing with the comment.

> +	this_cpu_dec(nr_dentry_unused);
> +	spin_unlock(&dentry->d_lock);

No need to decrement the per-cpu counter while still having the lock
held.

> @@ -1094,11 +1069,10 @@ resume:
>  		/*
>  		 * move only zero ref count dentries to the dispose list.
>  		 */
> +		dentry_lru_del(dentry);
>  		if (!dentry->d_count) {
> -			dentry_lru_move_list(dentry, dispose);
> +			list_add_tail(&dentry->d_lru, dispose);
>  			found++;
> -		} else {
> -			dentry_lru_del(dentry);

I'd rather move this hunk to the previous patch, as it fits into the
logical change done there.

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