linux-kernel - Re: [PATCH 04/18] fs: Implement lazy LRU updates for inodes.

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Message-ID: <20101008070856.GC23595@lst.de>
Date:	Fri, 8 Oct 2010 03:08:56 -0400
From:	Christoph Hellwig <hch@....de>
To:	Dave Chinner <david@...morbit.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/18] fs: Implement lazy LRU updates for inodes.

Looks good,

Reviewed-by: Christoph Hellwig <hch@....de>

A few nipicks on the comments or lack thereof below:

> @@ -489,8 +484,15 @@ static void prune_icache(int nr_to_scan)
>  
>  		inode = list_entry(inode_unused.prev, struct inode, i_list);
>  
> -		if (inode->i_state || atomic_read(&inode->i_count)) {
> +		if (atomic_read(&inode->i_count) ||
> +		    (inode->i_state & ~I_REFERENCED)) {
> +			list_del_init(&inode->i_list);
> +			percpu_counter_dec(&nr_inodes_unused);
> +			continue;
> +		}
> +		if (inode->i_state & I_REFERENCED) {
>  			list_move(&inode->i_list, &inode_unused);
> +			inode->i_state &= ~I_REFERENCED;
>  			continue;

I think this code could use some comments explaining the lazy LRU
scheme.

> -			if (inode != list_entry(inode_unused.next,
> -						struct inode, i_list))
> -				continue;	/* wrong inode or list_empty */
> -			if (!can_unuse(inode))
> +			/*
> +			 * if we can't reclaim this inod immediately, give it
> +			 * another pass through the free list so we don't spin
> +			 * on it.

s/inod/inode/

> +
> +	/*
> +	 * We avoid moving dirty inodes back onto the LRU now because I_FREEING
> +	 * is set and hence writeback_single_inode() won't move the inode
> +	 * around.
> +	 */
> +	if (!list_empty(&inode->i_list)) {
> +		list_del_init(&inode->i_list);
> +		percpu_counter_dec(&nr_inodes_unused);
> +	}
> +

The comment is a bit misleading.  We do not only avoid moving it to the
LRU, but actively delete the inode from the LRU here.  I don't think the
I_FREEING check isn't the only reason - the LRU code traditionally
couldn't deal with unlinked inodes at all, although the switch to
->evict_inode probably has fixed that.


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