Re: [PATCH v4 05/31] dcache: remove dentries from LRU before putting on dispose list

[Mel Gorman <mgorman@suse.de>]

On Sat, Apr 27, 2013 at 03:19:01AM +0400, Glauber Costa wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> One of the big problems with modifying the way the dcache shrinker
> and LRU implementation works is that the LRU is abused in several
> ways. One of these is shrink_dentry_list().
> 
> Basically, we can move a dentry off the LRU onto a different list
> without doing any accounting changes, and then use dentry_lru_prune()
> to remove it from what-ever list it is now on to do the LRU
> accounting at that point.
> 
> This makes it -really hard- to change the LRU implementation. The
> use of the per-sb LRU lock serialises movement of the dentries
> between the different lists and the removal of them, and this is the
> only reason that it works. If we want to break up the dentry LRU
> lock and lists into, say, per-node lists, we remove the only
> serialisation that allows this lru list/dispose list abuse to work.
> 
> To make this work effectively, the dispose list has to be isolated
> from the LRU list - dentries have to be removed from the LRU
> *before* being placed on the dispose list. This means that the LRU
> accounting and isolation is completed before disposal is started,
> and that means we can change the LRU implementation freely in
> future.
> 
> This means that dentries *must* be marked with DCACHE_SHRINK_LIST
> when they are placed on the dispose list so that we don't think that
> parent dentries found in try_prune_one_dentry() are on the LRU when
> the are actually on the dispose list. This would result in
> accounting the dentry to the LRU a second time. Hence
> dentry_lru_prune() has to handle the DCACHE_SHRINK_LIST case
> differently because the dentry isn't on the LRU list.
> 
> [ v2: don't decrement nr unused twice, spotted by Sha Zhengju ]
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> Signed-off-by: Glauber Costa <glommer@openvz.org>

Acked-by: Mel Gorman <mgorman@suse.de>

-- 
Mel Gorman
SUSE Labs

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