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Re: [PATCH 1/5] inode: Make unused inode LRU per superblock

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 1/5] inode: Make unused inode LRU per superblock
From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Date: Thu, 27 May 2010 13:32:30 -0700
Cc: linux-kernel@xxxxxxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, linux-mm@xxxxxxxxx, xfs@xxxxxxxxxxx
In-reply-to: <1274777588-21494-2-git-send-email-david@xxxxxxxxxxxxx>
References: <1274777588-21494-1-git-send-email-david@xxxxxxxxxxxxx> <1274777588-21494-2-git-send-email-david@xxxxxxxxxxxxx>
On Tue, 25 May 2010 18:53:04 +1000
Dave Chinner <david@xxxxxxxxxxxxx> wrote:

> From: Dave Chinner <dchinner@xxxxxxxxxx>
> The inode unused list is currently a global LRU. This does not match
> the other global filesystem cache - the dentry cache - which uses
> per-superblock LRU lists. Hence we have related filesystem object
> types using different LRU reclaimatin schemes.
> To enable a per-superblock filesystem cache shrinker, both of these
> caches need to have per-sb unused object LRU lists. Hence this patch
> converts the global inode LRU to per-sb LRUs.
> The patch only does rudimentary per-sb propotioning in the shrinker
> infrastructure, as this gets removed when the per-sb shrinker
> callouts are introduced later on.
> ...
> +                     list_move(&inode->i_list, &inode->i_sb->s_inode_lru);

It's a shape that s_inode_lru is still protected by inode_lock.  One
day we're going to get in trouble over that lock.  Migrating to a
per-sb lock would be logical and might help.

Did you look into this?  I expect we'd end up taking both inode_lock
and the new sb->lru_lock in several places, which wouldn't be of any
help, at least in the interim.  Long-term, the locking for
fs-writeback.c should move to the per-superblock one also, at which
time this problem largely goes away I think.  Unfortunately the
writeback inode lists got moved into the backing_dev_info, whcih messes
things up a bit.

>       inodes_stat.nr_unused--;
> +     inode->i_sb->s_nr_inodes_unused--;

It's regrettable to be counting the same thing twice.  Did you look
into removing (or no longer using) inodes_stat.nr_unused?

> +             /* Now, we reclaim unused dentrins with fairness.

May as well fix the typo while we're there.

Please review all these comments to ensure that they are still accurate
and complete.

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