linux-kernel - Re: [PATCH v2 3/4] limit nr

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Date:	Fri, 12 Aug 2011 15:56:52 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Glauber Costa <glommer@...allels.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	containers@...ts.linux-foundation.org,
	Pavel Emelyanov <xemul@...allels.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Hugh Dickins <hughd@...gle.com>,
	Nick Piggin <npiggin@...nel.dk>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	James Bottomley <JBottomley@...allels.com>,
	David Chinner <david@...morbit.com>
Subject: Re: [PATCH v2 3/4] limit nr_dentries per superblock

Le vendredi 05 août 2011 à 04:35 +0400, Glauber Costa a écrit :
> This patch lays the foundation for us to limit the dcache size.
> Each super block can have only a maximum amount of dentries under its
> sub-tree. Allocation fails if we we're over limit and the cache
> can't be pruned to free up space for the newcomers.
> 
> Signed-off-by: Glauber Costa <glommer@...allels.com>
> CC: Dave Chinner <david@...morbit.com>
> ---
>  fs/dcache.c        |   25 +++++++++++++++++++++++++
>  fs/super.c         |    1 +
>  include/linux/fs.h |    1 +
>  3 files changed, 27 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/dcache.c b/fs/dcache.c
> index ac19d24..52a0faf 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -1180,6 +1180,28 @@ void shrink_dcache_parent(struct dentry * parent)
>  }
>  EXPORT_SYMBOL(shrink_dcache_parent);
>  
> +static int dcache_mem_check(struct super_block *sb)
> +{
> +	int i;
> +	int nr_dentry;
> +	struct shrink_control sc = {
> +		.gfp_mask = GFP_KERNEL,
> +	};
> +
> +	do {
> +		nr_dentry = 0;
> +		for_each_possible_cpu(i)
> +			nr_dentry += per_cpu(*sb->s_nr_dentry, i);

You seriously want to call this for every __d_alloc() invocation,
even if s_nr_dentry_max is the default value (INT_MAX) ?

On a 4096 cpu machine, it will be _very_ slow.

A percpu_counter would be the thing to consider, since you can avoid the
for_each_possible_cpu(i) loop if percpu_counter_read() is smaller than
sb->s_nr_dentry_max.

Check how its done in include/net/tcp.h, tcp_too_many_orphans()



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