Re: [PATCH v2 3/4] limit nr_dentries per superblock

From: Glauber Costa
Date: Fri Aug 12 2011 - 15:19:15 EST

On 08/12/2011 10:56 AM, Eric Dumazet wrote:
Le vendredi 05 aoÃt 2011 Ã 04:35 +0400, Glauber Costa a Ãcrit :
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@xxxxxxxxxxxxx>
CC: Dave Chinner<david@xxxxxxxxxxxxx>
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)

+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) ?

Well, I guess that special-casing INT_MAX is a good thing.
I can include it in the next submission, I like it. Thanks.

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

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

Yeah, I guess I could do that. In fact, my first series used percpu_counters, and then I switched. But looking back, percpu_counters
are indeed more suitable. The goal back then was trying to avoid percpu_counter_add, but as it is right now, we trade it for an even worse thing.

Thank you for your comments.
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