On Wed, Jul 04, 2012 at 11:56:28AM +0900, Kamezawa Hiroyuki wrote:
> I think usage > limit means a sign of BUG. But, sometimes,
> res_counter_charge_nofail() is very convenient. tcp_memcg uses it.
> And I'd like to use it for helping page migration.
>
> This patch adds res_counter_usage_safe() which returns min(usage,limit).
> By this we can use res_counter_charge_nofail() without breaking
> user experience.
>
> Changelog:
> - read res_counter directrly under lock.
> - fixed comment.
>
> Acked-by: Glauber Costa <glommer@xxxxxxxxxxxxx>
> Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
> Reviewed-by: Michal Hocko <mhocko@xxxxxxx>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> ---
> include/linux/res_counter.h | 2 ++
> kernel/res_counter.c | 18 ++++++++++++++++++
> net/ipv4/tcp_memcontrol.c | 2 +-
> 3 files changed, 21 insertions(+), 1 deletions(-)
>
> diff --git a/include/linux/res_counter.h b/include/linux/res_counter.h
> index 7d7fbe2..a6f8cc5 100644
> --- a/include/linux/res_counter.h
> +++ b/include/linux/res_counter.h
> @@ -226,4 +226,6 @@ res_counter_set_soft_limit(struct res_counter *cnt,
> return 0;
> }
>
> +u64 res_counter_usage_safe(struct res_counter *cnt);
> +
> #endif
> diff --git a/kernel/res_counter.c b/kernel/res_counter.c
> index ad581aa..f0507cd 100644
> --- a/kernel/res_counter.c
> +++ b/kernel/res_counter.c
> @@ -171,6 +171,24 @@ u64 res_counter_read_u64(struct res_counter *counter, int member)
> }
> #endif
>
> +/*
> + * Returns usage. If usage > limit, limit is returned.
> + * This is useful not to break user experiance if the excess
> + * is temporary.
> + */
> +u64 res_counter_usage_safe(struct res_counter *counter)
> +{
> + unsigned long flags;
> + u64 usage, limit;
> +
> + spin_lock_irqsave(&counter->lock, flags);
> + limit = counter->limit;
> + usage = counter->usage;
> + spin_unlock_irqrestore(&counter->lock, flags);
> +
> + return min(usage, limit);
> +}
> +
> int res_counter_memparse_write_strategy(const char *buf,
> unsigned long long *res)
> {
> diff --git a/net/ipv4/tcp_memcontrol.c b/net/ipv4/tcp_memcontrol.c
> index b6f3583..a73dce6 100644
> --- a/net/ipv4/tcp_memcontrol.c
> +++ b/net/ipv4/tcp_memcontrol.c
> @@ -180,7 +180,7 @@ static u64 tcp_read_usage(struct mem_cgroup *memcg)
> return atomic_long_read(&tcp_memory_allocated) << PAGE_SHIFT;
>
> tcp = tcp_from_cgproto(cg_proto);
> - return res_counter_read_u64(&tcp->tcp_memory_allocated, RES_USAGE);
> + return res_counter_usage_safe(&tcp->tcp_memory_allocated);
> }
Hm, it depends on what you consider more important.
Personally, I think it's more useful to report the truth rather than
pretending we'd enforce an invariant that we actually don't. And I
think it can just be documented that we have to charge memory over the
limit in certain contexts, so people/scripts should expect usage to
exceed the limit.
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