Re: [PATCH] introduce res_counter_charge_nofail() for socket allocations

[Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>]

On Wed, Jan 18, 2012 at 07:15:58PM +0400, Glauber Costa wrote:
> There is a case in __sk_mem_schedule(), where an allocation
> is beyond the maximum, but yet we are allowed to proceed.
> It happens under the following condition:
> 
> 	sk->sk_wmem_queued + size >= sk->sk_sndbuf
> 
> The network code won't revert the allocation in this case,
> meaning that at some point later it'll try to do it. Since
> this is never communicated to the underlying res_counter
> code, there is an inbalance in res_counter uncharge operation.
> 
> I see two ways of fixing this:
> 
> 1) storing the information about those allocations somewhere
>    in memcg, and then deducting from that first, before
>    we start draining the res_counter,
> 2) providing a slightly different allocation function for
>    the res_counter, that matches the original behavior of
>    the network code more closely.
> 
> I decided to go for #2 here, believing it to be more elegant,
> since #1 would require us to do basically that, but in a more
> obscure way.
> 
> I will eventually submit it through Dave for the -net tree,
> but I wanted to query you guys first, to see if this approach
> is acceptable or if you'd prefer me to try something else.
> 
> Thanks
> 
> Signed-off-by: Glauber Costa <glommer-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>
> Cc: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>
> Cc: Michal Hocko <mhocko-AlSwsSmVLrQ@public.gmane.org>
> ---
>  include/linux/res_counter.h |    6 ++++++
>  include/net/sock.h          |   10 ++++------
>  kernel/res_counter.c        |   25 +++++++++++++++++++++++++
>  net/core/sock.c             |    4 ++--
>  4 files changed, 37 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/res_counter.h b/include/linux/res_counter.h
> index c9d625c..32a7b02 100644
> --- a/include/linux/res_counter.h
> +++ b/include/linux/res_counter.h
> @@ -109,12 +109,18 @@ void res_counter_init(struct res_counter *counter, struct res_counter *parent);
>   *
>   * returns 0 on success and <0 if the counter->usage will exceed the
>   * counter->limit _locked call expects the counter->lock to be taken
> + *
> + * charge_nofail works the same, except that it charges the resource
> + * counter unconditionally, and returns < 0 if the after the current
> + * charge we are over limit.
>   */

res_counter_margin() assumes usage <= limit is always true.  Just make
sure you return 0 if that is not the case, or the charge path can get
confused, thinking there is enough room and retry needlessly.

Otherwise, looks good.