Re: [patch 2/3] memcg: prevent endless loop on huge page charge

[Gleb Natapov <gleb@redhat.com>]

On Thu, Jan 27, 2011 at 02:46:45PM +0100, Johannes Weiner wrote:
> The charging code can encounter a charge size that is bigger than a
> regular page in two situations: one is a batched charge to fill the
> per-cpu stocks, the other is a huge page charge.
> 
> This code is distributed over two functions, however, and only the
> outer one is aware of huge pages.  In case the charging fails, the
> inner function will tell the outer function to retry if the charge
> size is bigger than regular pages--assuming batched charging is the
> only case.  And the outer function will retry forever charging a huge
> page.
> 
> This patch makes sure the inner function can distinguish between batch
> charging and a single huge page charge.  It will only signal another
> attempt if batch charging failed, and go into regular reclaim when it
> is called on behalf of a huge page.
> 
Yeah, that is exactly the case I am debugging right now. Came up with
different solution: pass page_size to __mem_cgroup_do_charge() and
compare csize with page_size (not CHARGE_SIZE). Not sure which solution
it more correct.

> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
>  mm/memcontrol.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 8fa4be3..17c4e36 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1847,7 +1847,7 @@ static int __mem_cgroup_do_charge(struct mem_cgroup *mem, gfp_t gfp_mask,
>  	} else
>  		mem_over_limit = mem_cgroup_from_res_counter(fail_res, res);
>  
> -	if (csize > PAGE_SIZE) /* change csize and retry */
> +	if (csize == CHARGE_SIZE) /* retry without batching */
>  		return CHARGE_RETRY;
>  
>  	if (!(gfp_mask & __GFP_WAIT))
> -- 
> 1.7.3.5
> 
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--
			Gleb.