Re: [PATCH] mm, oom: Give __GFP_NOFAIL allocations access to memory reserves

[Vlastimil Babka <vbabka@suse.cz>]

On 11.11.2015 14:48, mhocko@kernel.org wrote:
>  mm/page_alloc.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 8034909faad2..d30bce9d7ac8 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2766,8 +2766,16 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
>  			goto out;
>  	}
>  	/* Exhausted what can be done so it's blamo time */
> -	if (out_of_memory(&oc) || WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL))
> +	if (out_of_memory(&oc) || WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL)) {
>  		*did_some_progress = 1;
> +
> +		if (gfp_mask & __GFP_NOFAIL) {
> +			page = get_page_from_freelist(gfp_mask, order,
> +					ALLOC_NO_WATERMARKS|ALLOC_CPUSET, ac);
> +			WARN_ONCE(!page, "Unable to fullfil gfp_nofail allocation."
> +				    " Consider increasing min_free_kbytes.\n");

It seems redundant to me to keep the WARN_ON_ONCE also above in the if () part?
Also s/gfp_nofail/GFP_NOFAIL/ for consistency?

Hm and probably out of scope of your patch, but I understand the WARN_ONCE
(WARN_ON_ONCE) to be _ONCE just to prevent a flood from a single task looping
here. But for distinct tasks and potentially far away in time, wouldn't we want
to see all the warnings? Would that be feasible to implement?

> +		}
> +	}
>  out:
>  	mutex_unlock(&oom_lock);
>  	return page;
>