Re: [PATCH 1/2] mm, oom: Give __GFP_NOFAIL allocations access to memory reserves

[David Rientjes]

On Wed, 25 Nov 2015, Michal Hocko wrote:

> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> __GFP_NOFAIL is a big hammer used to ensure that the allocation
> request can never fail. This is a strong requirement and as such
> it also deserves a special treatment when the system is OOM. The
> primary problem here is that the allocation request might have
> come with some locks held and the oom victim might be blocked
> on the same locks. This is basically an OOM deadlock situation.
> 
> This patch tries to reduce the risk of such a deadlocks by giving
> __GFP_NOFAIL allocations a special treatment and let them dive into
> memory reserves after oom killer invocation. This should help them
> to make a progress and release resources they are holding. The OOM
> victim should compensate for the reserves consumption.
> 
> Suggested-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
>  mm/page_alloc.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 8034909faad2..70db11c27046 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2766,8 +2766,13 @@ __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);
> +	}
>  out:
>  	mutex_unlock(&oom_lock);
>  	return page;

I don't understand why you're setting ALLOC_CPUSET if you're giving them 
"special treatment".  If you want to allow access to memory reserves to 
prevent an oom livelock, then why not also allow it access to allocate 
outside its cpuset?
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