linux-kernel - Re: [RFC PATCH 1/2] include/linux/gfp.h: Do not allocate pages on a offlined node

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Message-ID: <Ya3WZC1JWU5jdtlC@dhcp22.suse.cz>
Date:   Mon, 6 Dec 2021 10:22:44 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Nico Pache <npache@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, shakeelb@...gle.com,
        ktkhai@...tuozzo.com, shy828301@...il.com, guro@...com,
        vbabka@...e.cz, vdavydov.dev@...il.com, raquini@...hat.com,
        Rafael Aquini <aquini@...hat.com>
Subject: Re: [RFC PATCH 1/2] include/linux/gfp.h: Do not allocate pages on a
 offlined node

On Sun 05-12-21 22:33:37, Nico Pache wrote:
> We shouldn't allocate pages on a unavailable node. Add a check for this
> in __alloc_pages_node and return NULL to avoid issues further down the
> callchain.
> 
> Also update the VM_WARN_ON in __alloc_pages_node which could skip this
> warn if the gfp_mask is not GFP_THISNODE.

Page allocator trusts callers to know they are doing a right thing so
that no unnecessary branches have to be implemented and the fast path is
really optimized for performance. Allocating from an !node_online node
is a bug in the caller. One that is not really that hard to make
unfortunatelly but also one that is is not that common.

That being said I am not really sure we want to introduce this check.

> Co-developed-by: Rafael Aquini <aquini@...hat.com>
> Signed-off-by: Nico Pache <npache@...hat.com>
> ---
>  include/linux/gfp.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index b976c4177299..e7e18f6d0d9d 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -565,7 +565,10 @@ static inline struct page *
>  __alloc_pages_node(int nid, gfp_t gfp_mask, unsigned int order)
>  {
>  	VM_BUG_ON(nid < 0 || nid >= MAX_NUMNODES);
> -	VM_WARN_ON((gfp_mask & __GFP_THISNODE) && !node_online(nid));
> +	VM_WARN_ON(!node_online(nid));
> +
> +	if (!node_online(nid))
> +		return NULL;
>  
>  	return __alloc_pages(gfp_mask, order, nid, NULL);
>  }
> -- 
> 2.33.1

-- 
Michal Hocko
SUSE Labs