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

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Date:   Tue, 7 Dec 2021 16:24:23 -0500
From:   Nico Pache <npache@...hat.com>
To:     Michal Hocko <mhocko@...e.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 12/6/21 04:22, Michal Hocko wrote:
> 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.

Thank you for your review,

That makes sense. I will drop this patch on my second posting to avoid any
performance regression.

Cheers,
-- Nico

> 
> 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
>