linux-kernel - Re: [PATCH 2/2] mm: do not warn on offline nodes unless the specific node is explicitly requested

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Date:   Thu, 24 May 2018 10:00:11 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...hadventures.net>,
        Vlastimil Babka <vbabka@...e.cz>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Reza Arbab <arbab@...ux.vnet.ibm.com>,
        Igor Mammedov <imammedo@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH 2/2] mm: do not warn on offline nodes unless the specific
 node is explicitly requested

On Thu 24-05-18 08:52:14, Anshuman Khandual wrote:
> On 05/23/2018 07:36 PM, Michal Hocko wrote:
> > On Wed 23-05-18 19:15:51, Anshuman Khandual wrote:
> >> On 05/23/2018 06:25 PM, Michal Hocko wrote:
> >>> when adding memory to a node that is currently offline.
> >>>
> >>> The VM_WARN_ON is just too loud without a good reason. In this
> >>> particular case we are doing
> >>> 	alloc_pages_node(node, GFP_KERNEL|__GFP_RETRY_MAYFAIL|__GFP_NOWARN, order)
> >>>
> >>> so we do not insist on allocating from the given node (it is more a
> >>> hint) so we can fall back to any other populated node and moreover we
> >>> explicitly ask to not warn for the allocation failure.
> >>>
> >>> Soften the warning only to cases when somebody asks for the given node
> >>> explicitly by __GFP_THISNODE.
> >>
> >> node hint passed here eventually goes into __alloc_pages_nodemask()
> >> function which then picks up the applicable zonelist irrespective of
> >> the GFP flag __GFP_THISNODE.
> > 
> > __GFP_THISNODE should enforce the given node without any fallbacks
> > unless something has changed recently.
> 
> Right. I was just saying requiring given preferred node to be online
> whose zonelist (hence allocation zone fallback order) is getting picked
> up during allocation and warning when that is not online still makes
> sense.

Why? We have a fallback and that is expected to be used. How does
offline differ from depleted node from the semantical point of view?

> We should only hide the warning if the allocation request has
> __GFP_NOWARN.
> 
> > 
> >> Though we can go into zones of other
> >> nodes if the present node (whose zonelist got picked up) does not
> >> have any memory in it's zones. So warning here might not be without
> >> any reason.
> > 
> > I am not sure I follow. Are you suggesting a different VM_WARN_ON?
> 
> I am just suggesting this instead.
> 
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index 036846fc00a6..7f860ea29ec6 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -464,7 +464,7 @@ 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(!node_online(nid));
> +	VM_WARN_ON(!(gfp_mask & __GFP_NOWARN) && !node_online(nid));
>  
>  	return __alloc_pages(gfp_mask, order, nid);
>  }

I have considered that but I fail to see why should we warn about
regular GFP_KERNEL allocations as mentioned above. Just consider an
allocation for the preffered node. Do you want to warn just because that
node went offline?
-- 
Michal Hocko
SUSE Labs