linux-kernel - Re: [PATCH -next v2] mm/hotplug: fix a null-ptr-deref during NUMA boot

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Date:   Fri, 31 May 2019 11:03:07 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Pingfan Liu <kernelfans@...il.com>
Cc:     Qian Cai <cai@....pw>, Andrew Morton <akpm@...ux-foundation.org>,
        Barret Rhoden <brho@...gle.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Ingo Molnar <mingo@...e.hu>,
        Oscar Salvador <osalvador@...e.de>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next v2] mm/hotplug: fix a null-ptr-deref during NUMA
 boot

On Thu 30-05-19 20:55:32, Pingfan Liu wrote:
> On Wed, May 29, 2019 at 2:20 AM Michal Hocko <mhocko@...nel.org> wrote:
> >
> > [Sorry for a late reply]
> >
> > On Thu 23-05-19 11:58:45, Pingfan Liu wrote:
> > > On Wed, May 22, 2019 at 7:16 PM Michal Hocko <mhocko@...nel.org> wrote:
> > > >
> > > > On Wed 22-05-19 15:12:16, Pingfan Liu wrote:
> > [...]
> > > > > But in fact, we already have for_each_node_state(nid, N_MEMORY) to
> > > > > cover this purpose.
> > > >
> > > > I do not really think we want to spread N_MEMORY outside of the core MM.
> > > > It is quite confusing IMHO.
> > > > .
> > > But it has already like this. Just git grep N_MEMORY.
> >
> > I might be wrong but I suspect a closer review would reveal that the use
> > will be inconsistent or dubious so following the existing users is not
> > the best approach.
> >
> > > > > Furthermore, changing the definition of online may
> > > > > break something in the scheduler, e.g. in task_numa_migrate(), where
> > > > > it calls for_each_online_node.
> > > >
> > > > Could you be more specific please? Why should numa balancing consider
> > > > nodes without any memory?
> > > >
> > > As my understanding, the destination cpu can be on a memory less node.
> > > BTW, there are several functions in the scheduler facing the same
> > > scenario, task_numa_migrate() is an example.
> >
> > Even if the destination node is memoryless then any migration would fail
> > because there is no memory. Anyway I still do not see how using online
> > node would break anything.
> >
> Suppose we have nodes A, B,C, where C is memory less but has little
> distance to B, comparing with the one from A to B. Then if a task is
> running on A, but prefer to run on B due to memory footprint.
> task_numa_migrate() allows us to migrate the task to node C. Changing
> for_each_online_node will break this.

That would require the task to have preferred node to be C no? Or do I
missunderstand the task migration logic?
-- 
Michal Hocko
SUSE Labs