Date:   Wed, 15 Feb 2023 12:08:27 +0200
From:   Mike Rapoport <rppt@kernel.org>
To:     Qi Zheng <zhengqi.arch@bytedance.com>
Cc:     Michal Hocko <mhocko@suse.com>,
        David Hildenbrand <david@redhat.com>,
        Vlastimil Babka <vbabka@suse.cz>, akpm@linux-foundation.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Teng Hu <huteng.ht@bytedance.com>,
        Matthew Wilcox <willy@infradead.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        Oscar Salvador <osalvador@suse.de>,
        Muchun Song <muchun.song@linux.dev>, x86@kernel.org
Subject: Re: [PATCH] mm: page_alloc: don't allocate page from memoryless nodes
Message-ID: <Y+yvG66aalD0x9W3@kernel.org>
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On Wed, Feb 15, 2023 at 05:41:52PM +0800, Qi Zheng wrote:
> 
> 
> On 2023/2/15 17:30, Mike Rapoport wrote:
> > On Tue, Feb 14, 2023 at 02:38:44PM +0100, Michal Hocko wrote:
> > > On Tue 14-02-23 12:58:39, David Hildenbrand wrote:
> > > > On 14.02.23 12:48, David Hildenbrand wrote:
> > > > > On 14.02.23 12:44, Mike Rapoport wrote:
> > > > > > (added x86 folks)
> > > > > > 
> > > > > > On Tue, Feb 14, 2023 at 12:29:42PM +0100, David Hildenbrand wrote:
> > > > > > > On 14.02.23 12:26, Qi Zheng wrote:
> > > > > > > > On 2023/2/14 19:22, David Hildenbrand wrote:
> > > > > > > > > 
> > > > > > > > > TBH, this is the first time I hear of NODE_MIN_SIZE and it seems to be a
> > > > > > > > > pretty x86 specific thing.
> > > > > > > > > 
> > > > > > > > > Are we sure we want to get NODE_MIN_SIZE involved?
> > > > > > > > 
> > > > > > > > Maybe add an arch_xxx() to handle it?
> > > > > > > 
> > > > > > > I still haven't figured out what we want to achieve with NODE_MIN_SIZE at
> > > > > > > all. It smells like an arch-specific hack looking at
> > > > > > > 
> > > > > > > "Don't confuse VM with a node that doesn't have the minimum amount of
> > > > > > > memory"
> > > > > > > 
> > > > > > > Why shouldn't mm-core deal with that?
> > > > > > 
> > > > > > Well, a node with <4M RAM is not very useful and bears all the overhead of
> > > > > > an extra live node.
> > > > > 
> > > > > And totally not with 4.1M, haha.
> > > > > 
> > > > > I really like the "Might fix boot" in the commit description.
> > > > > 
> > > > > > 
> > > > > > But, hey, why won't we just drop that '< NODE_MIN_SIZE' and let people with
> > > > > > weird HW configurations just live with this?
> > > > > 
> > > > > 
> > > > > ;)
> > > > > 
> > > > 
> > > > Actually, remembering 09f49dca570a ("mm: handle uninitialized numa nodes
> > > > gracefully"), this might be the right thing to do. That commit assumes that
> > > > all offline nodes would get the pgdat allocated in free_area_init(). So that
> > > > we end up with an allocated pgdat for all possible nodes. The reasoning IIRC
> > > > was that we don't care about wasting memory in weird VM setups.
> > > 
> > > Yes, that is the case indeed. I suspect the NODE_MIN_SIZE is a relict of
> > > the past when some PXM entries were incorrect or fishy. I would just
> > > drop the check and see whether something breaks. Or make those involved
> > > back then remember whether this is addressing something that is relevant
> > > these days. Even 5MB node makes (as the memmap is allocated for the
> > > whole memory section anyway and that is 128MB) a very little sense if you ask me.
> > 
> > How about we try this:
> 
> I'm curious how we can test this? I guess no one remembers the
> historical background of NODE_MIN_SIZE. :(
 
At the very least we can verify that your setup works fine with this ;-)

Of course we cannot test the exact same configuration that NODE_MIN_SIZE
was supposed to fix, but there was a lot of effort to make core mm
initialization robust to cope with weird memory layouts and I'm quite
confident this won't break anything.
 
> >  From b670120bcacd3fe34a40d7179c70ca2ab69279e0 Mon Sep 17 00:00:00 2001
> > From: "Mike Rapoport (IBM)" <rppt@kernel.org>
> > Date: Wed, 15 Feb 2023 11:12:18 +0200
> > Subject: [PATCH] x86/mm: drop 4MB restriction on minimal NUMA node size
> > 
> > Qi Zheng reports crashes in a production environment and provides a
> > simplified example as a reproducer:
> > 
> >    For example, if we use qemu to start a two NUMA node kernel,
> >    one of the nodes has 2M memory (less than NODE_MIN_SIZE),
> >    and the other node has 2G, then we will encounter the
> >    following panic:
> > 
> >    [    0.149844] BUG: kernel NULL pointer dereference, address: 0000000000000000
> >    [    0.150783] #PF: supervisor write access in kernel mode
> >    [    0.151488] #PF: error_code(0x0002) - not-present page
> >    <...>
> >    [    0.156056] RIP: 0010:_raw_spin_lock_irqsave+0x22/0x40
> >    <...>
> >    [    0.169781] Call Trace:
> >    [    0.170159]  <TASK>
> >    [    0.170448]  deactivate_slab+0x187/0x3c0
> >    [    0.171031]  ? bootstrap+0x1b/0x10e
> >    [    0.171559]  ? preempt_count_sub+0x9/0xa0
> >    [    0.172145]  ? kmem_cache_alloc+0x12c/0x440
> >    [    0.172735]  ? bootstrap+0x1b/0x10e
> >    [    0.173236]  bootstrap+0x6b/0x10e
> >    [    0.173720]  kmem_cache_init+0x10a/0x188
> >    [    0.174240]  start_kernel+0x415/0x6ac
> >    [    0.174738]  secondary_startup_64_no_verify+0xe0/0xeb
> >    [    0.175417]  </TASK>
> >    [    0.175713] Modules linked in:
> >    [    0.176117] CR2: 0000000000000000
> > 
> > The crashes happen because of inconsistency between nodemask that has
> > nodes with less than 4MB as memoryless and the actual memory fed into
> > core mm.
> > 
> > The commit 9391a3f9c7f1 ("[PATCH] x86_64: Clear more state when ignoring
> > empty node in SRAT parsing") that introduced minimal size of a NUMA node
> > does not explain why a node size cannot be less than 4MB and what boot
> > failures this restriction might fix.
> > 
> > Since then a lot has changed and core mm won't confuse badly about small
> > node sizes.
> > 
> > Drop the limitation for the minimal node size.
> > 
> > Link: https://lore.kernel.org/all/20230212110305.93670-1-zhengqi.arch@bytedance.com/
> > Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
> > ---
> >   arch/x86/include/asm/numa.h | 7 -------
> >   arch/x86/mm/numa.c          | 7 -------
> >   2 files changed, 14 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/numa.h b/arch/x86/include/asm/numa.h
> > index e3bae2b60a0d..ef2844d69173 100644
> > --- a/arch/x86/include/asm/numa.h
> > +++ b/arch/x86/include/asm/numa.h
> > @@ -12,13 +12,6 @@
> >   #define NR_NODE_MEMBLKS		(MAX_NUMNODES*2)
> > -/*
> > - * Too small node sizes may confuse the VM badly. Usually they
> > - * result from BIOS bugs. So dont recognize nodes as standalone
> > - * NUMA entities that have less than this amount of RAM listed:
> > - */
> > -#define NODE_MIN_SIZE (4*1024*1024)
> > -
> >   extern int numa_off;
> >   /*
> > diff --git a/arch/x86/mm/numa.c b/arch/x86/mm/numa.c
> > index 2aadb2019b4f..55e3d895f15c 100644
> > --- a/arch/x86/mm/numa.c
> > +++ b/arch/x86/mm/numa.c
> > @@ -601,13 +601,6 @@ static int __init numa_register_memblks(struct numa_meminfo *mi)
> >   		if (start >= end)
> >   			continue;
> > -		/*
> > -		 * Don't confuse VM with a node that doesn't have the
> > -		 * minimum amount of memory:
> > -		 */
> > -		if (end && (end - start) < NODE_MIN_SIZE)
> > -			continue;
> > -
> >   		alloc_node_data(nid);
> >   	}
> 
> -- 
> Thanks,
> Qi

-- 
Sincerely yours,
Mike.