Date:   Wed, 15 Feb 2023 11:30:19 +0200
From:   Mike Rapoport <rppt@kernel.org>
To:     Michal Hocko <mhocko@suse.com>
Cc:     David Hildenbrand <david@redhat.com>,
        Qi Zheng <zhengqi.arch@bytedance.com>,
        Qi Zheng <arch0.zheng@gmail.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
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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:

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);
 	}
 
-- 
2.35.1


> -- 
> Michal Hocko
> SUSE Labs

-- 
Sincerely yours,
Mike.