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@xxxxxxxxxx>
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@xxxxxxxxxxxxx/
Signed-off-by: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>
---
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.
