Re: [PATCH] mm: page_alloc: don't allocate page from memoryless nodes

[David Hildenbrand <david@redhat.com>]

On 15.02.23 10:30, 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:
> 
>  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);
>   	}
>   

Hopefully it fixes the issue.

Acked-by: David Hildenbrand <david@redhat.com>


The 4 MiB looks like the magical MAX_ORDER (and/or pageblock) thingy to 
me. I recall that there were issues in the past when memory exposed to 
the buddy would only be partially covering a pageblock. IIRC, memblock 
should already take care to not expose memory to the buddy that is not 
aligned to MAX_ORDER boundaries -- correct?

-- 
Thanks,

David / dhildenb