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Message-ID: <605cc166-e731-e7d1-25d7-b6797a802e6f@bytedance.com>
Date: Wed, 18 Oct 2023 20:26:31 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Ingo Molnar <mingo@...nel.org>, Mike Rapoport <rppt@...nel.org>,
David Hildenbrand <david@...hat.com>,
Michal Hocko <mhocko@...e.com>,
Andrew Morton <akpm@...ux-foundation.org>
Cc: x86@...nel.org, Andy Lutomirski <luto@...nel.org>,
Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>,
"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2] x86/mm: Drop 4MB restriction on minimal NUMA node
memory size
Hi all,
On 2023/10/18 18:42, Ingo Molnar wrote:
>
> * Mike Rapoport <rppt@...nel.org> wrote:
>
>> From: "Mike Rapoport (IBM)" <rppt@...nel.org>
>>
>> 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.
>
> Presumably the core MM got fixed too to not just crash, but provide some
> sort of warning?
>
>> 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.
>
> Core MM won't get confused ... other than by the above weird Qemu topology,
> to which it responds with a ... NULL pointer dereference?
>
> Seems quite close to the literal definition of 'get confused badly' to me,
> and doesn't give me the warm fuzzy feeling that giving the core MM even
> *more* weird topologies is super safe ... :-/
>
>> Drop the limitation for the minimal node size.
>
> While I agree with dropping the limitation, and I agree that 9391a3f9c7f1
> should have provided more of a justification, I believe a core MM fix is in
> order as well, for it to not crash. [ If it's fixed upstream already,
> please reference the relevant commit ID. ]
Agree. I posted a fixed patchset[1] before, maybe we can reconsider
it. :)
[1].
https://lore.kernel.org/lkml/20230215152412.13368-1-zhengqi.arch@bytedance.com/
For memoryless node, this patchset skip it and fallback to other nodes
when build its zonelists.
Say we have node0 and node1, and node0 is memoryless, then:
[ 0.102400] Fallback order for Node 0: 1
[ 0.102931] Fallback order for Node 1: 1
In this way, we will not allocate pages from memoryless node0. Then
the crash problem under the weird Qemu topology will be fixed.
Thanks,
Qi
>
> Also, the changelog spelling & general presentation were quite low quality
> - I've fixed it up a bit below, please carry this version going forward.
> Please spell-check your patches before sending out Nth versions of it,
> maybe maintainers are skipping them for a reason!
>
> Thanks,
>
> Ingo
>
> =================>
> From: "Mike Rapoport (IBM)" <rppt@...nel.org>
> Date: Tue, 17 Oct 2023 09:22:15 +0300
> Subject: [PATCH] x86/mm: Drop 4MB restriction on minimal NUMA node memory size
>
> Qi Zheng reported crashes in a production environment and provided 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:
> |
> | BUG: kernel NULL pointer dereference, address: 0000000000000000
> | <...>
> | RIP: 0010:_raw_spin_lock_irqsave+0x22/0x40
> | <...>
> | Call Trace:
> | <TASK>
> | deactivate_slab()
> | bootstrap()
> | kmem_cache_init()
> | start_kernel()
> | secondary_startup_64_no_verify()
>
> The crashes happen because of inconsistency between the nodemask that
> has nodes with less than 4MB as memoryless, and the actual memory fed
> into the 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.
>
> In the 17 years since then a lot has changed and core mm won't get
> confused about small node sizes.
>
> Drop the limitation for the minimal node size.
>
> [ mingo: Improved changelog clarity. ]
>
> Reported-by: Qi Zheng <zhengqi.arch@...edance.com>
> Signed-off-by: Mike Rapoport (IBM) <rppt@...nel.org>
> Not-Yet-Signed-off-by: Ingo Molnar <mingo@...nel.org>
> Acked-by: David Hildenbrand <david@...hat.com>
> Acked-by: Michal Hocko <mhocko@...e.com>
> Link: https://lore.kernel.org/all/20230212110305.93670-1-zhengqi.arch@bytedance.com/
> ---
> 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 c01c5506fd4a..aa39d678fe81 100644
> --- a/arch/x86/mm/numa.c
> +++ b/arch/x86/mm/numa.c
> @@ -602,13 +602,6 @@ 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);
> }
>
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