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Message-ID: <251f7bdd-f431-4006-a560-3e208cd6f1d5@redhat.com>
Date:   Tue, 17 Oct 2023 09:35:39 +0200
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Subject: Re: [PATCH] x86/mm: drop 4MB restriction on minimal NUMA node size
Content-Language: en-US
From:   David Hildenbrand <david@redhat.com>
To:     Mike Rapoport <rppt@kernel.org>, x86@kernel.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Michal Hocko <mhocko@suse.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Qi Zheng <zhengqi.arch@bytedance.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
References: <20231017062215.171670-1-rppt@kernel.org>
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On 17.10.23 09:28, David Hildenbrand wrote:
> On 17.10.23 08:22, Mike Rapoport wrote:
>> From: "Mike Rapoport (IBM)" <rppt@kernel.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.
>>
>> 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.
>>
>> Reported-by: Qi Zheng <zhengqi.arch@bytedance.com>
>> Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
>> Acked-by: David Hildenbrand <david@redhat.com>
>> Acked-by: Michal Hocko <mhocko@suse.com>
>> Link: https://lore.kernel.org/all/20230212110305.93670-1-zhengqi.arch@bytedance.com/
> 
> That's just a resend I assume? Or has anything changed?

Saw the other mail now, so just a resend.

-- 
Cheers,

David / dhildenb