Re: [PATCH] x86/mm: drop 4MB restriction on minimal NUMA node size

[David Hildenbrand]

On 17.10.23 08:22, Mike Rapoport wrote:
> From: "Mike Rapoport (IBM)" <rppt@xxxxxxxxxx>
>
> 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@xxxxxxxxxxxxx>
> Signed-off-by: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> Link: https://lore.kernel.org/all/20230212110305.93670-1-zhengqi.arch@xxxxxxxxxxxxx/

That's just a resend I assume? Or has anything changed?

--
Cheers,

David / dhildenb