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

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