Messages in this thread | | | Date | Wed, 15 Feb 2023 18:19:56 +0800 | Subject | Re: [PATCH] mm: page_alloc: don't allocate page from memoryless nodes | From | Qi Zheng <> |
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On 2023/2/15 18:08, Mike Rapoport wrote: > On Wed, Feb 15, 2023 at 05:41:52PM +0800, Qi Zheng wrote: >> >> >> On 2023/2/15 17:30, 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: >> >> I'm curious how we can test this? I guess no one remembers the >> historical background of NODE_MIN_SIZE. :( > > At the very least we can verify that your setup works fine with this ;-) > > Of course we cannot test the exact same configuration that NODE_MIN_SIZE > was supposed to fix, but there was a lot of effort to make core mm > initialization robust to cope with weird memory layouts and I'm quite > confident this won't break anything.
Got it. And for the following patch: Reviewed-by: Qi Zheng <zhengqi.arch@bytedance.com>
In addition, with this patch, although the crash problem I mentioned will not exist, I still think it is necessary to modify the find_next_best_node() and offline_pages() mentioned in the summary in my other reply. What do you think?
> >>> 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); >>> } >> >> -- >> Thanks, >> Qi >
-- Thanks, Qi
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