[PATCH] mm/hugetlb: split hugetlb_cma in nodes with memory

[Anshuman Khandual]

Hello Barry,

On 07/08/2020 05:53 AM, Barry Song wrote:
> Rather than splitting huge_cma in online nodes, it is better to do it in
> nodes with memory.

Right, it makes sense to avoid nodes without memory, hence loosing portions
of CMA reservation intended for HugeTLB. N_MEMORY is better than N_ONLINE
and will help avoid this situation.

> For an ARM64 server with four numa nodes and only node0 has memory. If I
> set hugetlb_cma=4G in bootargs,
> 
> without this patch, I got the below printk:
> hugetlb_cma: reserve 4096 MiB, up to 1024 MiB per node
> hugetlb_cma: reserved 1024 MiB on node 0
> hugetlb_cma: reservation failed: err -12, node 1
> hugetlb_cma: reservation failed: err -12, node 2
> hugetlb_cma: reservation failed: err -12, node 3

As expected.

> 
> hugetlb_cma size is broken once the system has nodes without memory.

I would not say that it is 'broken'. It is just not optimal but still works
as designed.

> 
> With this patch, I got the below printk:
> hugetlb_cma: reserve 4096 MiB, up to 4096 MiB per node
> hugetlb_cma: reserved 4096 MiB on node 0

As expected, the per node CMA reservation quota has changed from N_ONLINE
to N_MEMORY.

> 
> So this patch fixes the broken hugetlb_cma size on arm64.

There is nothing arm64 specific here. A platform where N_ONLINE != N_MEMORY
i.e with some nodes without memory when CMA reservation gets called, will
have this problem.

> 
> Jonathan Cameron tested this patch on x86 platform. Jonathan figured out x86
> is much different with arm64. hugetlb_cma size has never broken on x86.
> On arm64 all nodes are marked online at the same time. On x86, only
> nodes with memory are initially marked as online:
> initmem_init()->x86_numa_init()->numa_init()->
> numa_register_memblks()->alloc_node_data()->node_set_online()
> So at time of the existing cma setup call only the memory containing nodes
> are online. The other nodes are brought up much later.

The problem is always there if N_ONLINE != N_MEMORY but in this case, it
is just hidden because N_ONLINE happen to match N_MEMORY during the boot
process when hugetlb_cma_reserve() gets called.

> 
> Thus, the change is simply to fix ARM64.  A change is needed to x86 only
> because the inherent assumptions in cma_hugetlb_reserve() have changed.

cma_hugetlb_reserve() will now scan over N_MEMORY and hence expects all
platforms to have N_MEMORY initialized properly before calling it. This
needs to be well documented for the hugetlb_cma_reserve() function along
with it's call sites.

> 
> Fixes: cf11e85fc08c ("mm: hugetlb: optionally allocate gigantic hugepages using cma")

I would not call this a "Fix". The current code still works, though in
a sub optimal manner.

> Cc: Roman Gushchin <guro at fb.com>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: Will Deacon <will at kernel.org>
> Cc: Thomas Gleixner <tglx at linutronix.de>
> Cc: Ingo Molnar <mingo at redhat.com>
> Cc: Borislav Petkov <bp at alien8.de>
> Cc: H. Peter Anvin <hpa at zytor.com>
> Cc: Mike Kravetz <mike.kravetz at oracle.com>
> Cc: Mike Rapoport <rppt at linux.ibm.com>
> Cc: Andrew Morton <akpm at linux-foundation.org>
> Cc: Anshuman Khandual <anshuman.khandual at arm.com>
> Cc: Jonathan Cameron <jonathan.cameron at huawei.com>
> Signed-off-by: Barry Song <song.bao.hua at hisilicon.com>
> ---
>  arch/arm64/mm/init.c    | 18 +++++++++---------
>  arch/x86/kernel/setup.c | 13 ++++++++++---
>  mm/hugetlb.c            |  4 ++--
>  3 files changed, 21 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 1e93cfc7c47a..f6090ef6812b 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -420,15 +420,6 @@ void __init bootmem_init(void)
>  
>  	arm64_numa_init();
>  
> -	/*
> -	 * must be done after arm64_numa_init() which calls numa_init() to
> -	 * initialize node_online_map that gets used in hugetlb_cma_reserve()
> -	 * while allocating required CMA size across online nodes.
> -	 */
> -#ifdef CONFIG_ARM64_4K_PAGES
> -	hugetlb_cma_reserve(PUD_SHIFT - PAGE_SHIFT);
> -#endif
> -
>  	/*
>  	 * Sparsemem tries to allocate bootmem in memory_present(), so must be
>  	 * done after the fixed reservations.
> @@ -438,6 +429,15 @@ void __init bootmem_init(void)
>  	sparse_init();
>  	zone_sizes_init(min, max);
>  
> +	/*
> +	 * must be done after zone_sizes_init() which calls node_set_state() to
> +	 * setup node_states[N_MEMORY] that gets used in hugetlb_cma_reserve()
> +	 * while allocating required CMA size across nodes with memory.
> +	 */

Needs better wording here, in particular a reference to free_area_init()
that updates N_MEMORY via node_set_state(). Also mention the fact that
now hugetlb_cma_reserve() scans over N_MEMORY nodemask and hence expects
the platforms to have a properly initialized one.

> +#ifdef CONFIG_ARM64_4K_PAGES
> +	hugetlb_cma_reserve(PUD_SHIFT - PAGE_SHIFT);
> +#endif
> +
>  	memblock_dump_all();
>  }
>  
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index a3767e74c758..fdb3a934b6c6 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -1164,9 +1164,6 @@ void __init setup_arch(char **cmdline_p)
>  	initmem_init();
>  	dma_contiguous_reserve(max_pfn_mapped << PAGE_SHIFT);
>  
> -	if (boot_cpu_has(X86_FEATURE_GBPAGES))
> -		hugetlb_cma_reserve(PUD_SHIFT - PAGE_SHIFT);
> -
>  	/*
>  	 * Reserve memory for crash kernel after SRAT is parsed so that it
>  	 * won't consume hotpluggable memory.
> @@ -1180,6 +1177,16 @@ void __init setup_arch(char **cmdline_p)
>  
>  	x86_init.paging.pagetable_init();
>  
> +	/*
> +	 * must be done after zone_sizes_init() which calls node_set_state() to
> +	 * setup node_states[N_MEMORY] that gets used in hugetlb_cma_reserve()
> +	 * while allocating required CMA size across nodes with memory.
> +	 * And zone_sizes_init() is done in x86_init.paging.pagetable_init()
> +	 * which is typically paging_init().
> +	 */

Drop the last sentence here. Should have just the same comment as arm64.

> +	if (boot_cpu_has(X86_FEATURE_GBPAGES))
> +		hugetlb_cma_reserve(PUD_SHIFT - PAGE_SHIFT);
> +
>  	kasan_init();
>  
>  	/*
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index d293c823121e..3a0ad49187e4 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5699,12 +5699,12 @@ void __init hugetlb_cma_reserve(int order)
>  	 * If 3 GB area is requested on a machine with 4 numa nodes,
>  	 * let's allocate 1 GB on first three nodes and ignore the last one.
>  	 */
> -	per_node = DIV_ROUND_UP(hugetlb_cma_size, nr_online_nodes);
> +	per_node = DIV_ROUND_UP(hugetlb_cma_size, num_node_state(N_MEMORY));
>  	pr_info("hugetlb_cma: reserve %lu MiB, up to %lu MiB per node\n",
>  		hugetlb_cma_size / SZ_1M, per_node / SZ_1M);
>  
>  	reserved = 0;
> -	for_each_node_state(nid, N_ONLINE) {
> +	for_each_node_state(nid, N_MEMORY) {
>  		int res;
>  
>  		size = min(per_node, hugetlb_cma_size - reserved);
> 

The patch makes sense. But it needs better articulation of the problem in
the commit message, specifically pointing out the fact that it originates
primarily from a scenario where N_ONLINE != N_MEMORY because the presence
of memory less online nodes. It manifests itself on arm64 because of how
N_ONLINE and N_MEMORY gets initialized during boot but remains hidden on
x86 for the very same reason.