Re: [akpm-mm:mm-unstable 313/322] include/linux/huge_mm.h:274:16: error: fields must have a constant size: 'variable length array in structure' extension will never be supported

[Yujie Liu <yujie.liu@intel.com>]

Hi Barry,

On Sun, Apr 07, 2024 at 04:22:47PM +1200, Barry Song wrote:
> >> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-unstable
> >> head:   f43b3aae94511d62174c3b29239da0dd22d0eeb3
> >> commit: d4358ee0a075e232114dfec0cd162860cfa2771b [313/322] mm: add per-order mTHP anon_alloc and anon_alloc_fallback counters
> >> config: powerpc-allyesconfig (https://download.01.org/0day-ci/archive/20240406/202404061754.n8jmZ6s3-lkp@intel.com/config)
> >> compiler: clang version 19.0.0git (https://github.com/llvm/llvm-project 546dc2245ffc4cccd0b05b58b7a5955e355a3b27)
> >> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20240406/202404061754.n8jmZ6s3-lkp@intel.com/reproduce)
> >> 
> >> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> >> the same patch/commit), kindly add following tags
> >> | Reported-by: kernel test robot <lkp@intel.com>
> >> | Closes: https://lore.kernel.org/oe-kbuild-all/202404061754.n8jmZ6s3-lkp@intel.com/
> >> 
> >> All error/warnings (new ones prefixed by >>):
> >> 
> >>    In file included from arch/powerpc/kernel/asm-offsets.c:19:
> >>    In file included from include/linux/mman.h:5:
> >>    In file included from include/linux/mm.h:1115:
> >>>> include/linux/huge_mm.h:274:22: warning: variable length array used [-Wvla]
> >>      274 |         unsigned long stats[PMD_ORDER + 1][__MTHP_STAT_COUNT];
> >          |                             ^~~~~~~~~~~~~
> > 
> > This is amazing. powerpc's PMD_ORDER isn't const. I will
> > move to alloc_percpu instead of using a static array.
> 
> As replied in another thread, can you please test the below patch?
> this is v2 combining the fix and the fix of the fix.

This v2 patch fixes the errors/warnings reported in both this thread
and thread [1].

[1] https://lore.kernel.org/all/202404061627.ungVloow-lkp@intel.com/

Tested-by: Yujie Liu <yujie.liu@intel.com>
 
> From 6f8604a51ff3333be467e2a681a1f1e8a5d41c48 Mon Sep 17 00:00:00 2001
> From: Barry Song <v-songbaohua@oppo.com>
> Date: Sun, 7 Apr 2024 12:23:35 +1200
> Subject: [PATCH v2] mm: fix powerpc build issue
> 
> On powerpc, PMD_ORDER is not a constant variable. We should transition
> to using alloc_percpu instead of a static percpu array.
> 
> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> ---
>  include/linux/huge_mm.h |  9 ++++-----
>  mm/huge_memory.c        | 12 ++++++++++--
>  2 files changed, 14 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index c5d33017a4dd..0717063dc5e7 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -271,16 +271,15 @@ enum mthp_stat_item {
>  };
>  
>  struct mthp_stat {
> -	unsigned long stats[PMD_ORDER + 1][__MTHP_STAT_COUNT];
> +	unsigned long stats[0][__MTHP_STAT_COUNT];
>  };
>  
> -DECLARE_PER_CPU(struct mthp_stat, mthp_stats);
> +extern struct mthp_stat __percpu *mthp_stats;
>  
>  static inline void count_mthp_stat(int order, enum mthp_stat_item item)
>  {
> -	if (unlikely(order > PMD_ORDER))
> -		return;
> -	this_cpu_inc(mthp_stats.stats[order][item]);
> +	if (likely(order <= PMD_ORDER))
> +		raw_cpu_ptr(mthp_stats)->stats[order][item]++;
>  }
>  
>  #define transparent_hugepage_use_zero_page()				\
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 5b875f0fc923..7a72760dc1ce 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -526,7 +526,7 @@ static const struct kobj_type thpsize_ktype = {
>  	.sysfs_ops = &kobj_sysfs_ops,
>  };
>  
> -DEFINE_PER_CPU(struct mthp_stat, mthp_stats) = {{{0}}};
> +struct mthp_stat __percpu *mthp_stats;
>  
>  static unsigned long sum_mthp_stat(int order, enum mthp_stat_item item)
>  {
> @@ -534,7 +534,7 @@ static unsigned long sum_mthp_stat(int order, enum mthp_stat_item item)
>  	int cpu;
>  
>  	for_each_online_cpu(cpu) {
> -		struct mthp_stat *this = &per_cpu(mthp_stats, cpu);
> +		struct mthp_stat *this = per_cpu_ptr(mthp_stats, cpu);
>  
>  		sum += this->stats[order][item];
>  	}
> @@ -636,6 +636,13 @@ static int __init hugepage_init_sysfs(struct kobject **hugepage_kobj)
>  		goto remove_hp_group;
>  	}
>  
> +	mthp_stats = __alloc_percpu((PMD_ORDER + 1) * sizeof(mthp_stats->stats[0]),
> +			sizeof(unsigned long));
> +	if (!mthp_stats) {
> +		err = -ENOMEM;
> +		goto remove_hp_group;
> +	}
> +
>  	orders = THP_ORDERS_ALL_ANON;
>  	order = highest_order(orders);
>  	while (orders) {
> @@ -673,6 +680,7 @@ static void __init hugepage_exit_sysfs(struct kobject *hugepage_kobj)
>  	sysfs_remove_group(hugepage_kobj, &khugepaged_attr_group);
>  	sysfs_remove_group(hugepage_kobj, &hugepage_attr_group);
>  	kobject_put(hugepage_kobj);
> +	free_percpu(mthp_stats);
>  }
>  #else
>  static inline int hugepage_init_sysfs(struct kobject **hugepage_kobj)
> -- 
> 2.34.1
> 
> 
>