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

[Barry Song <21cnbao@gmail.com>]

>> 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.

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