[RFC v2 PATCH] mm/percpu.c: fix panic triggered by BUG_ON() falsely

[zijun_hu <zijun_hu@zoho.com>]

From: zijun_hu <zijun_hu@htc.com>

as shown by pcpu_build_alloc_info(), the number of units within a percpu
group is educed by rounding up the number of CPUs within the group to
@upa boundary, therefore, the number of CPUs isn't equal to the units's
if it isn't aligned to @upa normally. however, pcpu_page_first_chunk()
uses BUG_ON() to assert one number is equal the other roughly, so a panic
is maybe triggered by the BUG_ON() falsely.

in order to fix this issue, the number of CPUs is rounded up then compared
with units's, the BUG_ON() is replaced by warning and returning error code
as well to keep system alive as much as possible.

Signed-off-by: zijun_hu <zijun_hu@htc.com>
---
 Changes in v2:
  - fix build error

 mm/percpu.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/mm/percpu.c b/mm/percpu.c
index 32e2d8d128c1..ab1186c68ab6 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -2095,6 +2095,8 @@ int __init pcpu_page_first_chunk(size_t reserved_size,
 	size_t pages_size;
 	struct page **pages;
 	int unit, i, j, rc;
+	int upa;
+	int nr_g0_units;
 
 	snprintf(psize_str, sizeof(psize_str), "%luK", PAGE_SIZE >> 10);
 
@@ -2102,7 +2104,12 @@ int __init pcpu_page_first_chunk(size_t reserved_size,
 	if (IS_ERR(ai))
 		return PTR_ERR(ai);
 	BUG_ON(ai->nr_groups != 1);
-	BUG_ON(ai->groups[0].nr_units != num_possible_cpus());
+	upa = ai->alloc_size/ai->unit_size;
+	nr_g0_units = roundup(num_possible_cpus(), upa);
+	if (unlikely(WARN_ON(ai->groups[0].nr_units != nr_g0_units))) {
+		pcpu_free_alloc_info(ai);
+		return -EINVAL;
+	}
 
 	unit_pages = ai->unit_size >> PAGE_SHIFT;
 
@@ -2113,21 +2120,22 @@ int __init pcpu_page_first_chunk(size_t reserved_size,
 
 	/* allocate pages */
 	j = 0;
-	for (unit = 0; unit < num_possible_cpus(); unit++)
+	for (unit = 0; unit < num_possible_cpus(); unit++) {
+		unsigned int cpu = ai->groups[0].cpu_map[unit];
 		for (i = 0; i < unit_pages; i++) {
-			unsigned int cpu = ai->groups[0].cpu_map[unit];
 			void *ptr;
 
 			ptr = alloc_fn(cpu, PAGE_SIZE, PAGE_SIZE);
 			if (!ptr) {
 				pr_warn("failed to allocate %s page for cpu%u\n",
-					psize_str, cpu);
+						psize_str, cpu);
 				goto enomem;
 			}
 			/* kmemleak tracks the percpu allocations separately */
 			kmemleak_free(ptr);
 			pages[j++] = virt_to_page(ptr);
 		}
+	}
 
 	/* allocate vm area, map the pages and copy static data */
 	vm.flags = VM_ALLOC;
-- 
1.9.1

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