fork: improve error message for corrupted page tables

When a user process exits, the kernel cleans up the mm_struct of the user
process and during cleanup, check_mm() checks the page tables of the user
process for corruption (E.g: unexpected page flags set/cleared).  For
corrupted page tables, the error message printed by check_mm() isn't very
clear as it prints the loop index instead of page table type (E.g:
Resident file mapping pages vs Resident shared memory pages).  The loop
index in check_mm() is used to index rss_stat[] which represents
individual memory type stats.  Hence, instead of printing index, print
memory type, thereby improving error message.

Without patch:
--------------
[  204.836425] mm/pgtable-generic.c:29: bad p4d 0000000089eb4e92(800000025f941467)
[  204.836544] BUG: Bad rss-counter state mm:00000000f75895ea idx:0 val:2
[  204.836615] BUG: Bad rss-counter state mm:00000000f75895ea idx:1 val:5
[  204.836685] BUG: non-zero pgtables_bytes on freeing mm: 20480

With patch:
-----------
[   69.815453] mm/pgtable-generic.c:29: bad p4d 0000000084653642(800000025ca37467)
[   69.815872] BUG: Bad rss-counter state mm:00000000014a6c03 type:MM_FILEPAGES val:2
[   69.815962] BUG: Bad rss-counter state mm:00000000014a6c03 type:MM_ANONPAGES val:5
[   69.816050] BUG: non-zero pgtables_bytes on freeing mm: 20480

Also, change print function (from printk(KERN_ALERT, ..) to pr_alert()) so
that it matches the other print statement.

Link: http://lkml.kernel.org/r/da75b5153f617f4c5739c08ee6ebeb3d19db0fbc.1565123758.git.sai.praneeth.prakhya@intel.com
Signed-off-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Suggested-by: Dave Hansen <dave.hansen@intel.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Dave Hansen <dave.hansen@intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mm_types_task.h

@@ -36,6 +36,10 @@ struct vmacache {
 	struct vm_area_struct *vmas[VMACACHE_SIZE];
 };
 
+/*
+ * When updating this, please also update struct resident_page_types[] in
+ * kernel/fork.c
+ */
 enum {
 	MM_FILEPAGES,	/* Resident file mapping pages */
 	MM_ANONPAGES,	/* Resident anonymous pages */

kernel/fork.c

@@ -125,6 +125,15 @@ int nr_threads;			/* The idle threads do not count.. */
 
 static int max_threads;		/* tunable limit on nr_threads */
 
+#define NAMED_ARRAY_INDEX(x)	[x] = __stringify(x)
+
+static const char * const resident_page_types[] = {
+	NAMED_ARRAY_INDEX(MM_FILEPAGES),
+	NAMED_ARRAY_INDEX(MM_ANONPAGES),
+	NAMED_ARRAY_INDEX(MM_SWAPENTS),
+	NAMED_ARRAY_INDEX(MM_SHMEMPAGES),
+};
+
 DEFINE_PER_CPU(unsigned long, process_counts) = 0;
 
 __cacheline_aligned DEFINE_RWLOCK(tasklist_lock);  /* outer */
@@ -645,12 +654,15 @@ static void check_mm(struct mm_struct *mm)
 {
 	int i;
 
+	BUILD_BUG_ON_MSG(ARRAY_SIZE(resident_page_types) != NR_MM_COUNTERS,
+			 "Please make sure 'struct resident_page_types[]' is updated as well");
+
 	for (i = 0; i < NR_MM_COUNTERS; i++) {
 		long x = atomic_long_read(&mm->rss_stat.count[i]);
 
 		if (unlikely(x))
-			printk(KERN_ALERT "BUG: Bad rss-counter state "
-					  "mm:%p idx:%d val:%ld\n", mm, i, x);
+			pr_alert("BUG: Bad rss-counter state mm:%p type:%s val:%ld\n",
+				 mm, resident_page_types[i], x);
 	}
 
 	if (mm_pgtables_bytes(mm))