[PATCH V3] fork: Improve error message for corrupted page tables

[PATCH V3] fork: Improve error message for corrupted page tables

[Sai Praneeth Prakhya]

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.

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

Changes from V2 to V3:
----------------------
1. Add comment that suggests to update resident_page_types[] if there are any
   changes to exisiting page types in <linux/mm_types_task.h>
2. Add a build check to enforce resident_page_types[] is always in sync
3. Use a macro to populate elements of resident_page_types[]

Changes from V1 to V2:
----------------------
1. Move struct definition from header file to fork.c file, so that it won't be
   included in every compilation unit. As this struct is used *only* in fork.c,
   include the definition in fork.c itself.
2. Index the struct to match respective macros.
3. Mention about print function change in commit message.

 include/linux/mm_types_task.h |  4 ++++
 kernel/fork.c                 | 16 ++++++++++++++--
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm_types_task.h b/include/linux/mm_types_task.h
index d7016dcb245e..c1bc6731125c 100644
--- a/include/linux/mm_types_task.h
+++ b/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 */
diff --git a/kernel/fork.c b/kernel/fork.c
index d8ae0f1b4148..7583e0fde0ed 100644
--- a/kernel/fork.c
+++ b/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))
-- 
2.7.4