[PATCH V2] 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@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Anshuman Khandual <anshuman.khandual@xxxxxxx>
Acked-by: Dave Hansen <dave.hansen@xxxxxxxxx>
Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxx>
Signed-off-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@xxxxxxxxx>
---

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.

 kernel/fork.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index d8ae0f1b4148..f34f441c50c0 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -125,6 +125,13 @@ int nr_threads;			/* The idle threads do not count.. */
 
 static int max_threads;		/* tunable limit on nr_threads */
 
+static const char * const resident_page_types[NR_MM_COUNTERS] = {
+	[MM_FILEPAGES]		= "MM_FILEPAGES",
+	[MM_ANONPAGES]		= "MM_ANONPAGES",
+	[MM_SWAPENTS]		= "MM_SWAPENTS",
+	[MM_SHMEMPAGES]		= "MM_SHMEMPAGES",
+};
+
 DEFINE_PER_CPU(unsigned long, process_counts) = 0;
 
 __cacheline_aligned DEFINE_RWLOCK(tasklist_lock);  /* outer */
@@ -649,8 +656,8 @@ static void check_mm(struct mm_struct *mm)
 		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