linux-kernel - [PATCH] fork: Improve error message for corrupted page tables

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Date:   Tue, 30 Jul 2019 15:18:20 -0700
From:   Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>
To:     linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:     dave.hansen@...el.com,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH] 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). Hence, improve the
error message so that it's more informative.

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

Cc: Ingo Molnar <mingo@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Suggested-by/Acked-by: Dave Hansen <dave.hansen@...el.com>
Signed-off-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>
---
 include/linux/mm_types_task.h | 7 +++++++
 kernel/fork.c                 | 4 ++--
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm_types_task.h b/include/linux/mm_types_task.h
index d7016dcb245e..881f4ea3a1b5 100644
--- a/include/linux/mm_types_task.h
+++ b/include/linux/mm_types_task.h
@@ -44,6 +44,13 @@ enum {
 	NR_MM_COUNTERS
 };
 
+static const char * const resident_page_types[NR_MM_COUNTERS] = {
+	"MM_FILEPAGES",
+	"MM_ANONPAGES",
+	"MM_SWAPENTS",
+	"MM_SHMEMPAGES",
+};
+
 #if USE_SPLIT_PTE_PTLOCKS && defined(CONFIG_MMU)
 #define SPLIT_RSS_COUNTING
 /* per-thread cached information, */
diff --git a/kernel/fork.c b/kernel/fork.c
index 2852d0e76ea3..6aef5842d4e0 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -649,8 +649,8 @@ 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))
-- 
2.19.1