linux-kernel - Re: [PATCH V2] fork: Improve error message for corrupted page tables

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Date:   Tue, 6 Aug 2019 09:53:46 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:     dave.hansen@...el.com, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH V2] fork: Improve error message for corrupted page tables


On 8/6/19 5:05 AM, Sai Praneeth Prakhya wrote:
> 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@...nel.org>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Anshuman Khandual <anshuman.khandual@....com>
> Acked-by: Dave Hansen <dave.hansen@...el.com>
> Suggested-by: Dave Hansen <dave.hansen@...el.com>
> Signed-off-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

I would also add something like this to reduce risk of breaking it in the
future:

----8<----
diff --git a/include/linux/mm_types_task.h b/include/linux/mm_types_task.h
index d7016dcb245e..a6f83cbe4603 100644
--- a/include/linux/mm_types_task.h
+++ b/include/linux/mm_types_task.h
@@ -36,6 +36,9 @@ struct vmacache {
 	struct vm_area_struct *vmas[VMACACHE_SIZE];
 };
 
+/*
+ * When touching this, update also resident_page_types in kernel/fork.c
+ */
 enum {
 	MM_FILEPAGES,	/* Resident file mapping pages */
 	MM_ANONPAGES,	/* Resident anonymous pages */