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

[Anshuman Khandual]

On 08/06/2019 01:23 PM, Vlastimil Babka wrote:
> 
> 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@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>
> 
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> 
> 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 */
> 

Agreed and with that

Reviewed-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>