linux-kernel - Re: [PATCH] mm: warn only once if page table misaccounting is detected

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Date:   Tue, 27 Nov 2018 08:55:47 -0700
From:   William Kucharski <william.kucharski@...cle.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Heiko Carstens <heiko.carstens@...ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-mm@...ck.org, "Kirill A . Shutemov" <kirill@...temov.name>,
        Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH] mm: warn only once if page table misaccounting is
 detected



> On Nov 27, 2018, at 8:52 AM, Sean Christopherson <sean.j.christopherson@...el.com> wrote:
> 
> On Tue, Nov 27, 2018 at 09:36:03AM +0100, Heiko Carstens wrote:
>> Use pr_alert_once() instead of pr_alert() if page table misaccounting
>> has been detected.
>> 
>> If this happens once it is very likely that there will be numerous
>> other occurrence as well, which would flood dmesg and the console with
>> hardly any added information. Therefore print the warning only once.
>> 
>> Cc: Kirill A. Shutemov <kirill@...temov.name>
>> Cc: Martin Schwidefsky <schwidefsky@...ibm.com>
>> Signed-off-by: Heiko Carstens <heiko.carstens@...ibm.com>
>> ---
>> kernel/fork.c | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index 07cddff89c7b..c887e9eba89f 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -647,8 +647,8 @@ static void check_mm(struct mm_struct *mm)
>> 	}
>> 
>> 	if (mm_pgtables_bytes(mm))
>> -		pr_alert("BUG: non-zero pgtables_bytes on freeing mm: %ld\n",
>> -				mm_pgtables_bytes(mm));
>> +		pr_alert_once("BUG: non-zero pgtables_bytes on freeing mm: %ld\n",
>> +			      mm_pgtables_bytes(mm));
> 
> I found the print-always behavior to be useful when developing a driver
> that mucked with PTEs directly via vmf_insert_pfn() and had issues with
> racing against exit_mmap().  It was nice to be able to recompile only
> the driver and rely on dmesg to let me know when I messed up yet again.
> 
> Would pr_alert_ratelimited() suffice?

Actually, I really like that idea.

There are certainly times when it is useful to see a cascade of messages, within reason;
one there are so many they overflow the dmesg buffer they're of limited usefulness.

Something like a pr_alert() that could rate limit to a preset value, perhaps a default of
fifty or so, could prove quite useful indeed without being an all or once choice.

    William Kucharski