linux-kernel - Re: [PATCH AUTOSEL 3.18 8/9] mm/vmstat.c: assert that vmstat_text is in sync with stat_items

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Date:   Thu, 15 Nov 2018 17:37:18 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     stable@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jann Horn <jannh@...gle.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Oleg Nesterov <oleg@...hat.com>,
        Christoph Lameter <clameter@....com>,
        Kemi Wang <kemi.wang@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm@...ck.org
Subject: Re: [PATCH AUTOSEL 3.18 8/9] mm/vmstat.c: assert that vmstat_text is
 in sync with stat_items_size

On Thu, Nov 15, 2018 at 02:08:10PM -0800, Andrew Morton wrote:
>On Tue, 13 Nov 2018 00:52:51 -0500 Sasha Levin <sashal@...nel.org> wrote:
>
>> From: Jann Horn <jannh@...gle.com>
>>
>> [ Upstream commit f0ecf25a093fc0589f0a6bc4c1ea068bbb67d220 ]
>>
>> Having two gigantic arrays that must manually be kept in sync, including
>> ifdefs, isn't exactly robust.  To make it easier to catch such issues in
>> the future, add a BUILD_BUG_ON().
>>
>> ...
>>
>> --- a/mm/vmstat.c
>> +++ b/mm/vmstat.c
>> @@ -1189,6 +1189,8 @@ static void *vmstat_start(struct seq_file *m, loff_t *pos)
>>  	stat_items_size += sizeof(struct vm_event_state);
>>  #endif
>>
>> +	BUILD_BUG_ON(stat_items_size !=
>> +		     ARRAY_SIZE(vmstat_text) * sizeof(unsigned long));
>>  	v = kmalloc(stat_items_size, GFP_KERNEL);
>>  	m->private = v;
>>  	if (!v)
>
>I don't think there's any way in which this can make a -stable kernel
>more stable!
>
>
>Generally, I consider -stable in every patch I merge, so for each patch
>which doesn't have cc:stable, that tag is missing for a reason.
>
>In other words, your criteria for -stable addition are different from
>mine.
>
>And I think your criteria differ from those described in
>Documentation/process/stable-kernel-rules.rst.
>
>So... what is your overall thinking on patch selection?

Indeed, this doesn't fix anything.

My concern is that in the future, we will pull a patch that will cause
the issue described here, and that issue will only be relevant on
stable. It is very hard to debug this, and I suspect that stable kernels
will still pass all their tests with flying colors.

As an example, consider the case where commit 28e2c4bb99aa ("mm/vmstat.c:
fix outdated vmstat_text") is backported to a kernel that doesn't have
commit 7a9cdebdcc17 ("mm: get rid of vmacache_flush_all() entirely").

I also felt safe with this patch since it adds a single BUILD_BUG_ON()
which does nothing during runtime, so the chances it introduces anything
beyond a build regression seemed to be slim to none.

--
Thanks,
Sasha