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

[Sasha Levin <sashal@kernel.org>]

On Fri, Nov 16, 2018 at 09:55:25AM +0100, Michal Hocko wrote:
>On Thu 15-11-18 18:01:18, Sasha Levin wrote:
>> On Thu, Nov 15, 2018 at 02:47:19PM -0800, Andrew Morton wrote:
>> > On Thu, 15 Nov 2018 17:37:18 -0500 Sasha Levin <sashal@kernel.org> wrote:
>> >
>> > > 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@kernel.org> wrote:
>> > > >
>> > > >> From: Jann Horn <jannh@google.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 fix outdated vmstat_text") is backported to a kernel that doesn't have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.
>> >
>> > Well OK.  But my question was general and covers basically every
>> > autosel patch which originated in -mm.
>>
>> Sure. I picked 3 patches that show up on top when I google for AUTOSEL
>> in linux-mm, maybe they'll be a good example to help me understand why
>> they were not selected.
>>
>> This one fixes a case where too few struct pages are allocated when
>> using mirrorred memory:
>>
>> 	https://marc.info/?l=linux-mm&m=154211933211147&w=2
>
>Let me quote "I found this bug by reading the code." I do not think
>anybody has ever seen this in practice.
>
>> Race condition with memory hotplug due to missing locks:
>>
>> 	https://marc.info/?l=linux-mm&m=154211934011188&w=2
>
>Memory hotplug locking is dubious at best and this patch doesn't really
>fix it. It fixes a theoretical problem. I am not aware anybody would be
>hitting in practice. We need to rework the locking quite extensively.

The word "theoretical" used in the stable rules file does not mean
that we need to have actual reports of users hitting bugs before we
start backporting the relevant patch, it simply suggests that there
needs to be a reasonable explanation of how this issue can be hit.

For this memory hotplug patch in particular, I use the hv_balloon driver
at this very moment (running a linux guest on windows, with "dynamic
memory" enabled). Should I wait for it to crash before I can fix it?

Is the upstream code perfect? No, but that doesn't mean that it's not
working at all, and if there are users they expect to see fixes going in
and not just sitting idly waiting for a big rewrite that will come in a
few years.

Memory hotplug fixes are not something you think should go to stable?
Andrew sent a few of them to stable, so that can't be the case.

>> Raising an OOM event that causes issues in userspace when no OOM has
>> actually occured:
>>
>> 	https://marc.info/?l=linux-mm&m=154211939811582&w=2
>
>The patch makes sense I just do not think this is a stable material. The
>semantic of the event was and still is suboptimal.

I really fail to understand your reasoning about -stable here. This
patch is something people actually hit in the field, spent time on
triaging and analysing it, and submitting a fix which looks reasonably
straightforward.

That fix was acked by quite a few folks (including yourself) and merged
in. And as far as we can tell, it actually fixed the problem.

Why is it not stable material?

My understanding is that you're concerned with the patch itself being
"suboptimal", but in that case - why did you ack it?

>> I think that all 3 cases represent a "real" bug users can hit, and I
>> honestly don't know why they were not tagged for stable.
>
>It would be much better to ask in the respective email thread rather
>than spamming mailing with AUTOSEL patches which rarely get any
>attention.

I actually tried it, but the comments I got is that it gets in the way
and people preferred something they can filter.

>We have been through this discussion several times already and I thought
>we have agreed that those subsystems which are seriously considering stable
>are opted out from the AUTOSEL automagic. Has anything changed in that
>regards.

I checked in with Andrew to get his input on this, he suggested that
these patches should be sent to linux-mm and he'll give it a close look.

Ultimately this is the subsystem's decision, yes, but I was under the
impression that this decision wasn't made yet.

I guess that I'm really failing to understand why patches like the third
one here (the OOM one) are being kept out.

--
Thanks,
Sasha