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

[Sasha Levin]

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@xxxxxxxxxx> 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@xxxxxxxxxx> wrote:
> > >
> > >> From: Jann Horn <jannh@xxxxxxxxxx>
> > >>
> > >> [ 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 fix outdated vmstat_text") is backported to a kernel that doesn't have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

Race condition with memory hotplug due to missing locks:

	https://marc.info/?l=linux-mm&m=154211934011188&w=2

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


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.

--
Thanks,
Sasha