linux-kernel - Re: [PATCH v2] mm: vmstat: fix /proc/sys/vm/stat

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Date:   Thu, 25 Feb 2021 09:21:04 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     Roman Gushchin <guro@...com>
cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        kernel-team@...com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm: vmstat: fix /proc/sys/vm/stat_refresh generating
 false warnings

On Wed, 24 Feb 2021, Roman Gushchin wrote:
> On Tue, Feb 23, 2021 at 11:24:23PM -0800, Hugh Dickins wrote:
> > On Thu, 6 Aug 2020, Andrew Morton wrote:
> > > On Thu, 6 Aug 2020 16:38:04 -0700 Roman Gushchin <guro@...com> wrote:
> > 
> > August, yikes, I thought it was much more recent.
> > 
> > > 
> > > > it seems that Hugh and me haven't reached a consensus here.
> > > > Can, you, please, not merge this patch into 5.9, so we would have
> > > > more time to find a solution, acceptable for all?
> > > 
> > > No probs.  I already had a big red asterisk on it ;)
> > 
> > I've a suspicion that Andrew might be tiring of his big red asterisk,
> > and wanting to unload
> > mm-vmstat-fix-proc-sys-vm-stat_refresh-generating-false-warnings.patch
> > mm-vmstat-fix-proc-sys-vm-stat_refresh-generating-false-warnings-fix.patch
> > mm-vmstat-fix-proc-sys-vm-stat_refresh-generating-false-warnings-fix-2.patch
> > into 5.12.
> > 
> > I would prefer not, and reiterate my Nack: but no great harm will
> > befall the cosmos if he overrules that, and it does go through to
> > 5.12 - I'll just want to revert it again later.  And I do think a
> > more straightforward way of suppressing those warnings would be just
> > to delete the code that issues them, rather than brushing them under
> > a carpet of overtuning.
> 
> I'm actually fine with either option. My only concern is that if somebody
> will try to use the hugetlb_cma boot option AND /proc/sys/vm/stat_refresh
> together, they will get a false warning and report them to mm@ or will
> waste their time trying to debug a non-existing problem. It's not the end
> of the world.
> We can also make the warning conditional on CONFIG_DEBUG_VM, for example.
> 
> Please, let me know what's your preferred way to go forward.

My preferred way forward (for now: since we're all too busy to fix
the misbehaving stats) is for Andrew to drop your patch, and I'll post
three patches against current 5.12 in a few hours: one to restore the
check on the missing NR_VM_NODE_STAT_ITEMS, one to remove the -EINVAL
(which upsets test scripts at our end), one to suppress the warning on
nr_zone_write_pending, nr_writeback and nr_free_cma.

Hugh