linux-kernel - Re: [PATCH] mm: Fix vmstat after removing NR

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Message-ID: <scchmajjawfmmoreihui4yyzuyutzf3evhmmx2j4f2lhu6r62n@ovaamezmqgun>
Date: Thu, 29 May 2025 13:51:48 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Vlastimil Babka <vbabka@...e.cz>, 
	Konstantin Khlebnikov <koct9i@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	David Hildenbrand <david@...hat.com>, lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, 
	rppt@...nel.org, surenb@...gle.com, mhocko@...e.com, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Hongyu Ning <hongyu.ning@...ux.intel.com>, 
	stable@...r.kernel.org, Christoph Hellwig <hch@....de>, Hannes Reinecke <hare@...e.de>, 
	Johannes Thumshirn <johannes.thumshirn@....com>, Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] mm: Fix vmstat after removing NR_BOUNCE

On Thu, May 29, 2025 at 01:47:10PM +0300, Kirill A. Shutemov wrote:
> On Thu, May 29, 2025 at 12:40:21PM +0200, Vlastimil Babka wrote:
> > On 5/29/25 12:38, Kirill A. Shutemov wrote:
> > > Hongyu noticed that the nr_unaccepted counter kept growing even in the
> > > absence of unaccepted memory on the machine.
> > > 
> > > This happens due to a commit that removed NR_BOUNCE: it removed the
> > > counter from the enum zone_stat_item, but left it in the vmstat_text
> > > array.
> > > 
> > > As a result, all counters below nr_bounce in /proc/vmstat are
> > > shifted by one line, causing the numa_hit counter to be labeled as
> > > nr_unaccepted.
> > > 
> > > To fix this issue, remove nr_bounce from the vmstat_text array.
> > > 
> > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > > Reported-by: Hongyu Ning <hongyu.ning@...ux.intel.com>
> > > Fixes: 194df9f66db8 ("mm: remove NR_BOUNCE zone stat")
> > > Cc: stable@...r.kernel.org
> > > Cc: Christoph Hellwig <hch@....de>
> > > Cc: Hannes Reinecke <hare@...e.de>
> > > Cc: Johannes Thumshirn <johannes.thumshirn@....com>
> > > Cc: Jens Axboe <axboe@...nel.dk>
> > 
> > Is there a way to add a BUILD_BUG_ON to catch a future case like this one?
> 
> There's
> 
> 	BUILD_BUG_ON(ARRAY_SIZE(vmstat_text) < NR_VMSTAT_ITEMS);
> 
> in vmstat_start().
> 
> Making it strict != seems to do the trick for my config. But it requires
> wider testing.
> 
> I can prepare a patch for that.

There was a strict check before 9d7ea9a297e6 ("mm/vmstat: add helpers to
get vmstat item names for each enum type"). Not sure if changing != to <
was intentional.

Konstantin?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov