linux-kernel - Re: [PATCH] mm/vmstat: Fix build with MEMCG=y and VM_EVENT

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Message-ID: <69a4b093-1fbd-4178-906b-960a3d8bc529@suse.cz>
Date: Thu, 5 Jun 2025 15:56:13 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.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, hannes@...xchg.org, shakeel.butt@...ux.dev,
 muchun.song@...ux.dev, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Randy Dunlap <rdunlap@...radead.org>,
 Konstantin Khlebnikov <koct9i@...il.com>
Subject: Re: [PATCH] mm/vmstat: Fix build with MEMCG=y and VM_EVENT_COUNTERS=n

On 6/5/25 13:53, Kirill A. Shutemov wrote:
> On Thu, Jun 05, 2025 at 08:19:28AM +0200, Vlastimil Babka wrote:
>> On 6/4/25 23:20, Andrew Morton wrote:
>> > On Wed, 4 Jun 2025 11:56:42 +0200 Vlastimil Babka <vbabka@...e.cz> wrote:
>> > 
>> >> > There is no need to backport this fix to stable trees. Without the
>> >> > strict BUILD_BUG_ON(), the issue is not harmful. The elements in
>> >> > question would only be read by the memcg code, not by /proc/vmstat.
>> >> > 
>> >> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
>> >> > Reported-by: Randy Dunlap <rdunlap@...radead.org>
>> >> > Fixes: ebc5d83d0443 ("mm/memcontrol: use vmstat names for printing statistics")
>> >> 
>> >> Well in that case I think we should put Fixes: to the BUILD_BUG_ON() change.
>> >> And if it's not yet a stable sha1, squash that together with this?
>> >> It doesn't seem ebc5d83d0443 alone needs this fix.
>> > 
>> > I shuffled things around.
>> > 
>> > I moved "mm: strictly check vmstat_text array size" from mm-hotfixes
>> > and back into mm-new for the next cycle.
>> > 
>> > I reworked "mm/vmstat: fix build with MEMCG=y and VM_EVENT_COUNTERS=n"
>> > so it precedes "mm: strictly check vmstat_text array size".
>> > 
>> > I reworked "mm/vmstat: utilize designated initializers for the
>> > vmstat_text array" so it comes last.
>> > 
>> > 
>> > So the applying order is now
>> 
>> I checked and in general it looks good, except a nit below.
>> 
>> > mm-hotfixes:
>> > mm-fix-vmstat-after-removing-nr_bounce.patch
>> > 
>> > mm-new:
>> > mm-vmstat-fix-build-with-memcg=y-and-vm_event_counters=n.patch
>> > mm-strictly-check-vmstat_text-array-size.patch
>> 
>> The changelogs of these two don't reflect the new ordering though, maybe
>> Kirill can provide updated ones?
> 
> Maybe something like this, for the first patch?

LGTM, thanks!

> > mm/vmstat: Make MEMCG select VM_EVENT_COUNTERS
> 
> The vmstat_text array contains labels for counters displayed in /proc/vmstat.
> It is important to keep the labels in sync with the counters.
> 
> There is a BUILD_BUG_ON() check in vmstat_start() that ensures the size of the
> vmstat_text is not smaller than VM_EVENT_COUNTERS. This helps to catch cases
> where a new counter is added but the label is not. However, it does not help if
> a counter is removed but the label remains.
> 
> It would be nice to make the BUILD_BUG_ON() check more strict to catch such
> cases. However, when compiling with MEMCG enabled but VM_EVENT_COUNTERS
> disabled, the vmstat_text array is larger than NR_VMSTAT_ITEMS.
> 
> This issue arises because some elements of the vmstat_text array are present
> when either MEMCG or VM_EVENT_COUNTERS is enabled, but NR_VMSTAT_ITEMS only
> accounts for these elements if VM_EVENT_COUNTERS is enabled.
> 
> Instead of adjusting the NR_VMSTAT_ITEMS definition to account for MEMCG, make
> MEMCG select VM_EVENT_COUNTERS. VM_EVENT_COUNTERS is enabled in most
> configurations anyway.
>