Re: [PATCH] mm: Add AnonZero accounting for zero-filled anonymous pages

[Wenchao Hao]

On Mon, Feb 16, 2026 at 8:15 PM David Hildenbrand (Arm)
<david@xxxxxxxxxx> wrote:
>
> On 2/14/26 09:45, Wenchao Hao wrote:
> > Add kernel command line option "count_zero_page" to track anonymous pages
> > have been allocated and mapped to userspace but zero-filled.
>
> "count_zero_page" is rather sub-optimal parameter name. "anonzero_in_smaps" or sth like that?
>
Your naming suggestion is indeed better than mine. If this patch is still needed
for further development, I will modify it according to your advice.

> Still wondering if there could be a better way to enable this dynamically.
>
> In particular, not using a core parameter.
>
> If you use a module parameter, you can just set on the cmdline
>
>         proc.anonzero_in_smaps=1
>
> And dynamically set/observe it in
>
>         /sys/module/proc/parameters/anonzero_in_smaps
>
Regarding the use of module parameters versus core_param, I think
either approach is ok.
Currently, the core_param I'm using also supports two modification methods:

1. Command line parameter: count_zero_page=Y
2. After system boot, view or modify it via
/sys/module/kernel/parameters/count_zero_page

It is true that this modification would be more appropriately placed
in the proc module,
which aligns with your earlier suggestion.

If there is a possibility of further iteration on this change, I will
move it to the proc module instead
of using a core_param

> >
> > This feature is mainly used to debug large folio mechanism, which
> > pre-allocates and map more pages than actually needed, leading to memory
> > waste from unaccessed pages.
> >
> > Export the result in /proc/pid/smaps as "AnonZero" field.
> >
> > Link: https://lore.kernel.org/linux-mm/20260210043456.2137482-1-haowenchao22@xxxxxxxxx/
> > Signed-off-by: Wenchao Hao <haowenchao22@xxxxxxxxx>
> > ---
> >   Documentation/filesystems/proc.rst |  5 +++++
> >   fs/proc/task_mmu.c                 | 10 ++++++++++
> >   2 files changed, 15 insertions(+)
> >
> > diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> > index b0c0d1b45b99..573c8b015e39 100644
> > --- a/Documentation/filesystems/proc.rst
> > +++ b/Documentation/filesystems/proc.rst
> > @@ -545,6 +545,11 @@ replaced by copy-on-write) part of the underlying shmem object out on swap.
> >   does not take into account swapped out page of underlying shmem objects.
> >   "Locked" indicates whether the mapping is locked in memory or not.
> >
> > +"AnonZero" shows the size of anonymous pages that have never been accessed
> > +after mapping, and it can reflect the memory waste caused by huge pages.
>
> That's not correct. They could be read/written, but with zeroes.
>
> > +Implemented by scanning the size of zero-filled pages of the VMA. It
> > +is default disabled, and enabled via cmdline param "count_zero_page=true".
>
> Probably best to keep it simpler:
>
> "AnonZero" shows the size of anonymous pages that contain zeroes. Zero-filled
> pages can indicate memory waste caused by memory-overallocation with (m)THPs.
> Availability is controlled through XYZ.
>
As with your earlier suggestion, if this change is to be iterated on
further, I will update it according
to your suggestion.

Thanks.

> --
> Cheers,
>
> David