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

[Wenchao Hao]

On Mon, Feb 16, 2026 at 7:58 PM Kiryl Shutsemau <kirill@xxxxxxxxxxxxx> wrote:
>
> On Mon, Feb 16, 2026 at 12:45:13PM +0100, David Hildenbrand (Arm) wrote:
> > On 2/16/26 12:34, Kiryl Shutsemau wrote:
> > > On Sat, Feb 14, 2026 at 04:45:14PM +0800, 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.
> > > >
> > > > 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.
> > >
> > > I expect it to slowdown /proc/pid/smaps read substantially. I don't
> > > think this line in smaps worth it.
> > >
> >
> > That's why it's enabled through a command line parameter.
>
> One users want the stat and all users on the machine pay the price?
> That's a poor trade off.
>
> In general, smaps scales poorly. It collects a lot of stats and most of
> them are ignored by user. We need something like statx(2) where user can
> declare what he is interested in, so kernel won't waste cycles.
>

I initially considered two approaches:

First, exposing the needed information via smaps. This does incur some
performance cost but is the simplest to implement. The new feature can be
dynamically toggled via a command-line parameter. When disabled, the
overhead is negligible—only a minor if check, which is insignificant compared
to the full smaps cost.

Second, adding a new system call or extending madvise with a new command
like MADV_GET_ZEROANON. Userspace tools can then use it to measure
memory waste from zero-filled anonymous huge pages.

This is slightly more complex but minimizes system impact: environments that
don’t care about zero-filled anonymous pages pay zero overhead when the
command is not used.

The exact implementation approach can be discussed after we confirm whether
the upstream kernel needs this debugging feature.

Thanks.

> Kernel cmdline parameter is the wrong place to declare what stats you
> want to see.
>
> --
>   Kiryl Shutsemau / Kirill A. Shutemov