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

[Kiryl Shutsemau <kas@kernel.org>]

On Tue, Feb 17, 2026 at 09:29:02PM +0100, David Hildenbrand (Arm) wrote:
> On 2/17/26 16:22, Wenchao Hao wrote:
> > On Sat, Feb 14, 2026 at 4:45 PM Wenchao Hao <haowenchao22@gmail.com> 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.
> > > 
> > > Link: https://lore.kernel.org/linux-mm/20260210043456.2137482-1-haowenchao22@gmail.com/
> > 
> > Sorry for the late reply. We are now on Chinese New Year holiday, so...
> > 
> > The original goal of this patch is to measure memory waste from anonymous
> > THPs - pages pre-allocated on fault but never accessed.
> > 
> > On memory-sensitive devices like mobile phones, this helps us make better
> > decisions about when and how to enable THP. I think this is useful for
> > guiding THP policies, even as a debugging feature.
> > 
> > Let me summarize the discussion so far:
> > - Matthew Wilcox questioned the value and raised concerns fork but haven't
> >    exec path
> > - Michal Hocko criticized the inefficiency of scanning zero-filled pages.
> > - Kiryl Shutsemau prefers a system-call-based interface.
> > - David Hildenbrand acknowledged the value and suggested implementation
> >    improvements.
> > Please correct me if I missed or misrepresented anything.
> > 
> > I suggest we first agree whether this functionality is useful for upstream,
> > before discussing implementation details.
> > 
> > Reasons why this should go upstream from me:
> > 
> > - Anonymous THP can introduce real memory waste, but we currently have no
> >    good way to measure it.
> > - With accurate metrics, we can make better THP policy: disable for
> >    low-utilization cases, or early-unmap to relieve memory pressure and so
> >    on. This is especially valuable for mobile/embedded devices.
> > 
> > Possible implementations:
> > 
> > 1. A new smaps counter (default-off) to count zero-filled pages.
> > 2. A new madvise command like MADV_GET_ZEROPAGE
> > 3. A dedicated system call
> 
> I understand Kiyls point about smaps providing too much information users
> might not be interested in already. So sorting that out might provide a real
> benefit to other users that are only interested in specific stats (e.g.,
> Rss).

You can also limit the range of virtual address space you want to look
at.

> Providing a system call where one can specify/filter in theory sounds like a
> good idea. A syscall implies that one has to write a tool to obtain these
> metrics.
> 
> The nice thing about smaps/smaps_rollup is that it can be easily consumed on
> any system while debugging.
> 
> I wonder if there could be a way to achieve something similar with a file.
> Likely not, but maybe someone reading along can surprise me :)

I guess you can open a file write to it what you want to get and then
read. It is awkward from shell to keep file descriptor around, but doable.

> Otherwise we'd have to go with a tool.

A tool might be more ergonomic.

To minimize friction, it would be nice to put the tool into util-linux
(or whatever trendy Rust-rewrite called), so it would find its way to
every machine. Eventually.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov