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

[Wenchao Hao <haowenchao22@gmail.com>]

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 welcome feedback on whether this is useful, and any better approaches.

Thank you.