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

["David Hildenbrand (Arm)" <david@kernel.org>]

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).

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 :)

Otherwise we'd have to go with a tool.

-- 
Cheers,

David