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

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

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?

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


diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index d7d52e259055..0301b9cd28f8 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -34,6 +34,10 @@
  
  #define SEQ_PUT_DEC(str, val) \
                 seq_put_decimal_ull_width(m, str, (val) << (PAGE_SHIFT-10), 8)
+
+static bool anonzero_in_smaps;
+module_param(anonzero_in_smaps, bool, 0644);
+
  void task_mem(struct seq_file *m, struct mm_struct *mm)
  {
         unsigned long text, lib, swap, anon, file, shmem;


> 
> 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/
> Signed-off-by: Wenchao Hao <haowenchao22@gmail.com>
> ---
>   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.

-- 
Cheers,

David