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

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

On 2/16/26 17:42, Michal Hocko wrote:
> On Mon 16-02-26 23:59:50, Wenchao Hao wrote:
>> On Mon, Feb 16, 2026 at 7:58 PM Kiryl Shutsemau <kirill@shutemov.name> wrote:
>>>
>>>
>>> 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.
> 
> You are comparing content of all anon pages, aren't you? Depending on
> the content this can add a lot of overhead.
> 
>> 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.
> 
> MADV_GET_ZEROPAGE would make more sense to me. But a more fundamental
> question is whether this metric is really what you want long term.
> Kernel can do all sorts of optimizations behind userspace back - e.g.
> map shared zero page - so just learning that a process has a lot of
> pages filled up with zeroes doesn't tell you all that much. Or does it?

If a sysadmin wants to see where THP hurt (zero-filled pages), surely 
MADV_GET_ZEROPAGE is the wrong (ugly) interface.

All we want are per-process stats. What am I missing?


KSM could deduplicate them, the deferred shrinker could remove them. 
Unless both mechanisms are not desired or are ineffective for another 
reason (e.g., page pinning).

-- 
Cheers,

David