Re: [RFC PATCH] mm: only set fault addrsss' access bit in do_anonymous_page

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

On 2/13/26 10:02, Wenchao Hao wrote:
> On Thu, Feb 12, 2026 at 4:54 PM David Hildenbrand (Arm)
> <david@kernel.org> wrote:
>>
>> On 2/12/26 02:57, Wenchao Hao wrote:
>>> On Wed, Feb 11, 2026 at 5:05 PM David Hildenbrand (Arm)
>>> <david@kernel.org> wrote:
>>>
>>> Thanks for your explanation. I now understand the design logic.
>>>
>>> What I’m proposing is mainly for debugging. After enabling 64K large folio
>>> on Android, we observed increased application memory footprint, especially
>>> for anonymous pages.
>>>
>>> Since Android app memory usage depends on runtime scenarios, we cannot
>>> confirm if the growth is directly caused by large folio. We want to
>>> analyze memory
>>> usage via the `Referenced` field in `/proc/[pid]/smaps`.
>>
>> Scanning for zero-filled pages will be much easier and more reliable.
>> For a debug feature good enough.
>>
>> I'm wondering what the best interface for something like that could be:
>> we don't want to make "/proc/[pid]/smaps" slower for all users.
>>
>> Maybe we could for debug kernels.
>>
>> For example, adding with CONFIG_DEBUG_KERNEL a new entry
>>
>>          Anon_Zero:
>>
>> counter that just tests whether the page content of an anonymous page is
>> all zeroes could be doable.
>>
> 
> Apologies for the delayed reply – I was just writing a demo to verify the
> approach you mentioned.
> 
> Using the CONFIG_DEBUG_KERNEL compile-time macro to isolate this feature
> is indeed an excellent idea.
> 
> However, in engineering practice, it requires recompiling and
> replacing the kernel,
> which can be cumbersome. Could we instead use a dynamic switch to control
> whether scan for zero-filled pages when reading /proc/[pid]/smaps?

Maybe a kernel cmdline option could do? Selectively enabling it for some 
PIDs only is not really possible, but also, maybe it's not really needed.

-- 
Cheers,

David