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

[David Hildenbrand (Arm)]

On 2/13/26 10:02, Wenchao Hao wrote:
> On Thu, Feb 12, 2026 at 4:54 PM David Hildenbrand (Arm)
> <david@xxxxxxxxxx> wrote:
> > On 2/12/26 02:57, Wenchao Hao wrote:
> > > On Wed, Feb 11, 2026 at 5:05 PM David Hildenbrand (Arm)
> > > <david@xxxxxxxxxx> 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