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

[Wenchao Hao <haowenchao22@gmail.com>]

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:
> >>
> >> On 2/11/26 01:49, Wenchao Hao wrote:
> >>> On Tue, Feb 10, 2026 at 5:07 PM David Hildenbrand (Arm)
> >>> <david@kernel.org> wrote:
> >>>
> >>> We have enabled 64KB large folios on Android devices, which may introduce
> >>> some memory waste. I want to figure out the proportion of memory waste
> >>> caused by large folios. Reading the "Referenced" field from /proc/pid/smaps
> >>> is a relatively low-cost method.
> >>
> >> Right. And that imprecision is to be expected when you opt-in into
> >> something that manages memory in other granularity and only has a single
> >> a/d bit: a large folio.
> >>
> >> Sure, individual PTEs *might* have independent a/d bits, but the
> >> underlying thing (folio) has only a single one. And optimizations that
> >> build on top (pte coalescing) reuse that principle that having a single
> >> logical a/d bit is fine.
> >>
> >>>
> >>> Additionally, considering future hot/cold page identification, we aim to
> >>> detect 64KB large folios where some pages are actually unaccessed and split
> >>> them into normal pages to avoid memory waste.
> >>>
> >>> However, the current large folio implementation sets the access bit for all
> >>> page table entries (PTEs) of the large folio in the do_anonymous_page
> >>> function, making it hard to distinguish whether pre-allocated pages were
> >>> truly accessed.
> >>
> >> The deferred shrinker uses a much simpler mechanism: if the page content
> >> is zero, likely it was over-allocated and never used later.
> >>
> >> It's not completely lightweight (scan pages for 0 content), but is
> >> reliable, independent of the mapping type (PMD, cont-pte, whatever) and
> >> independent of any access/dirty bits, leaving performance unharmed.
> >>
> >> When you say "I want to figure out the proportion of memory waste", are
> >> we talking about a debug feature?
> >>
> >
> > 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?

> --
> Cheers,
>
> David