Re: [PATCH v7 06/12] khugepaged: introduce khugepaged_scan_bitmap for mTHP support

[Nico Pache <npache@redhat.com>]

On Tue, May 20, 2025 at 4:09 AM Baolin Wang
<baolin.wang@linux.alibaba.com> wrote:
>
> Sorry for late reply.
>
> On 2025/5/17 14:47, Nico Pache wrote:
> > On Thu, May 15, 2025 at 9:20 PM Baolin Wang
> > <baolin.wang@linux.alibaba.com> wrote:
> >>
> >>
> >>
> >> On 2025/5/15 11:22, Nico Pache wrote:
> >>> khugepaged scans anons PMD ranges for potential collapse to a hugepage.
> >>> To add mTHP support we use this scan to instead record chunks of utilized
> >>> sections of the PMD.
> >>>
> >>> khugepaged_scan_bitmap uses a stack struct to recursively scan a bitmap
> >>> that represents chunks of utilized regions. We can then determine what
> >>> mTHP size fits best and in the following patch, we set this bitmap while
> >>> scanning the anon PMD. A minimum collapse order of 2 is used as this is
> >>> the lowest order supported by anon memory.
> >>>
> >>> max_ptes_none is used as a scale to determine how "full" an order must
> >>> be before being considered for collapse.
> >>>
> >>> When attempting to collapse an order that has its order set to "always"
> >>> lets always collapse to that order in a greedy manner without
> >>> considering the number of bits set.
> >>>
> >>> Signed-off-by: Nico Pache <npache@redhat.com>
> >>
> >> Sigh. You still haven't addressed or explained the issues I previously
> >> raised [1], so I don't know how to review this patch again...
> > Can you still reproduce this issue?
>
> Yes, I can still reproduce this issue with today's (5/20) mm-new branch.
>
> I've disabled PMD-sized THP in my system:
> [root]# cat /sys/kernel/mm/transparent_hugepage/enabled
> always madvise [never]
> [root]# cat /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled
> always inherit madvise [never]
>
> And I tried calling madvise() with MADV_COLLAPSE for anonymous memory,
> and I can still see it collapsing to a PMD-sized THP.
Hi Baolin ! Thank you for your reply and willingness to test again :)

I didn't realize we were talking about madvise collapse-- this makes
sense now. I also figured out why I could "reproduce" it before. My
script was always enabling the THP settings in two places, and I only
commented out one to test this. But this time I was doing more manual
testing.

The original design of madvise_collapse ignores the sysfs and
collapses even if you have an order disabled. I believe this behavior
is wrong, but by design. I spent some time playing around with madvise
collapses with and w/o my changes. This is not a new thing, I
reproduced the issue in 6.11 (Fedora 41), and I think its been
possible since the inception of madvise collapse 3 years ago. I
noticed a similar behavior on one of my RFC since it was "breaking"
selftests, and the fix was to reincorporate this broken sysfs
behavior.

7d8faaf15545 ("mm/madvise: introduce MADV_COLLAPSE sync hugepage collapse")
"This call is independent of the system-wide THP sysfs settings, but
will fail for memory marked VM_NOHUGEPAGE."

The second condition holds true (and fails for VM_NOHUGEPAGE), but I
dont know if we actually want madvise_collapse to be independent of
the system-wide.

So I'll ask the authors
+David Rientjes +zokeefe@google.com
Was this brought up as a concern when this feature was first
introduced, was there any pushback, what was the outcome of the
discussion if so?
I can easily fix this and it would further simplify the code (by
removing the is_khugepaged and friends). As David H. has brought up in
other discussions around similar topics, never should mean never, is
this the only exception we should allow?

Thanks!
>
> > I can no longer reproduce this issue, that's why I posted... although
> > I should have followed up, and looked into what the original issue
> > was. Nothing really sticks out so perhaps something in mm-new was
> > broken and pulled out... not sure.
> >
> > It should now follow the expected behavior, which is that no mTHP
> > collapse occurs because if the PMD size is disabled so is khugepaged
> > collapse.
> >
> > Lmk if you are still experiencing this issue please.
> >
> > Cheers,
> > -- Nico
> >>
> >> [1]
> >> https://lore.kernel.org/all/83a66442-b7c7-42e7-af4e-fd211d8ed6f8@linux.alibaba.com/
> >>
>