linux-kernel - Re: [PATCH v7 06/12] khugepaged: introduce khugepaged_scan

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Message-ID: <CAA1CXcC9MB2Nw4MmGajESfH8DhAsh4QvTj4ABG3+Rg2iPi087w@mail.gmail.com>
Date: Sat, 17 May 2025 00:47:35 -0600
From: Nico Pache <npache@...hat.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: linux-mm@...ck.org, linux-doc@...r.kernel.org, 
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Subject: Re: [PATCH v7 06/12] khugepaged: introduce khugepaged_scan_bitmap for
 mTHP support

On Thu, May 15, 2025 at 9:20 PM Baolin Wang
<baolin.wang@...ux.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@...hat.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?
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/
>