linux-kernel - Re: [PATCH v10 13/13] Documentation: mm: update the admin guide for mTHP collapse

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Message-ID: <383886bf-b502-46c2-8703-37941caaf2a8@lucifer.local>
Date: Thu, 21 Aug 2025 16:03:47 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Nico Pache <npache@...hat.com>
Cc: linux-mm@...ck.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
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        Bagas Sanjaya <bagasdotme@...il.com>
Subject: Re: [PATCH v10 13/13] Documentation: mm: update the admin guide for
 mTHP collapse

On Tue, Aug 19, 2025 at 08:17:42AM -0600, Nico Pache wrote:
> Now that we can collapse to mTHPs lets update the admin guide to
> reflect these changes and provide proper guidence on how to utilize it.
>
> Reviewed-by: Bagas Sanjaya <bagasdotme@...il.com>
> Signed-off-by: Nico Pache <npache@...hat.com>
> ---
>  Documentation/admin-guide/mm/transhuge.rst | 19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
>
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index b85547ac4fe9..1f9e6a32052c 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -63,7 +63,7 @@ often.
>  THP can be enabled system wide or restricted to certain tasks or even
>  memory ranges inside task's address space. Unless THP is completely
>  disabled, there is ``khugepaged`` daemon that scans memory and
> -collapses sequences of basic pages into PMD-sized huge pages.
> +collapses sequences of basic pages into huge pages.

Maybe worth saying 'of either PMD size or mTHP sizes, if the system is
configured to do so.' to really spell it out.

>
>  The THP behaviour is controlled via :ref:`sysfs <thp_sysfs>`
>  interface and using madvise(2) and prctl(2) system calls.
> @@ -149,6 +149,18 @@ hugepage sizes have enabled="never". If enabling multiple hugepage
>  sizes, the kernel will select the most appropriate enabled size for a
>  given allocation.
>
> +khugepaged uses max_ptes_none scaled to the order of the enabled mTHP size
> +to determine collapses. When using mTHPs it's recommended to set
> +max_ptes_none low-- ideally less than HPAGE_PMD_NR / 2 (255 on 4k page
> +size). This will prevent undesired "creep" behavior that leads to

Woah wait what??

OK sorry but this is crazy - I've sent a reply to the 00/13 patch as this is
really concerning, this suggests to me that this sysctl is completely broken for
this purpose.

I think the series has to be altered to introduce an mthp-specific ratio, anwyay
let's discuss that on the 00/13 reply thread please.

> +continuously collapsing to the largest mTHP size; when we collapse, we are
> +bringing in new non-zero pages that will, on a subsequent scan, cause the
> +max_ptes_none check of the +1 order to always be satisfied. By limiting
> +this to less than half the current order, we make sure we don't cause this
> +feedback loop. max_ptes_shared and max_ptes_swap have no effect when
> +collapsing to a mTHP, and mTHP collapse will fail on shared or swapped out
> +pages.

In general, though I actually want you fundamentally change your approach so you
don't need to document this at all, this whole sentence is far too dense, it
needs paragraphs and some text around it so it's easier to read, right now it's
really quite difficult.

> +
>  It's also possible to limit defrag efforts in the VM to generate
>  anonymous hugepages in case they're not immediately free to madvise
>  regions or to never try to defrag memory and simply fallback to regular
> @@ -264,11 +276,6 @@ support the following arguments::
>  Khugepaged controls
>  -------------------
>
> -.. note::
> -   khugepaged currently only searches for opportunities to collapse to
> -   PMD-sized THP and no attempt is made to collapse to other THP
> -   sizes.
> -
>  khugepaged runs usually at low frequency so while one may not want to
>  invoke defrag algorithms synchronously during the page faults, it
>  should be worth invoking defrag at least in khugepaged. However it's
> --
> 2.50.1
>

Thanks, Lorenzo