linux-kernel - Re: [PATCH v4 12/12] Documentation: mm: update the admin guide for mTHP collapse

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <b22e222f-1d6a-4685-871c-1aaee319b744@gmail.com>
Date: Thu, 24 Apr 2025 16:03:49 +0100
From: Usama Arif <usamaarif642@...il.com>
To: Nico Pache <npache@...hat.com>, linux-mm@...ck.org,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org
Cc: akpm@...ux-foundation.org, corbet@....net, rostedt@...dmis.org,
 mhiramat@...nel.org, mathieu.desnoyers@...icios.com, david@...hat.com,
 baohua@...nel.org, baolin.wang@...ux.alibaba.com, ryan.roberts@....com,
 willy@...radead.org, peterx@...hat.com, ziy@...dia.com,
 wangkefeng.wang@...wei.com, sunnanyong@...wei.com, vishal.moola@...il.com,
 thomas.hellstrom@...ux.intel.com, yang@...amperecomputing.com,
 kirill.shutemov@...ux.intel.com, aarcange@...hat.com, raquini@...hat.com,
 dev.jain@....com, anshuman.khandual@....com, catalin.marinas@....com,
 tiwai@...e.de, will@...nel.org, dave.hansen@...ux.intel.com, jack@...e.cz,
 cl@...two.org, jglisse@...gle.com, surenb@...gle.com, zokeefe@...gle.com,
 hannes@...xchg.org, rientjes@...gle.com, mhocko@...e.com,
 rdunlap@...radead.org
Subject: Re: [PATCH v4 12/12] Documentation: mm: update the admin guide for
 mTHP collapse



On 17/04/2025 01:02, 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.
> 
> Signed-off-by: Nico Pache <npache@...hat.com>
> ---
>  Documentation/admin-guide/mm/transhuge.rst | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index dff8d5985f0f..06814e05e1d5 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.
>  
>  The THP behaviour is controlled via :ref:`sysfs <thp_sysfs>`
>  interface and using madvise(2) and prctl(2) system calls.
> @@ -144,6 +144,14 @@ 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 continuously collapsing to a
> +larger mTHP size. 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.
> +

Hi Nico,

Could you add a bit more explanation of the creep behaviour here in documentation.
I remember you explained in one of the earlier versions that if more than half of the
collapsed mTHP is zero-filled, it for some reason becomes eligible for collapsing to
larger order, but if less than half is zero-filled its not eligible? I cant exactly
remember what the reason was :) Would be good to have it documented more if possible.

Thanks

>  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