linux-kernel - Re: [PATCH] docs: update THP documentation to clarify sysfs "never" setting

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

Message-ID: <CAGsJ_4wvWZwG6agXOzDoYBD_vnN6k4TRJjhFfR5dw5pQrk2mwQ@mail.gmail.com>
Date: Tue, 22 Jul 2025 11:37:07 +0800
From: Barry Song <21cnbao@...il.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	David Hildenbrand <david@...hat.com>, Zi Yan <ziy@...dia.com>, 
	"Liam R . Howlett" <Liam.Howlett@...cle.com>, Nico Pache <npache@...hat.com>, 
	Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>, 
	Jonathan Corbet <corbet@....net>, linux-mm@...ck.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] docs: update THP documentation to clarify sysfs "never" setting

On Tue, Jul 22, 2025 at 10:33 AM Baolin Wang
<baolin.wang@...ux.alibaba.com> wrote:
>
>
>
> On 2025/7/22 10:23, Barry Song wrote:
> > On Tue, Jul 22, 2025 at 9:30 AM Baolin Wang
> > <baolin.wang@...ux.alibaba.com> wrote:
> >>
> >>
> >>
> >> On 2025/7/21 23:55, Lorenzo Stoakes wrote:
> >>> Rather confusingly, setting all Transparent Huge Page sysfs settings to
> >>> "never" does not in fact result in THP being globally disabled.
> >>>
> >>> Rather, it results in khugepaged being disabled, but one can still obtain
> >>> THP pages using madvise(..., MADV_COLLAPSE).
> >>>
> >>> This is something that has remained poorly documented for some time, and it
> >>> is likely the received wisdom of most users of THP that never does, in
> >>> fact, mean never.
> >>>
> >>> It is therefore important to highlight, very clearly, that this is not the
> >>> ase.
> >>>
> >>> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> >>> ---
> >>>    Documentation/admin-guide/mm/transhuge.rst | 11 +++++++++--
> >>>    1 file changed, 9 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> >>> index dff8d5985f0f..182519197ef7 100644
> >>> --- a/Documentation/admin-guide/mm/transhuge.rst
> >>> +++ b/Documentation/admin-guide/mm/transhuge.rst
> >>> @@ -107,7 +107,7 @@ sysfs
> >>>    Global THP controls
> >>>    -------------------
> >>>
> >>> -Transparent Hugepage Support for anonymous memory can be entirely disabled
> >>> +Transparent Hugepage Support for anonymous memory can be disabled
> >>>    (mostly for debugging purposes) or only enabled inside MADV_HUGEPAGE
> >>>    regions (to avoid the risk of consuming more memory resources) or enabled
> >>>    system wide. This can be achieved per-supported-THP-size with one of::
> >>> @@ -119,6 +119,11 @@ system wide. This can be achieved per-supported-THP-size with one of::
> >>>    where <size> is the hugepage size being addressed, the available sizes
> >>>    for which vary by system.
> >>>
> >>> +.. note:: Setting "never" in all sysfs THP controls does **not** disable
> >>> +          Transparent Huge Pages globally. This is because ``madvise(...,
> >>> +          MADV_COLLAPSE)`` ignores these settings and collapses ranges to
> >>> +          PMD-sized huge pages unconditionally.
> >>> +
> >>>    For example::
> >>>
> >>>        echo always >/sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled
> >>> @@ -187,7 +192,9 @@ madvise
> >>>        behaviour.
> >>>
> >>>    never
> >>> -     should be self-explanatory.
> >>> +     should be self-explanatory. Note that ``madvise(...,
> >>> +     MADV_COLLAPSE)`` can still cause transparent huge pages to be
> >>> +     obtained even if this mode is specified everywhere.
> >>
> >> I hope this part of the explanation is also copy-pasted into the
> >> 'Hugepages in tmpfs/shmem' section. Otherwise look good to me. Thanks.
> >
> > Apologies if this is a silly question, but regarding this patchset:
> > https://lore.kernel.org/linux-mm/cover.1750815384.git.baolin.wang@linux.alibaba.com/
> >
> > It looks like the intention is to disable hugepages even for
> > `MADV_COLLAPSE` when the user has set the policy to 'never'. However,
> > based on Lorenzo's documentation update, it seems we still want to allow
> > hugepages for `MADV_COLLAPSE` even if 'never' is set?
> >
> > Could you clarify what the intended behavior is? It seems we've decided
> > to keep the existing behavior unchanged—am I understanding that
> > correctly?
>
> Yes, Hugh has already explicitly opposed the current changes to the
> MADV_COLLAPSE logic[1], although there are still some disagreements that
> cannot be resolved.
>
> At least we reached the consensus to update the documentation to reflect
> the current sysfs THP control logic first, to avoid the misunderstanding
> that 'sysfs THP controls can disable Transparent Huge Pages globally'.

Nice, thanks! Personally, I prefer this approach as well. Updating the
man page feels a bit odd, since it's something people are already
familiar with and may have memorized.

>
> [1]
> https://lore.kernel.org/linux-mm/75c02dbf-4189-958d-515e-fa80bb2187fc@google.com/

Best regards
Barry