linux-kernel - Re: [PATCH v12 mm-new 15/15] 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] [thread-next>] [day] [month] [year] [list]

Message-ID: <666ee834-396d-4a7c-be89-96c58b5c2ea8@lucifer.local>
Date: Thu, 23 Oct 2025 09:00:10 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: "Christoph Lameter (Ampere)" <cl@...two.org>,
        Nico Pache <npache@...hat.com>, linux-kernel@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-doc@...r.kernel.org, ziy@...dia.com,
        baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
        ryan.roberts@....com, dev.jain@....com, corbet@....net,
        rostedt@...dmis.org, mhiramat@...nel.org,
        mathieu.desnoyers@...icios.com, akpm@...ux-foundation.org,
        baohua@...nel.org, willy@...radead.org, peterx@...hat.com,
        wangkefeng.wang@...wei.com, usamaarif642@...il.com,
        sunnanyong@...wei.com, vishal.moola@...il.com,
        thomas.hellstrom@...ux.intel.com, yang@...amperecomputing.com,
        kas@...nel.org, aarcange@...hat.com, raquini@...hat.com,
        anshuman.khandual@....com, catalin.marinas@....com, tiwai@...e.de,
        will@...nel.org, dave.hansen@...ux.intel.com, jack@...e.cz,
        jglisse@...gle.com, surenb@...gle.com, zokeefe@...gle.com,
        hannes@...xchg.org, rientjes@...gle.com, mhocko@...e.com,
        rdunlap@...radead.org, hughd@...gle.com, richard.weiyang@...il.com,
        lance.yang@...ux.dev, vbabka@...e.cz, rppt@...nel.org,
        jannh@...gle.com, pfalcato@...e.de,
        Bagas Sanjaya <bagasdotme@...il.com>
Subject: Re: [PATCH v12 mm-new 15/15] Documentation: mm: update the admin
 guide for mTHP collapse

On Wed, Oct 22, 2025 at 10:22:08PM +0200, David Hildenbrand wrote:
> On 22.10.25 21:52, Christoph Lameter (Ampere) wrote:
> > On Wed, 22 Oct 2025, Nico Pache wrote:
> >
> > > Currently, madvise_collapse only supports collapsing to PMD-sized THPs +
> > > and does not attempt mTHP collapses. +
> >
> > madvise collapse is frequently used as far as I can tell from the THP
> > loads being tested. Could we support madvise collapse for mTHP?
>
> The big question is still how user space can communicate the desired order,
> and how we can not break existing users.

Yes, and let's go one step at a time, this series still needs careful scrutiny
and we need to ensure the _fundamentals_ are in place for khugepaged before we
get into MADV_COLLAPSE :)

>
> So I guess there will definitely be some support to trigger collapse to mTHP
> in the future, the big question is through which interface. So it will
> happen after this series.

Yes.

>
> Maybe through process_madvise() where we have an additional parameter, I
> think that was what people discussed in the past.

I wouldn't absolutely love us doing that, given it is a general parameter so
would seem applicable to any madvise() option and could lead to confusion, also
process_madvise() was originally for cross-process madvise vector operations.

I expanded this to make it applicable to the current process (and introduced
PIDFD_SELF to make that more sane), and SJ has optimised it across vector
operations (thanks SJ! :), but in general - it seems very weird to have
madvise() provide an operation that process_madvise() providse another version
of that has an extra parameter.

As usual we've painted ourselves into a corner with an API... :)

Perhaps we'll to accept the process_madvise() compromise and add
MADV_COLLAPSE_MHTP that only works with it or something.

Of course adding a new syscall isn't impossible... madvise2() not very appealing
however...

TL;DR I guess we'll deal with that when we come to it :)

>
> --
> Cheers
>
> David / dhildenb
>

Cheers, Lorenzo