linux-kernel - Re: [PATCH v4 7/7] selftests: prctl: introduce tests for disabling THPs except for madvise

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Message-ID: <de8da320-3286-4639-8f61-b99d1186ca41@lucifer.local>
Date: Thu, 14 Aug 2025 16:02:10 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Mark Brown <broonie@...nel.org>
Cc: David Hildenbrand <david@...hat.com>, Usama Arif <usamaarif642@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, corbet@....net, rppt@...nel.org,
        surenb@...gle.com, mhocko@...e.com, hannes@...xchg.org,
        baohua@...nel.org, shakeel.butt@...ux.dev, riel@...riel.com,
        ziy@...dia.com, laoar.shao@...il.com, dev.jain@....com,
        baolin.wang@...ux.alibaba.com, npache@...hat.com,
        Liam.Howlett@...cle.com, ryan.roberts@....com, vbabka@...e.cz,
        jannh@...gle.com, Arnd Bergmann <arnd@...db.de>, sj@...nel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        kernel-team@...a.com
Subject: Re: [PATCH v4 7/7] selftests: prctl: introduce tests for disabling
 THPs except for madvise

On Thu, Aug 14, 2025 at 02:08:57PM +0100, Mark Brown wrote:
> On Thu, Aug 14, 2025 at 02:59:13PM +0200, David Hildenbrand wrote:
> > On 14.08.25 14:09, Mark Brown wrote:
>
> > > Perhaps this is something that needs considering in the ABI, so
> > > userspace can reasonably figure out if it failed to configure whatever
> > > is being configured due to a missing feature (in which case it should
> > > fall back to not using that feature somehow) or due to it messing
> > > something else up?  We might be happy with the tests being version
> > > specific but general userspace should be able to be a bit more robust.
>
> > Yeah, the whole prctl() ship has sailed, unfortunately :(
>
> Perhaps a second call or sysfs file or something that returns the
> supported mask?  You'd still have a boostrapping issue with existing
> versions but at least at any newer stuff would be helped.

Ack yeah I do wish we had better APIs for expressing what was
available/not. Will put this sort of thing on the TODO...

Overall I don't want to hold this up unnecesarily, and I bow to the
consensus if others feel we ought not to _assume_ same kernel at least best
effort.

Usama - It's ok to leave it as is in this case since obviously only tip
kernel will have this feature.

Cheers, Lorenzo