RFC: malloc automatic THP opt-in breaks madvise contract for container hosts

[H.J. Lu]

On Fri, Apr 17, 2026 at 3:59 PM Sun, Yutong <yutongsu@amazon.com> wrote:
>
> > One question is if malloc THP usage should be opt-in or opt-out.
> > If THP usage in malloc improves performance for majority cases,
> > it should be opt-out.
>
> We'd vote opt-in. On container hosts most processes are small (shims,
> agents, runtime components) and don't benefit from THP. We're seeing
> this across managed services using our distro. It feels like the default
> should follow the kernel madvise mode semantics.

I see.

> > No. Yours is the first one I heard. But I am not surprised, given
> > its side effects.
>
> I expect more reports as distros pick up 2.43.
>
> --
> Yutong
>
>
>
> On 4/16/26, 10:56 PM, "H.J. Lu" <hjl.tools@gmail.com <mailto:hjl.tools@gmail.com>> wrote:
>
>
> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
>
>
>
>
>
>
> On Fri, Apr 17, 2026 at 1:25 PM Sun, Yutong <yutongsu@amazon.com <mailto:yutongsu@amazon.com>> wrote:
> >
> >
> >
> > On 4/16/26, 10:23 PM, "Sun, Yutong" <yutongsu@amazon.com <mailto:yutongsu@amazon.com> <mailto:yutongsu@amazon.com <mailto:yutongsu@amazon.com>>> wrote:
> >
> >
> > Thanks for the pointer. The per-binary opt-in idea is interesting.
> >
> >
> > To clarify our case — the issue is specifically malloc's thp_init()
> > auto-calling madvise(MADV_HUGEPAGE) when the kernel is in `madvise`
> > mode. Your proposal covers PT_LOAD segments which seems broader.
> > Would a binary without the property effectively get no THP at all
> > under `madvise` mode?
>
>
> My proposal is specific to THP usage on read-only segments. THP usage
> in malloc isn't impacted. A different bit is needed for malloc. One question
> is if malloc THP usage should be opt-in or opt-out. If THP usage in malloc
> improves performance for majority cases, it should be opt-out.
>
>
> >
> > Also, are you hearing similar reports from other distros as 2.43 rolls out?
>
>
> No. Yours is the first one I heard. But I am not surprised, given
> its side effects.
>
>
> >
> > Again, as I mentioned we're carrying a revert patch [1] for now and happy to move to
> > whatever upstream solution lands.
> >
>
>
>
>
> --
> H.J.
>
>
>


-- 
H.J.