Re: [PATCH v4 0/2] fix MADV_COLLAPSE issue if THP settings are disabled

[David Hildenbrand]

On 25.06.25 13:03, Usama Arif wrote:
> On 25/06/2025 08:34, David Hildenbrand wrote:
> > > > We would all prefer a less messy world of THP tunables.  I certainly
> > > > find plenty to dislike there too; and wish that a less assertive name
> > > > than "never" had been chosen originally for the default off position.
> > > >
> > > > But please don't break the accepted and documented behaviour of
> > > > MADV_COLLAPSE now.
> > >
> > > Again see above, I absolutely disagree this is documented _clearly_. And
> > > that's the underlying issue here.
> > > > I feel like if you polled 100 system administrators (assuming they knew
> > > about THP) as to how you globally disable THP, probably all 100 would say
> > > you do it via:
> > >
> > > # echo never > /sys/kernel/mm/transparent_hugepage/enabled
> >
> > Yes. One big problem is that the documentation was not updated.
> >
> > Changing the meaning of "entirely disabled" to "entirely disabled automatically (page faults, khugepaged)"
> >
> > > So shouldn't 'never break userspace' be based on practical reality rather
> > > than a theorised interpretation of documents that sadly are not clear
> > > enough?
> >
> > I think the problem is that there might indeed be more users out there relying on "never+MADV_COLLPASE" to now place THPs than "never+MADV_COLLPASE" to no place THPs.
> >
> > What is the harm when not placing THPs? Performance degradation for some apps?
>
> I think a bigger issue than performance degradation is someone upgrading the kernel and not
> seeing MADV_COLLAPSE working as it has since the beginning and not knowing that its due
> to a kernel change.
>
> I feel transparent_hugepage/enabled is too messed up, and its difficult to fix it without
> breaking it for someone? I still find it weird that we can set transparent_hugepage/enabled
> to never and transparent_hugepage/hugepages-2048kB/enabled to madvise and still get hugepages.
> (And we actually use this configuration in production for our ARM servers).
>
> Introducing deny for global and page size I feel will over complicate it because of the issue in
> the previous paragraph, page size setting overrides global setting. so even if
> transparent_hugepage/enabled is deny, we might still get a THP if the page setting is not.
> Someone needs to file to deny, which is the same as setting every file to never.
>
> So I just wanted to throw another bad idea in the mix, what if we introduce another sysfs file
> (I hate introducing sysfs :)), something like /sys/kernel/mm/thp_allowed (or some other alternate name)
> which is default 1.

Let's rather not :)

--
Cheers,

David / dhildenb