Re: [PATCH] Revert "mm, hugetlb: remove hugepages_treat_as_movable sysctl"

Michal Hocko
2025-10-08T15:23:47Z

On Wed 08-10-25 17:14:26, David Hildenbrand wrote:
> On 08.10.25 16:59, Michal Hocko wrote:
> > On Wed 08-10-25 10:58:23, David Hildenbrand wrote:
> > > On 07.10.25 23:44, Gregory Price wrote:
> > [...]
> > > > @@ -926,7 +927,8 @@ static inline gfp_t htlb_alloc_mask(struct hstate *h)
> > > > {
> > > > gfp_t gfp = __GFP_COMP | __GFP_NOWARN;
> > > > - gfp |= hugepage_movable_supported(h) ? GFP_HIGHUSER_MOVABLE : GFP_HIGHUSER;
> > > > + gfp |= (hugepage_movable_supported(h) || hugepages_treat_as_movable) ?
> > > > + GFP_HIGHUSER_MOVABLE : GFP_HIGHUSER;
> > >
> > > I mean, this is as ugly as it gets.
> > >
> > > Can't we just let that old approach RIP where it belongs? :)
> > >
> > > If something unmovable, it does not belong on ZONE_MOVABLE, as simple as that.
> >
> > yes, I do agree. This is just muddying the semantic of the zone.
> >
> > Maybe what we really want is to have a configurable zone rather than a
> > very specific consumer of it instead. What do I mean by that? We clearly
> > have physically (DMA, DMA32) and usability (NORMAL, MOVABLE) constrained
> > zones. So rather than having a MOVABLE zone we can have a single zone
> > $FOO_NAME zone with configurable attributes - like allocation
> > constrains (kernel, user, movable, etc). Now that we can overlap zones
> > this should allow for quite a lot flexibility. Implementation wise this
> > would require some tricks as we have 2 zone types for potentially 3
> > different major usecases (kernel allocations, userspace reserved ranges
> > without movability and movable allocations). I haven't thought this
> > through completely and mostly throwing this as an idea (maybe won't
> > work). Does that make sense?
>
> I suggested something called PREFER_MOVABLE in the past, that would prefer
> movable allocations but nothing would stop unmovable allocations to end up
> on it. But only as a last resort or when explicitly requested (e.g.,
> gigantic pages).
>
> Maybe that's similar to what you have in mind?

Slightly different because what I was thinking about was more towards
guarantee/predictability. Last resort is quite hard to plan around. It
might be a peak memory pressure to eat up portion of a memory block and
then fragmenting it to prevent other use planned for that memroy block.
That is why I called it user allocations because those are supposed to
be configured for userspace consumation and planned for that use. So you
would get pretty much a guarantee that no kernel allocations will fall
there.

--
Michal Hocko
SUSE Labs