Re: [LSF/MM/BPF TOPIC][RFC PATCH v4 00/27] Private Memory Nodes (w/ Compressed RAM)

[Gregory Price <gourry@gourry.net>]

On Mon, Feb 23, 2026 at 02:07:15PM +0100, David Hildenbrand (Arm) wrote:
> > 
> > Gregory Price (27):
> >    numa: introduce N_MEMORY_PRIVATE node state
> >    mm,cpuset: gate allocations from N_MEMORY_PRIVATE behind __GFP_PRIVATE
> >    mm/page_alloc: add numa_zone_allowed() and wire it up
> >    mm/page_alloc: Add private node handling to build_zonelists
> >    mm: introduce folio_is_private_managed() unified predicate
> >    mm/mlock: skip mlock for managed-memory folios
> >    mm/madvise: skip madvise for managed-memory folios
> >    mm/ksm: skip KSM for managed-memory folios
> >    mm/khugepaged: skip private node folios when trying to collapse.
> >    mm/swap: add free_folio callback for folio release cleanup
> >    mm/huge_memory.c: add private node folio split notification callback
> >    mm/migrate: NP_OPS_MIGRATION - support private node user migration
> >    mm/mempolicy: NP_OPS_MEMPOLICY - support private node mempolicy
> >    mm/memory-tiers: NP_OPS_DEMOTION - support private node demotion
> >    mm/mprotect: NP_OPS_PROTECT_WRITE - gate PTE/PMD write-upgrades
> 
> I'm concerned about adding more special-casing (similar to what we already
> added for ZONE_DEVICE) all over the place.
> 
> Like the whole folio_managed_() stuff in mprotect.c
> 
> Having that said, sounds like a reasonable topic to discuss.
> 

It's a valid concern - and is why I tried to re-use as many of the
zone_device hooks as possible.  It does not seem zone_device has quite
the same semantics for a case like this, so I had to make something new.

DEVICE_COHERENT injects a temporary swap entry to allow the device to do
a large atomic operation - then the page table is restored and the CPU
is free to change entries as it pleases.

Another option would be to add the hook to vma_wants_writenotify()
instead of the page table code - and mask MM_CP_TRY_CHANGE_WRITABLE.

This would require adding a vma flag - or maybe a count of protected /
device pages.

int mprotect_fixup() {
    ...
    if (vma_wants_manual_pte_write_upgrade(vma))
        mm_cp_flags |= MM_CP_TRY_CHANGE_WRITABLE;
}

bool vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot)
{
    if (vma->managed_wrprotect)
        return true;
}

That would localize the change in folio_managed_fixup_migration_pte() :

static inline pte_t folio_managed_fixup_migration_pte(struct page *new,
                                                      pte_t pte,
                                                      pte_t old_pte,
                                                      struct vm_area_struct *vma)
{
    ...
    } else if (folio_managed_wrprotect(page_folio(new))) {
        pte = pte_wrprotect(pte);
+       atomic_inc(&vma->managed_wrprotect);
    }
    return pte;
}

This would cover both the huge_memory.c and mprotect, and maybe that's
just generally cleaner? I can try that to see if it actually works.

~Gregory