Re: [PATCH v3 3/5] loongarch/mm: disable device DAX vmemmap optimization

[Lance Yang <lance.yang@linux.dev>]

On Sat, Nov 07, 2026 at 12:31:20PM +0800, Muchun Song wrote:
>LoongArch selects ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP but not
>ARCH_WANT_OPTIMIZE_DAX_VMEMMAP. Its vmemmap_free() implementation is also
>empty, so it cannot tear down optimized Device DAX mappings.
>
>A later change removes the generic Device DAX architecture gate and lets
>vmemmap_populate() decide whether a section can use optimization. Clear the
>section compound order in the LoongArch path so Device DAX keeps using
>unoptimized mappings.
>
>This does not affect HugeTLB HVO: LoongArch does not support gigantic
>pages, so HugeTLB does not use section-based vmemmap optimization there.
>
>Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>---
>v2:
>- New patch using section_vmemmap_optimization_disable() to keep Device
>  DAX vmemmap optimization disabled on LoongArch (reported by Sashiko)
>---
> arch/loongarch/mm/init.c | 2 ++
> 1 file changed, 2 insertions(+)
>
>diff --git a/arch/loongarch/mm/init.c b/arch/loongarch/mm/init.c
>index f801f7097f03..ea769e479241 100644
>--- a/arch/loongarch/mm/init.c
>+++ b/arch/loongarch/mm/init.c
>@@ -15,6 +15,7 @@
> #include <linux/memblock.h>
> #include <linux/memremap.h>
> #include <linux/mm.h>
>+#include <linux/vmemmap-optimization.h>
> #include <linux/mman.h>
> #include <linux/highmem.h>
> #include <linux/swap.h>
>@@ -143,6 +144,7 @@ void __meminit vmemmap_set_pmd(pmd_t *pmd, void *p, int node,
> int __meminit vmemmap_populate(unsigned long start, unsigned long end,
> 			       int node, struct vmem_altmap *altmap)
> {
>+	section_vmemmap_optimization_disable(start, end);

Yeah, clearing the order here keeps Device DAX optimization disabled once
the generic gate goes away ;)

Nothing jumped out at me, feel free to add:

Reviewed-by: Lance Yang <lance.yang@linux.dev>