Re: [PATCHv2] mm/huge_memory: set the mmu_gather page size in zap_huge_pud()

Unmapping a device-dax mapping with 1G alignment on powerpc radix kernel
built with CONFIG_DEBUG_VM triggers this warning on every munmap:

    WARNING: mm/mmu_gather.c:176 at __tlb_remove_folio_pages_size+0x38/0x230
    NIP [c00000000063d508] __tlb_remove_folio_pages_size+0x38/0x230
    LR [c0000000006fb688] zap_huge_pud+0x1b8/0x2d0
    Call Trace:
     zap_huge_pud+0x170/0x2d0 (unreliable)
     __zap_vma_range+0x1604/0x1e20
     unmap_vmas+0xa0/0x1f0
     unmap_region+0xb0/0x130
     vms_complete_munmap_vmas+0x120/0x310
     do_vmi_align_munmap+0x1ec/0x240
     __vm_munmap+0x148/0x220
     sys_munmap+0x28/0x40

To trigger this - it is enough to mmap() /dev/dax0.0, touch the first
byte so that the whole 1G is mapped by a single PUD and then munmap()
it.

PowerPC needs MMU_GATHER_PAGE_SIZE in which mmu_gather needs to record
the page size for which tlb flush is needed. zap_huge_pmd(),
zap_pte_range(), change_huge_pud() etc all do this already. However,
since commit:
  dbe54153296d ("mm/huge_memory: add vmf_insert_folio_pud()")

a device-dax PUD maps a real folio and zap_huge_pud() now hands it to
tlb_remove_page_size(tlb, page, HPAGE_PUD_SIZE). At that point
tlb->page_size is still whatever the previous user left behind, which is
0 for the fresh gather set up by munmap() and so the check fires.

Fix this in the same way like how other similar paths do.

Fixes: dbe54153296d0 ("mm/huge_memory: add vmf_insert_folio_pud()")
Reported-by: Pavithra <redacted>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
---
[v1]->v2:
=========
- Split the 2 part patch series. Patch-1 is now superseded by a different patch
  from Muchun which he already posted [2].
- No difference in this patch from v1.

[v1]:
https://lore.kernel.org/all/97f85592bb78a273be8519e6897e7f9b9bd31469.1793070429.git.ritesh.list@gmail.com/ (local)
[2]:
https://lore.kernel.org/all/20261029063601.1961415-1-songmuchun@bytedance.com/ (local)

 mm/huge_memory.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index bd44d58e8684..7e589a2fb02d 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c

@@ -3040,6 +3040,8 @@ int zap_huge_pud(struct mmu_gather *tlb, struct

vm_area_struct *vma,
 	spinlock_t *ptl;
 	pud_t orig_pud;

+	tlb_change_page_size(tlb, HPAGE_PUD_SIZE);
+
 	ptl = __pud_trans_huge_lock(pud, vma);
 	if (!ptl)
 		return 0;
--
2.39.5