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

[pavrampu <pavrampu@linux.ibm.com>]

On 2026-10-30 05:37, Ritesh Harjani (IBM) wrote:
> 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 <pavrampu@linux.ibm.com>
> 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 Patch-1 is now superseded by a 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/
> [2]:
> https://lore.kernel.org/all/20261029063601.1961415-1-songmuchun@bytedance.com/
> 
>  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

Tested on Power10 (ppc64le), kernel v7.3

Tested-by: Pavithra Prakash <pavrampu@linux.ibm.com>