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

[Muchun Song <muchun.song@linux.dev>]

> On Oct 30, 2026, at 08:07, Ritesh Harjani (IBM) <ritesh.list@gmail.com> 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>

Acked-by: Muchun Song <muchun.song@linux.dev>

Thanks.