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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 10/30/26 11:15, Ritesh Harjani (IBM) wrote:
> "David Hildenbrand (Arm)" <david@kernel.org> writes:
> 
>> On 10/30/26 01:07, 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 Patch-1 is now superseded by a different patch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
>>>
>>
>>
>> Matches zap_huge_pmd().
>>
>> I will wait a bit  and then queue this shortly.
>>
> 
> Thanks David, however, looks like Andrew might have queued this up in
> his mm tree [1], right?
> 
> [1]: https://lore.kernel.org/all/20261030021905.2DCA71F000C1@smtp.kernel.org/

No worries, one of us can drop it before sending it upstream.

I'm about to send out the MAINTAINTERS update in a bit, so right now it's still
confusing.

(but also there is no harm in having it in two trees temporarily)

> 
>> This sounds like stable material?
>>
> 
> Sorry, my bad. Yes, we should have this Cc: stable@vger.kernel.org.
> 
> Kindly let me know if I should update the tag and send v3, or can it be
> taken care? Thanks!

I would just apply that when picking, no need to resend.

Thanks!

-- 
Cheers,

David