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

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

"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 Patch-1 is now superseded by a different patch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/

> 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!

-ritesh