Re: [PATCH v4] mm: Use a folio in the softleaf_is_device_private path

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

On 8/17/26 18:39, Lorenzo Stoakes (ARM) wrote:
> On Mon, Aug 17, 2026 at 05:57:57PM +0200, David Hildenbrand (Arm) wrote:
>> On 8/17/26 17:52, David Hildenbrand (Arm) wrote:
>>>
>>> Okay, so this breaks the HMM selftests:
>>>
>>> # ok 30 hmm.hmm_device_private.migrate_anon_huge_fault
>>> # #  RUN           hmm.hmm_device_private.migrate_partial_unmap_fault ...
>>> [   14.589125] Internal error: Oops - BUG: 00000000f2000800 [#1]  SMP
>>> [   14.589178] Modules linked in: test_hmm
>>> [   14.589203] CPU: 25 UID: 0 PID: 653 Comm: hmm-tests Not tainted 7.2.0-rc5+ #14 PREEMPT
>>> [   14.589243] Hardware name: linux,dummy-virt (DT)
>>> [   14.589268] pstate: 63400005 (nZCv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--)
>>> [   14.589300] pc : folio_unlock+0x78/0x88
>>> [   14.589326] lr : folio_unlock+0x78/0x88
>>> [   14.589349] sp : ffff80008284bb90
>>> [   14.589368] x29: ffff80008284bb90 x28: ffff0000cc8ce180 x27: 0000000000000000
>>> [   14.589406] x26: 0000000000000000 x25: ffff0000ca395dc0 x24: 0000000000000000
>>> [   14.589447] x23: 0000000020100073 x22: ffff0000c23d35c0 x21: ffffffffbfff8000
>>> [   14.589485] x20: ffffffffbfff8000 x19: ffff80008284bca0 x18: 0000000000000010
>>> [   14.589523] x17: 3030303030303030 x16: 2066666666666666 x15: 6630303030303030
>>> [   14.589562] x14: 0000000000000000 x13: 29296f696c6f6628 x12: 64656b636f6c5f74
>>> [   14.589603] x11: fffffffffffc7100 x10: fffffffffffc70b0 x9 : ffffb0504ff88f70
>>> [   14.589640] x8 : 3fffffffffffdfff x7 : ffff000c79b24bc0 x6 : 00000000000001a7
>>> [   14.589680] x5 : ffff000c79b24bc0 x4 : 0000000000000000 x3 : 0000000000000000
>>> [   14.589718] x2 : 0000000000000000 x1 : ffff0000cc8ce180 x0 : 000000000000003f
>>> [   14.589756] Call trace:
>>> [   14.589769]  folio_unlock+0x78/0x88 (P)
>>> [   14.589792]  do_swap_page+0xf40/0x1580
>>> [   14.589815]  __handle_mm_fault+0x648/0xe68
>>> [   14.589838]  handle_mm_fault+0x9c/0x330
>>> [   14.589858]  do_page_fault+0x268/0x7e0
>>> [   14.589882]  do_translation_fault+0x5c/0x80
>>> [   14.589904]  do_mem_abort+0x48/0xa0
>>> [   14.589927]  el0_da+0x38/0xd0
>>> [   14.589948]  el0t_64_sync_handler+0xd0/0xe8
>>> [   14.589970]  el0t_64_sync+0x198/0x1a0
>>> [   14.589990] Code: d65f03c0 d000eec1 913c4021 94015993 (d4210000)
>>>
>>> I can only speculate that we need another
>>>
>>> 	folio = page_folio(vmf->page);
>>>
>>> after the migrate_to_ram() call.
>>>
>>> Let me try that real quick.
>>>
>>
>> Yeah, that fixes it:
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 4134ac607ee0..83d1a2be957c 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -4937,6 +4937,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>>                                 pte_unmap_unlock(vmf->pte, vmf->ptl);
>>                                 pgmap = page_pgmap(vmf->page);
>>                                 ret = pgmap->ops->migrate_to_ram(vmf);
>> +                               folio = page_folio(vmf->page);
> 
> So after migrate_to_ram() the folio might have been split or otherwise somehow
> the page doesn't belong to the same locked, refcount-incremented folio it did
> before?

I suspect a split.

> 
> That's kinda a footgun... but this documents it at least.
> 
> I think a comment explaining how this can happen would be helpful though as this
> doesn't seem intuitive.

I think, conceptually, calling into something that consumes a page (vmf->page)
always needs care when operating on folios.

Passing the vmf to some callback might be the odd thing here, because the
vmf->page contract is not really clear.

-- 
Cheers,

David