Re: [PATCH] mm/zone_device: Do not touch device folio after calling ->folio_free()

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

On 4/13/26 06:06, Alistair Popple wrote:
> On 2026-04-11 at 09:03 +1000, Matthew Brost <matthew.brost@intel.com> wrote...
>> The contents of a device folio can immediately change after calling
>> ->folio_free(), as the folio may be reallocated by a driver with a
>> different order. Instead of touching the folio again to extract the
>> pgmap, use the local stack variable when calling percpu_ref_put_many().
>>
>> Cc: David Hildenbrand <david@kernel.org>
>> Cc: Oscar Salvador <osalvador@suse.de>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Balbir Singh <balbirs@nvidia.com>
>> Cc: linux-mm@kvack.org
>> Cc: linux-cxl@vger.kernel.org
>> Cc: linux-kernel@vger.kernel.org
>> Fixes: d245f9b4ab80 ("mm/zone_device: support large zone device private folios")
>> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
>>
>> ---
>> Stack trace:
>>
>> [  631.875165] [IGT] xe_exec_system_allocator: starting subtest threads-many-new-prefetch
>> [  632.282992] Oops: general protection fault, probably for non-canonical address 0x900000000000000: 0000 [#1] SMP NOPTI
>> [  632.293469] CPU: 8 UID: 0 PID: 59267 Comm: xe_exec_system_ Not tainted 7.0.0-rc7-xe+ #281 PREEMPT(full)
>> [  632.316023] RIP: 0010:free_zone_device_folio+0x149/0x240
>> [  632.339782] RSP: 0000:ffffc90023d1fd00 EFLAGS: 00010206
>> [  632.344947] RAX: 0900000000000000 RBX: 0000000000000001 RCX: 0000000094472d4d
>> [  632.351991] RDX: ffffffff8155c76f RSI: 000000006f2213bf RDI: 000000008e84943a
>> [  632.359042] RBP: ffffea0ff4030001 R08: 0000000000000000 R09: 0000000000000001
>> [  632.366094] R10: 0000000000000028 R11: 0000000000000000 R12: ffff88811828e400
>> [  632.373145] R13: 0000000000000000 R14: 000fffffc0000000 R15: 0000000000100073
>> [  632.380194] FS:  00007f2f0fdfe6c0(0000) GS:ffff88890a7e7000(0000) knlGS:0000000000000000
>> [  632.388186] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> [  632.393870] CR2: 00007f2f002e90f8 CR3: 0000000106708002 CR4: 0000000000f70ef0
>> [  632.400919] PKRU: 55555554
>> [  632.403605] Call Trace:
>> [  632.406039]  <TASK>
>> [  632.408131]  do_swap_page+0x146d/0x18c0
>> [  632.411938]  ? __pte_offset_map+0x3e/0x190
>> [  632.415994]  __handle_mm_fault+0x6e8/0x8d0
>> [  632.420053]  handle_mm_fault+0xbf/0x250
>> [  632.423855]  ? lock_mm_and_find_vma+0x41/0x6f0
>> [  632.428256]  do_user_addr_fault+0x168/0x690
>> [  632.432399]  exc_page_fault+0x74/0x200
>> [  632.436117]  asm_exc_page_fault+0x26/0x30
>> [  632.440092] RIP: 0033:0x5587554ff70d
>> [  632.462142] RSP: 002b:00007f2f0fdfc970 EFLAGS: 00010246
>> [  632.467308] RAX: 0000000000003fc0 RBX: 00007f2f082e1fc0 RCX: 00007f2f12b3287d
>> [  632.474355] RDX: 0000000000000000 RSI: 00000000c048644a RDI: 0000000000000003
>> [  632.481404] RBP: 00007f2f082e1fc0 R08: 00007f2f0fdfc958 R09: 0000000000000066
>> [  632.488450] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
>> [  632.495495] R13: 00007f2f082de000 R14: 0000000000c00002 R15: 00007f2f1319e000
>> [  632.502547]  </TASK>
> 
> I'm not sure, but I think Andrew likes the stack traces included in the actual
> commit messages. I've certainly found it helpful when debugging traces reported
> from the field so would prefer it there.

Agreed.

> 
>> ---
>>  mm/memremap.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/memremap.c b/mm/memremap.c
>> index ac7be07e3361..053842d45cb1 100644
>> --- a/mm/memremap.c
>> +++ b/mm/memremap.c
>> @@ -454,7 +454,7 @@ void free_zone_device_folio(struct folio *folio)
>>  		if (WARN_ON_ONCE(!pgmap->ops || !pgmap->ops->folio_free))
>>  			break;
>>  		pgmap->ops->folio_free(folio);
>> -		percpu_ref_put_many(&folio->pgmap->ref, nr);
>> +		percpu_ref_put_many(&pgmap->ref, nr);
> 

I assume the ref keeps pgmap alive, such that that cannot go away after
the folio_free().

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David