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

David Hildenbrand (Arm)
2026-04-16T08:52:12Z

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