Re: [PATCH v2] dma-buf/udmabuf: reinstate a reasonable default size limit

["Christian König" <christian.koenig@amd.com>]

On 9/22/26 07:17, Kasireddy, Vivek wrote:
> Hi Christian,
> 
>> Subject: Re: [PATCH v2] dma-buf/udmabuf: reinstate a reasonable
>> default size limit
>>
>> On 9/19/26 02:42, Jiakai Xu wrote:
>>> Commit 44e9eb5a7621 ("dma-buf/udmabuf: Disable the size limit by
>>> default") raised the default size_limit_mb from 64 to INT_MAX.  With
>>> 4K pages, creating a buffer of more than INT_MAX / 8 pages (about 1
>> TB
>>> of pinned memory) now makes the two per-page pointer arrays in
>>> init_udmabuf() exceed what kvmalloc() can handle: kvmalloc() warns
>> and
>>> fails when the requested size exceeds INT_MAX bytes
>> (WARN_ON_ONCE in
>>> __kvmalloc_node_noprof), which is trivially triggerable from
>>> unprivileged users through the UDMABUF_CREATE ioctls:
>>>
>>>   WARNING: CPU: 0 PID: 412 at mm/slub.c:7021
>> __kvmalloc_node_noprof
>>>   Call Trace:
>>>    udmabuf_create
>>>    udmabuf_ioctl_create_list
>>>
>>> Restore a reasonable default limit of 256 MB, which is the value
>>> proposed in the RFC discussion that led to commit 44e9eb5a7621
>>
>> That is way to low, we want to allocate multiple gigabytes using that
>> API.
> I am wondering, what is the use-case that would require pinning multiple
> Gigabytes of memory via udmabuf?

We use that as replacement for driver allocated memory on acceleration devices.

Regards,
Christian.

> 
> Thanks,
> Vivek
> 
>>
>> Please make that (((INT_MAX / sizeof(struct folio *)) - 1) >> 20) <<
>> PAGE_SHIFT.
>>
>> That should allow to allocate a bit below 1TiB again without triggering
>> the issue.
>>
>> Regards,
>> Christian.
>>
>>> ("dma-buf/udmabuf: Disable the size limit by default") and
>> comfortably
>>> covers the 8K video use cases cited there, while keeping the
>>> kvmalloc() request far below its threshold.  The limit remains
>>> runtime-tunable via the size_limit_mb module parameter for setups
>>> that need more.
>>>
>>> Fixes: 44e9eb5a7621 ("dma-buf/udmabuf: Disable the size limit by
>> default")
>>> Assisted-by: OpenCode:DeepSeek-V4-Flash
>>> Signed-off-by: Jiakai Xu <xujiakai24@mails.ucas.ac.cn>
>>> ---
>>> V1 -> V2:
>>> - Instead of adding a pgcnt check in udmabuf_create(), restore a
>>>   reasonable default size limit (256 MB), as suggested by Christian
>>>   König.
>>> - Drop the second Fixes tag; the regression is solely due to
>>>   44e9eb5a7621.
>>> - Drop the "Cc: stable" tag; 44e9eb5a7621 is only in v7.3-rc1 and has
>>>   not shipped in any released kernel yet.
>>>
>>> V1: https://lore.kernel.org/all/20260918013434.1849135-1-
>> xujiakai24@mails.ucas.ac.cn/
>>> ---
>>>  drivers/dma-buf/udmabuf.c | 4 ++--
>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
>>> index df6dd00462423..4d90c2cb14225 100644
>>> --- a/drivers/dma-buf/udmabuf.c
>>> +++ b/drivers/dma-buf/udmabuf.c
>>> @@ -20,9 +20,9 @@ static int list_limit = 1024;
>>>  module_param(list_limit, int, 0644);
>>>  MODULE_PARM_DESC(list_limit, "udmabuf_create_list->count limit.
>> Default is 1024.");
>>>
>>> -static int size_limit_mb = INT_MAX;
>>> +static int size_limit_mb = 256;
>>>  module_param(size_limit_mb, int, 0644);
>>> -MODULE_PARM_DESC(size_limit_mb, "Max size of a dmabuf, in
>> megabytes. Default is INT_MAX.");
>>> +MODULE_PARM_DESC(size_limit_mb, "Max size of a dmabuf, in
>> megabytes. Default is 256.");
>>>
>>>  struct udmabuf {
>>>         pgoff_t pagecount;
>>> --
>>> 2.34.1
>>>
>>> --
>>> Below is the crash report:
>>> ------------[ cut here ]------------
>>> WARNING: mm/slub.c:7013 at __kvmalloc_node_noprof+0x516/0x5d0
>> home/zzzrrll/tmp/kf_src/linux-7.3-rc2/mm/slub.c:7013, CPU#0:
>> syz.5.27938/167363
>>> Modules linked in:
>>> CPU: 0 UID: 0 PID: 167363 Comm: syz.5.27938 Tainted: G        W    L
>> 7.3.0-rc2 #1 PREEMPT(full)
>>> Tainted: [W]=WARN, [L]=SOFTLOCKUP
>>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-
>> 1 04/01/2014
>>> RIP: 0010:__kvmalloc_node_noprof+0x516/0x5d0
>> home/zzzrrll/tmp/kf_src/linux-7.3-rc2/mm/slub.c:7013
>>> Code: 05 bf fa 2d 08 49 8b 0f c6 04 08 00 65 48 ff 0d 90 fa 2d 08 0f 85
>> 4c fd ff ff e8 05 43 8c ff e9 42 fd ff ff e8 9b 86 0e 04 90 <0f> 0b 90 31 db
>> e9 d2 fe ff ff f3 0f 1e fa 65 8b 05 8d fa 2d 08 89
>>> RSP: 0018:ffffc90002bc3cf8 EFLAGS: 00010246
>>> RAX: 0000000000000100 RBX: 0000000000000000 RCX:
>> 0000008000000000
>>> RDX: 0000008000000000 RSI: 0000000000000000 RDI: ffffffff835e3226
>>> RBP: 00000000ffffffff R08: 00000000000028c0 R09: 00000000ffffffff
>>> R10: 0000000000000006 R11: 0000000000000002 R12:
>> 0000008000000000
>>> R13: 00000000000028c0 R14: 0000001000000000 R15:
>> 0000008000000000
>>> FS:  00007fc89ae45640(0000) GS:ffff8880f3de4000(0000)
>> knlGS:0000000000000000
>>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>> CR2: 000000110c2ff305 CR3: 000000002b3d4000 CR4:
>> 0000000000752ef0
>>> DR0: 0000000000000000 DR1: 0000000000000000 DR2:
>> 0000000000006000
>>> DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7:
>> 0000000000000400
>>> PKRU: 80000000
>>> Call Trace:
>>>  <TASK>
>>>  init_udmabuf home/zzzrrll/tmp/kf_src/linux-7.3-rc2/drivers/dma-
>> buf/udmabuf.c:193 [inline]
>>>  udmabuf_create+0x1b6/0x750 home/zzzrrll/tmp/kf_src/linux-7.3-
>> rc2/drivers/dma-buf/udmabuf.c:386
>>>  udmabuf_ioctl_create home/zzzrrll/tmp/kf_src/linux-7.3-
>> rc2/drivers/dma-buf/udmabuf.c:465 [inline]
>>>  udmabuf_ioctl+0x132/0x1d0 home/zzzrrll/tmp/kf_src/linux-7.3-
>> rc2/drivers/dma-buf/udmabuf.c:496
>>>  vfs_ioctl home/zzzrrll/tmp/kf_src/linux-7.3-rc2/fs/ioctl.c:51 [inline]
>>>  __do_sys_ioctl home/zzzrrll/tmp/kf_src/linux-7.3-rc2/fs/ioctl.c:597
>> [inline]
>>>  __se_sys_ioctl+0xbc/0x130 home/zzzrrll/tmp/kf_src/linux-7.3-
>> rc2/fs/ioctl.c:583
>>>  do_syscall_x64 home/zzzrrll/tmp/kf_src/linux-7.3-
>> rc2/arch/x86/entry/syscall_64.c:61 [inline]
>>>  do_syscall_64+0x12b/0x350 home/zzzrrll/tmp/kf_src/linux-7.3-
>> rc2/arch/x86/entry/syscall_64.c:84
>>>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>>> RIP: 0033:0x590d6d
>>> Code: 02 b8 ff ff ff ff c3 66 0f 1f 44 00 00 f3 0f 1e fa 48 89 f8 48 89 f7 48
>> 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff
>> 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
>>> RSP: 002b:00007fc89ae44fd8 EFLAGS: 00000216 ORIG_RAX:
>> 0000000000000010
>>> RAX: ffffffffffffffda RBX: 0000000000611e7f RCX: 0000000000590d6d
>>> RDX: 00002000000023c0 RSI: 0000000040187542 RDI:
>> 000000000000000a
>>> RBP: 00007fc89ae45010 R08: 0000000000000000 R09:
>> 0000000000000000
>>> R10: 0000000000000000 R11: 0000000000000216 R12:
>> 00007fc89ae45640
>>> R13: 000000000000000b R14: 0000000000528d40 R15:
>> 00007fc89ae25000
>>>  </TASK>
>>> ---[ end trace 0000000000000000 ]---
>>> ---
>>>
>