Re: [v7 00/16] mm: support device-private THP

[Balbir Singh <balbirs@nvidia.com>]

On 11/20/25 13:50, Balbir Singh wrote:
> On 11/20/25 13:40, Matthew Brost wrote:
>> On Wed, Nov 12, 2025 at 10:52:43AM +1100, Balbir Singh wrote:
>>> On 11/12/25 10:43, Andrew Morton wrote:
>>>> On Thu, 9 Oct 2025 03:33:33 -0700 Matthew Brost <matthew.brost@intel.com> wrote:
>>>>
>>>>>>>> This patch series introduces support for Transparent Huge Page
>>>>>>>> (THP) migration in zone device-private memory. The implementation enables
>>>>>>>> efficient migration of large folios between system memory and
>>>>>>>> device-private memory
>>>>>>>
>>>>>>> Lots of chatter for the v6 series, but none for v7.  I hope that's a
>>>>>>> good sign.
>>>>>>>
>>>>>>
>>>>>> I hope so too, I've tried to address the comments in v6.
>>>>>>
>>>>>
>>>>> Circling back to this series, we will itegrate and test this version.
>>>>
>>>> How'd it go?
>>>>
>>
>> My apologies for the delay—I got distracted by other tasks in Xe (my
>> driver) and was out for a bit. Unfortunately, this series breaks
>> something in the existing core MM code for the Xe SVM implementation. I
>> have an extensive test case that hammers on SVM, which fully passes
>> prior to applying this series, but fails randomly with the series
>> applied (to drm-tip-rc6) due to the below kernel lockup.
>>
>> I've tried to trace where the migration PTE gets installed but not
>> removed or isolate a test case which causes this failure but no luck so
>> far. I'll keep digging as I have time.
>>
>> Beyond that, if I enable Xe SVM + THP, it seems to mostly work (though
>> the same issue as above eventually occurs), but I do need two additional
>> core MM patches—one is new code required for Xe, and the other could be
>> considered a bug fix. Those patches can included when Xe merges SVM THP
>> support but we need at least not break Xe SVM before this series merges.
>>
>> Stack trace:
>>
>> INFO: task kworker/u65:2:1642 blocked for more than 30
>> seconds.
>> [  212.624286]       Tainted: G S      W           6.18.0-rc6-xe+ #1719
>> [  212.630561] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
>> disables this message.
>> [  212.638285] task:kworker/u65:2   state:D stack:0     pid:1642
>> tgid:1642  ppid:2      task_flags:0x4208060 flags:0x00080000
>> [  212.638288] Workqueue: xe_page_fault_work_queue
>> xe_pagefault_queue_work [xe]
>> [  212.638323] Call Trace:
>> [  212.638324]  <TASK>
>> [  212.638325]  __schedule+0x4b0/0x990
>> [  212.638330]  schedule+0x22/0xd0
>> [  212.638331]  io_schedule+0x41/0x60
>> [  212.638333]  migration_entry_wait_on_locked+0x1d8/0x2d0
>> [  212.638336]  ? __pfx_wake_page_function+0x10/0x10
>> [  212.638339]  migration_entry_wait+0xd2/0xe0
>> [  212.638341]  hmm_vma_walk_pmd+0x7c9/0x8d0
>> [  212.638343]  walk_pgd_range+0x51d/0xa40
>> [  212.638345]  __walk_page_range+0x75/0x1e0
>> [  212.638347]  walk_page_range_mm+0x138/0x1f0
>> [  212.638349]  hmm_range_fault+0x59/0xa0
>> [  212.638351]  drm_gpusvm_get_pages+0x194/0x7b0 [drm_gpusvm_helper]
>> [  212.638354]  drm_gpusvm_range_get_pages+0x2d/0x40 [drm_gpusvm_helper]
>> [  212.638355]  __xe_svm_handle_pagefault+0x259/0x900 [xe]
>> [  212.638375]  ? update_load_avg+0x7f/0x6c0
>> [  212.638377]  ? update_curr+0x13d/0x170
>> [  212.638379]  xe_svm_handle_pagefault+0x37/0x90 [xe]
>> [  212.638396]  xe_pagefault_queue_work+0x2da/0x3c0 [xe]
>> [  212.638420]  process_one_work+0x16e/0x2e0
>> [  212.638422]  worker_thread+0x284/0x410
>> [  212.638423]  ? __pfx_worker_thread+0x10/0x10
>> [  212.638425]  kthread+0xec/0x210
>> [  212.638427]  ? __pfx_kthread+0x10/0x10
>> [  212.638428]  ? __pfx_kthread+0x10/0x10
>> [  212.638430]  ret_from_fork+0xbd/0x100
>> [  212.638433]  ? __pfx_kthread+0x10/0x10
>> [  212.638434]  ret_from_fork_asm+0x1a/0x30
>> [  212.638436]  </TASK>
>>
> 
> Hi, Matt
> 
> Thanks for the report, two questions
> 
> 1. Are you using mm/mm-unstable, we've got some fixes in there (including fixes to remove_migration_pmd())
>    - Generally a left behind migration entry is a symptom of a failed migration that did not clean up
>      after itself.
> 2. The stack trace is from hmm_range_fault(), not something that this code touches.
> 
> The stack trace shows your code is seeing a migration entry and waiting on it.
> Can you please provide a reproducer for the issue? In the form of a test in hmm-tests.c
> 
> Have you been able to bisect the issue?

Also could you please try with 10b9feee2d0d ("mm/hmm: populate PFNs from PMD swap entry")
reverted?

> 
> Balbir
> 
> 
>> Matt 
>>
>>>> Balbir, what's the status here?  It's been a month and this series
>>>> still has a "needs a new version" feeling to it.  If so, very soon
>>>> please.
>>>>
>>>
>>> I don't think this needs a new revision, I've been testing frequently
>>> at my end to see if I can catch any regressions. I have a patch update for
>>> mm-migrate_device-add-thp-splitting-during-migration.patch, it can be applied
>>> on top or I can send a new version of the patch. I was waiting
>>> on any feedback before I sent the patch out, but I'll do it now.
>>>
>>>> TODOs which I have noted are
>>>>
>>>> https://lkml.kernel.org/r/aOePfeoDuRW+prFq@lstrano-desk.jf.intel.com
>>>
>>> This was a clarification on the HMM patch mentioned in the changelog
>>>
>>>> https://lkml.kernel.org/r/CABzRoyZZ8QLF5PSeDCVxgcnQmF9kFQ3RZdNq0Deik3o9OrK+BQ@mail.gmail.com
>>>
>>> That's a minor comment on not using a temporary declaration, I don't think we need it, let me know if you feel strongly
>>>
>>>> https://lkml.kernel.org/r/D2A4B724-E5EF-46D3-9D3F-EBAD9B22371E@nvidia.com
>>>
>>> I have a patch for this, which I posted, I can do an update and resend it if required (the one mentioned above)
>>>
>>>> https://lkml.kernel.org/r/62073ca1-5bb6-49e8-b8d4-447c5e0e582e@
>>>>
>>>
>>> I can't seem to open this
>>>
>>>> plus a general re-read of the
>>>> mm-migrate_device-add-thp-splitting-during-migration.patch review
>>>> discussion.
>>>>
>>> That's the patch I have
>>>
>>> Thanks for following up
>>> Balbir
>