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 <[email protected]>
>>>> 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/[email protected]
>>>
>>> This was a clarification on the HMM patch mentioned in the changelog
>>>
>>>> https://lkml.kernel.org/r/cabzroyzz8qlf5psedcvxgcnqmf9kfq3rzdnq0deik3o9ork...@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/[email protected]
>>>
>>> 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
>
