linux-kernel - Re: [v1 resend 00/12] THP support for zone device page migration

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Message-ID: <0340e202-8ec8-4226-b136-e924eca00309@nvidia.com>
Date: Tue, 22 Jul 2025 09:48:18 +1000
From: Balbir Singh <balbirs@...dia.com>
To: Matthew Brost <matthew.brost@...el.com>
Cc: linux-mm@...ck.org, akpm@...ux-foundation.org,
 linux-kernel@...r.kernel.org, Karol Herbst <kherbst@...hat.com>,
 Lyude Paul <lyude@...hat.com>, Danilo Krummrich <dakr@...nel.org>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Jérôme Glisse <jglisse@...hat.com>,
 Shuah Khan <shuah@...nel.org>, David Hildenbrand <david@...hat.com>,
 Barry Song <baohua@...nel.org>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Ryan Roberts <ryan.roberts@....com>, Matthew Wilcox <willy@...radead.org>,
 Peter Xu <peterx@...hat.com>, Zi Yan <ziy@...dia.com>,
 Kefeng Wang <wangkefeng.wang@...wei.com>, Jane Chu <jane.chu@...cle.com>,
 Alistair Popple <apopple@...dia.com>, Donet Tom <donettom@...ux.ibm.com>
Subject: Re: [v1 resend 00/12] THP support for zone device page migration

On 7/18/25 14:57, Matthew Brost wrote:
> On Fri, Jul 18, 2025 at 01:57:13PM +1000, Balbir Singh wrote:
>> On 7/18/25 09:40, Matthew Brost wrote:
>>> On Fri, Jul 04, 2025 at 09:34:59AM +1000, Balbir Singh wrote:
>> ...
>>>>
>>>> The nouveau dmem code has been enhanced to use the new THP migration
>>>> capability.
>>>>
>>>> Feedback from the RFC [2]:
>>>>
>>>
>>> Thanks for the patches, results look very promising. I wanted to give
>>> some quick feedback:
>>>
>>
>> Are you seeing improvements with the patchset?
>>
> 
> We're nowhere near stable yet, but basic testing shows that CPU time
> from the start of migrate_vma_* to the end drops from ~300µs to ~6µs on
> a 2MB GPU fault. A lot of this improvement is dma-mapping at 2M
> grainularity for the CPU<->GPU copy rather than mapping 512 4k pages
> too.
> 
>>> - You appear to have missed updating hmm_range_fault, specifically
>>> hmm_vma_handle_pmd, to check for device-private entries and populate the
>>> HMM PFNs accordingly. My colleague François has a fix for this if you're
>>> interested.
>>>
>>
>> Sure, please feel free to post them. 
>>
>>> - I believe copy_huge_pmd also needs to be updated to avoid installing a
>>> migration entry if the swap entry is device-private. I don't have an
>>> exact fix yet due to my limited experience with core MM. The test case
>>> that triggers this is fairly simple: fault in a 2MB device page on the
>>> GPU, then fork a process that reads the page — the kernel crashes in
>>> this scenario.
>>>
>>
>> I'd be happy to look at any traces you have or post any fixes you have
>>
> 
> I've got it so the kernel doesn't explode but still get warnings like:
> 
> [ 3564.850036] mm/pgtable-generic.c:54: bad pmd ffff8881290408e0(efffff80042bfe00)
> [ 3565.298186] BUG: Bad rss-counter state mm:ffff88810a100c40 type:MM_ANONPAGES val:114688
> [ 3565.306108] BUG: non-zero pgtables_bytes on freeing mm: 917504
> 
> I'm basically just skip is_swap_pmd clause if the entry is device
> private, and let the rest of the function execute. This avoids
> installing a migration entry—which isn't required and cause the
> crash—and allows the rmap code to run, which flips the pages to not
> anonymous exclusive (effectively making them copy-on-write (?), though
> that doesn't fully apply to device pages). It's not 100% correct yet,
> but it's a step in the right direction.
> 


Thanks, could you post the stack trace as well. This is usually a symptom of 
not freeing up the page table cleanly.

Do you have my latest patches that have

	if (is_swap_pmd(pmdval)) {
		swp_entry_t entry = pmd_to_swp_entry(pmdval);

		if (is_device_private_entry(entry))
			goto nomap;
	}

in __pte_offset_map()?

Balbir Singh