Re: [v2 02/11] mm/thp: zone_device awareness in THP handling code

[Zi Yan <ziy@nvidia.com>]

On 31 Jul 2025, at 3:15, David Hildenbrand wrote:

> On 30.07.25 18:29, Mika Penttilä wrote:
>>
>> On 7/30/25 18:58, Zi Yan wrote:
>>> On 30 Jul 2025, at 11:40, Mika Penttilä wrote:
>>>
>>>> On 7/30/25 18:10, Zi Yan wrote:
>>>>> On 30 Jul 2025, at 8:49, Mika Penttilä wrote:
>>>>>
>>>>>> On 7/30/25 15:25, Zi Yan wrote:
>>>>>>> On 30 Jul 2025, at 8:08, Mika Penttilä wrote:
>>>>>>>
>>>>>>>> On 7/30/25 14:42, Mika Penttilä wrote:
>>>>>>>>> On 7/30/25 14:30, Zi Yan wrote:
>>>>>>>>>> On 30 Jul 2025, at 7:27, Zi Yan wrote:
>>>>>>>>>>
>>>>>>>>>>> On 30 Jul 2025, at 7:16, Mika Penttilä wrote:
>>>>>>>>>>>
>>>>>>>>>>>> Hi,
>>>>>>>>>>>>
>>>>>>>>>>>> On 7/30/25 12:21, Balbir Singh wrote:
>>>>>>>>>>>>> Make THP handling code in the mm subsystem for THP pages aware of zone
>>>>>>>>>>>>> device pages. Although the code is designed to be generic when it comes
>>>>>>>>>>>>> to handling splitting of pages, the code is designed to work for THP
>>>>>>>>>>>>> page sizes corresponding to HPAGE_PMD_NR.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Modify page_vma_mapped_walk() to return true when a zone device huge
>>>>>>>>>>>>> entry is present, enabling try_to_migrate() and other code migration
>>>>>>>>>>>>> paths to appropriately process the entry. page_vma_mapped_walk() will
>>>>>>>>>>>>> return true for zone device private large folios only when
>>>>>>>>>>>>> PVMW_THP_DEVICE_PRIVATE is passed. This is to prevent locations that are
>>>>>>>>>>>>> not zone device private pages from having to add awareness. The key
>>>>>>>>>>>>> callback that needs this flag is try_to_migrate_one(). The other
>>>>>>>>>>>>> callbacks page idle, damon use it for setting young/dirty bits, which is
>>>>>>>>>>>>> not significant when it comes to pmd level bit harvesting.
>>>>>>>>>>>>>
>>>>>>>>>>>>> pmd_pfn() does not work well with zone device entries, use
>>>>>>>>>>>>> pfn_pmd_entry_to_swap() for checking and comparison as for zone device
>>>>>>>>>>>>> entries.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Zone device private entries when split via munmap go through pmd split,
>>>>>>>>>>>>> but need to go through a folio split, deferred split does not work if a
>>>>>>>>>>>>> fault is encountered because fault handling involves migration entries
>>>>>>>>>>>>> (via folio_migrate_mapping) and the folio sizes are expected to be the
>>>>>>>>>>>>> same there. This introduces the need to split the folio while handling
>>>>>>>>>>>>> the pmd split. Because the folio is still mapped, but calling
>>>>>>>>>>>>> folio_split() will cause lock recursion, the __split_unmapped_folio()
>>>>>>>>>>>>> code is used with a new helper to wrap the code
>>>>>>>>>>>>> split_device_private_folio(), which skips the checks around
>>>>>>>>>>>>> folio->mapping, swapcache and the need to go through unmap and remap
>>>>>>>>>>>>> folio.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Cc: Karol Herbst <kherbst@redhat.com>
>>>>>>>>>>>>> Cc: Lyude Paul <lyude@redhat.com>
>>>>>>>>>>>>> Cc: Danilo Krummrich <dakr@kernel.org>
>>>>>>>>>>>>> Cc: David Airlie <airlied@gmail.com>
>>>>>>>>>>>>> Cc: Simona Vetter <simona@ffwll.ch>
>>>>>>>>>>>>> Cc: "Jérôme Glisse" <jglisse@redhat.com>
>>>>>>>>>>>>> Cc: Shuah Khan <shuah@kernel.org>
>>>>>>>>>>>>> Cc: David Hildenbrand <david@redhat.com>
>>>>>>>>>>>>> Cc: Barry Song <baohua@kernel.org>
>>>>>>>>>>>>> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
>>>>>>>>>>>>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>>>>>>>>>>>>> Cc: Matthew Wilcox <willy@infradead.org>
>>>>>>>>>>>>> Cc: Peter Xu <peterx@redhat.com>
>>>>>>>>>>>>> Cc: Zi Yan <ziy@nvidia.com>
>>>>>>>>>>>>> Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
>>>>>>>>>>>>> Cc: Jane Chu <jane.chu@oracle.com>
>>>>>>>>>>>>> Cc: Alistair Popple <apopple@nvidia.com>
>>>>>>>>>>>>> Cc: Donet Tom <donettom@linux.ibm.com>
>>>>>>>>>>>>> Cc: Mika Penttilä <mpenttil@redhat.com>
>>>>>>>>>>>>> Cc: Matthew Brost <matthew.brost@intel.com>
>>>>>>>>>>>>> Cc: Francois Dugast <francois.dugast@intel.com>
>>>>>>>>>>>>> Cc: Ralph Campbell <rcampbell@nvidia.com>
>>>>>>>>>>>>>
>>>>>>>>>>>>> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
>>>>>>>>>>>>> Signed-off-by: Balbir Singh <balbirs@nvidia.com>
>>>>>>>>>>>>> ---
>>>>>>>>>>>>>   include/linux/huge_mm.h |   1 +
>>>>>>>>>>>>>   include/linux/rmap.h    |   2 +
>>>>>>>>>>>>>   include/linux/swapops.h |  17 +++
>>>>>>>>>>>>>   mm/huge_memory.c        | 268 +++++++++++++++++++++++++++++++++-------
>>>>>>>>>>>>>   mm/page_vma_mapped.c    |  13 +-
>>>>>>>>>>>>>   mm/pgtable-generic.c    |   6 +
>>>>>>>>>>>>>   mm/rmap.c               |  22 +++-
>>>>>>>>>>>>>   7 files changed, 278 insertions(+), 51 deletions(-)
>>>>>>>>>>>>>
>>>>>>>>>>> <snip>
>>>>>>>>>>>
>>>>>>>>>>>>> +/**
>>>>>>>>>>>>> + * split_huge_device_private_folio - split a huge device private folio into
>>>>>>>>>>>>> + * smaller pages (of order 0), currently used by migrate_device logic to
>>>>>>>>>>>>> + * split folios for pages that are partially mapped
>>>>>>>>>>>>> + *
>>>>>>>>>>>>> + * @folio: the folio to split
>>>>>>>>>>>>> + *
>>>>>>>>>>>>> + * The caller has to hold the folio_lock and a reference via folio_get
>>>>>>>>>>>>> + */
>>>>>>>>>>>>> +int split_device_private_folio(struct folio *folio)
>>>>>>>>>>>>> +{
>>>>>>>>>>>>> +	struct folio *end_folio = folio_next(folio);
>>>>>>>>>>>>> +	struct folio *new_folio;
>>>>>>>>>>>>> +	int ret = 0;
>>>>>>>>>>>>> +
>>>>>>>>>>>>> +	/*
>>>>>>>>>>>>> +	 * Split the folio now. In the case of device
>>>>>>>>>>>>> +	 * private pages, this path is executed when
>>>>>>>>>>>>> +	 * the pmd is split and since freeze is not true
>>>>>>>>>>>>> +	 * it is likely the folio will be deferred_split.
>>>>>>>>>>>>> +	 *
>>>>>>>>>>>>> +	 * With device private pages, deferred splits of
>>>>>>>>>>>>> +	 * folios should be handled here to prevent partial
>>>>>>>>>>>>> +	 * unmaps from causing issues later on in migration
>>>>>>>>>>>>> +	 * and fault handling flows.
>>>>>>>>>>>>> +	 */
>>>>>>>>>>>>> +	folio_ref_freeze(folio, 1 + folio_expected_ref_count(folio));
>>>>>>>>>>>> Why can't this freeze fail? The folio is still mapped afaics, why can't there be other references in addition to the caller?
>>>>>>>>>>> Based on my off-list conversation with Balbir, the folio is unmapped in
>>>>>>>>>>> CPU side but mapped in the device. folio_ref_freeeze() is not aware of
>>>>>>>>>>> device side mapping.
>>>>>>>>>> Maybe we should make it aware of device private mapping? So that the
>>>>>>>>>> process mirrors CPU side folio split: 1) unmap device private mapping,
>>>>>>>>>> 2) freeze device private folio, 3) split unmapped folio, 4) unfreeze,
>>>>>>>>>> 5) remap device private mapping.
>>>>>>>>> Ah ok this was about device private page obviously here, nevermind..
>>>>>>>> Still, isn't this reachable from split_huge_pmd() paths and folio is mapped to CPU page tables as a huge device page by one or more task?
>>>>>>> The folio only has migration entries pointing to it. From CPU perspective,
>>>>>>> it is not mapped. The unmap_folio() used by __folio_split() unmaps a to-be-split
>>>>>>> folio by replacing existing page table entries with migration entries
>>>>>>> and after that the folio is regarded as “unmapped”.
>>>>>>>
>>>>>>> The migration entry is an invalid CPU page table entry, so it is not a CPU
>>>>>> split_device_private_folio() is called for device private entry, not migrate entry afaics.
>>>>> Yes, but from CPU perspective, both device private entry and migration entry
>>>>> are invalid CPU page table entries, so the device private folio is “unmapped”
>>>>> at CPU side.
>>>> Yes both are "swap entries" but there's difference, the device private ones contribute to mapcount and refcount.
>>> Right. That confused me when I was talking to Balbir and looking at v1.
>>> When a device private folio is processed in __folio_split(), Balbir needed to
>>> add code to skip CPU mapping handling code. Basically device private folios are
>>> CPU unmapped and device mapped.
>>>
>>> Here are my questions on device private folios:
>>> 1. How is mapcount used for device private folios? Why is it needed from CPU
>>>     perspective? Can it be stored in a device private specific data structure?
>>
>> Mostly like for normal folios, for instance rmap when doing migrate. I think it would make
>> common code more messy if not done that way but sure possible.
>> And not consuming pfns (address space) at all would have benefits.
>>
>>> 2. When a device private folio is mapped on device, can someone other than
>>>     the device driver manipulate it assuming core-mm just skips device private
>>>     folios (barring the CPU access fault handling)?
>>>
>>> Where I am going is that can device private folios be treated as unmapped folios
>>> by CPU and only device driver manipulates their mappings?
>>>
>> Yes not present by CPU but mm has bookkeeping on them. The private page has no content
>> someone could change while in device, it's just pfn.
>
> Just to clarify: a device-private entry, like a device-exclusive entry, is a *page table mapping* tracked through the rmap -- even though they are not present page table entries.
>
> It would be better if they would be present page table entries that are PROT_NONE, but it's tricky to mark them as being "special" device-private, device-exclusive etc. Maybe there are ways to do that in the future.
>
> Maybe device-private could just be PROT_NONE, because we can identify the entry type based on the folio. device-exclusive is harder ...
>
>
> So consider device-private entries just like PROT_NONE present page table entries. Refcount and mapcount is adjusted accordingly by rmap functions.

Thanks for the clarification.

So folio_mapcount() for device private folios should be treated the same
as normal folios, even if the corresponding PTEs are not accessible from CPUs.
Then I wonder if the device private large folio split should go through
__folio_split(), the same as normal folios: unmap, freeze, split, unfreeze,
remap. Otherwise, how can we prevent rmap changes during the split?


Best Regards,
Yan, Zi