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

[Mika Penttilä]

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@xxxxxxxxxx>
>>>>> Cc: Lyude Paul <lyude@xxxxxxxxxx>
>>>>> Cc: Danilo Krummrich <dakr@xxxxxxxxxx>
>>>>> Cc: David Airlie <airlied@xxxxxxxxx>
>>>>> Cc: Simona Vetter <simona@xxxxxxxx>
>>>>> Cc: "Jérôme Glisse" <jglisse@xxxxxxxxxx>
>>>>> Cc: Shuah Khan <shuah@xxxxxxxxxx>
>>>>> Cc: David Hildenbrand <david@xxxxxxxxxx>
>>>>> Cc: Barry Song <baohua@xxxxxxxxxx>
>>>>> Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
>>>>> Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
>>>>> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
>>>>> Cc: Peter Xu <peterx@xxxxxxxxxx>
>>>>> Cc: Zi Yan <ziy@xxxxxxxxxx>
>>>>> Cc: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
>>>>> Cc: Jane Chu <jane.chu@xxxxxxxxxx>
>>>>> Cc: Alistair Popple <apopple@xxxxxxxxxx>
>>>>> Cc: Donet Tom <donettom@xxxxxxxxxxxxx>
>>>>> Cc: Mika Penttilä <mpenttil@xxxxxxxxxx>
>>>>> Cc: Matthew Brost <matthew.brost@xxxxxxxxx>
>>>>> Cc: Francois Dugast <francois.dugast@xxxxxxxxx>
>>>>> Cc: Ralph Campbell <rcampbell@xxxxxxxxxx>
>>>>>
>>>>> Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
>>>>> Signed-off-by: Balbir Singh <balbirs@xxxxxxxxxx>
>>>>> ---
>>>>>  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?

>
>>>>> +	ret = __split_unmapped_folio(folio, 0, &folio->page, NULL, NULL, true);
>>>> Confusing to  __split_unmapped_folio() if folio is mapped...
>>> From driver point of view, __split_unmapped_folio() probably should be renamed
>>> to __split_cpu_unmapped_folio(), since it is only dealing with CPU side
>>> folio meta data for split.
>>>
>>>
>>> Best Regards,
>>> Yan, Zi
>> Best Regards,
>> Yan, Zi
>>

--Mika