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

["Mika Penttilä" <mpenttil@redhat.com>]

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..

>>>> +	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
>