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

[Balbir Singh <balbirs@nvidia.com>]

On 8/1/25 17:04, David Hildenbrand wrote:
> On 01.08.25 06:44, Balbir Singh wrote:
>> On 8/1/25 11:16, Mika Penttilä wrote:
>>> Hi,
>>>
>>> On 8/1/25 03:49, Balbir Singh wrote:
>>>
>>>> On 7/31/25 21:26, Zi Yan wrote:
>>>>> 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?
>>>>>
>>>> That is true in general, the special cases I mentioned are:
>>>>
>>>> 1. split during migration (where we the sizes on source/destination do not
>>>>     match) and so we need to split in the middle of migration. The entries
>>>>     there are already unmapped and hence the special handling
>>>> 2. Partial unmap case, where we need to split in the context of the unmap
>>>>     due to the isses mentioned in the patch. I expanded the folio split code
>>>>     for device private can be expanded into its own helper, which does not
>>>>     need to do the xas/mapped/lru folio handling. During partial unmap the
>>>>     original folio does get replaced by new anon rmap ptes (split_huge_pmd_locked)
>>>>
>>>> For (2), I spent some time examining the implications of not unmapping the
>>>> folios prior to split and in the partial unmap path, once we split the PMD
>>>> the folios diverge. I did not run into any particular race either with the
>>>> tests.
>>>
>>> 1) is totally fine. This was in v1 and lead to Zi's split_unmapped_folio()
>>>
>>> 2) is a problem because folio is mapped. split_huge_pmd() can be reached also from other than unmap path.
>>> It is vulnerable to races by rmap. And for instance this does not look right without checking:
>>>
>>>     folio_ref_freeze(folio, 1 + folio_expected_ref_count(folio));
>>>
>>
>> I can add checks to make sure that the call does succeed.
>>
>>> You mention 2) is needed because of some later problems in fault path after pmd split. Would it be
>>> possible to split the folio at fault time then?
>>
>> So after the partial unmap, the folio ends up in a little strange situation, the folio is large,
>> but not mapped (since large_mapcount can be 0, after all the folio_rmap_remove_ptes). Calling folio_split()
>> on partially unmapped fails because folio_get_anon_vma() fails due to the folio_mapped() failures
>> related to folio_large_mapcount. There is also additional complexity with ref counts and mapping.
> 
> I think you mean "Calling folio_split() on a *fully* unmapped folio fails ..."
> 
> A partially mapped folio still has folio_mapcount() > 0 -> folio_mapped() == true.
> 

Looking into this again at my end

>>
>>
>>> Also, didn't quite follow what kind of lock recursion did you encounter doing proper split_folio()
>>> instead?
>>>
>>>
>>
>> Splitting during partial unmap causes recursive locking issues with anon_vma when invoked from
>> split_huge_pmd_locked() path.
> 
> Yes, that's very complicated.
> 

Yes and I want to avoid going down that path.

>> Deferred splits do not work for device private pages, due to the
>> migration requirements for fault handling.
> 
> Can you elaborate on that?
> 

If a folio is under deferred_split() and is still pending a split. When a fault is handled on a partially
mapped folio, the expectation is that as a part of fault handling during migration, the code in migrate_folio_mapping()
assumes that the folio sizes are the same (via check for reference and mapcount)

Balbir