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

[David Hildenbrand <david@redhat.com>]

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.

> 
> 
>> 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.

> Deferred splits do not work for device private pages, due to the
> migration requirements for fault handling.

Can you elaborate on that?

-- 
Cheers,

David / dhildenb