Message-ID: <ee06bd19-4831-493f-ae88-f1d8a2fe9fa4@redhat.com>
Date: Fri, 1 Aug 2025 10:46:03 +0200
From: David Hildenbrand <david@...hat.com>
To: Balbir Singh <balbirs@...dia.com>, Mika Penttilä
 <mpenttil@...hat.com>, Zi Yan <ziy@...dia.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Karol Herbst <kherbst@...hat.com>, Lyude Paul <lyude@...hat.com>,
 Danilo Krummrich <dakr@...nel.org>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Jérôme Glisse
 <jglisse@...hat.com>, Shuah Khan <shuah@...nel.org>,
 Barry Song <baohua@...nel.org>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Ryan Roberts <ryan.roberts@....com>, Matthew Wilcox <willy@...radead.org>,
 Peter Xu <peterx@...hat.com>, Kefeng Wang <wangkefeng.wang@...wei.com>,
 Jane Chu <jane.chu@...cle.com>, Alistair Popple <apopple@...dia.com>,
 Donet Tom <donettom@...ux.ibm.com>, Matthew Brost <matthew.brost@...el.com>,
 Francois Dugast <francois.dugast@...el.com>,
 Ralph Campbell <rcampbell@...dia.com>
Subject: Re: [v2 02/11] mm/thp: zone_device awareness in THP handling code

On 01.08.25 10:01, Balbir Singh wrote:
> 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@...hat.com>
>>>>>>>>>>>>>>>>>>> Cc: Lyude Paul <lyude@...hat.com>
>>>>>>>>>>>>>>>>>>> Cc: Danilo Krummrich <dakr@...nel.org>
>>>>>>>>>>>>>>>>>>> Cc: David Airlie <airlied@...il.com>
>>>>>>>>>>>>>>>>>>> Cc: Simona Vetter <simona@...ll.ch>
>>>>>>>>>>>>>>>>>>> Cc: "Jérôme Glisse" <jglisse@...hat.com>
>>>>>>>>>>>>>>>>>>> Cc: Shuah Khan <shuah@...nel.org>
>>>>>>>>>>>>>>>>>>> Cc: David Hildenbrand <david@...hat.com>
>>>>>>>>>>>>>>>>>>> Cc: Barry Song <baohua@...nel.org>
>>>>>>>>>>>>>>>>>>> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
>>>>>>>>>>>>>>>>>>> Cc: Ryan Roberts <ryan.roberts@....com>
>>>>>>>>>>>>>>>>>>> Cc: Matthew Wilcox <willy@...radead.org>
>>>>>>>>>>>>>>>>>>> Cc: Peter Xu <peterx@...hat.com>
>>>>>>>>>>>>>>>>>>> Cc: Zi Yan <ziy@...dia.com>
>>>>>>>>>>>>>>>>>>> Cc: Kefeng Wang <wangkefeng.wang@...wei.com>
>>>>>>>>>>>>>>>>>>> Cc: Jane Chu <jane.chu@...cle.com>
>>>>>>>>>>>>>>>>>>> Cc: Alistair Popple <apopple@...dia.com>
>>>>>>>>>>>>>>>>>>> Cc: Donet Tom <donettom@...ux.ibm.com>
>>>>>>>>>>>>>>>>>>> Cc: Mika Penttilä <mpenttil@...hat.com>
>>>>>>>>>>>>>>>>>>> Cc: Matthew Brost <matthew.brost@...el.com>
>>>>>>>>>>>>>>>>>>> Cc: Francois Dugast <francois.dugast@...el.com>
>>>>>>>>>>>>>>>>>>> Cc: Ralph Campbell <rcampbell@...dia.com>
>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>> Signed-off-by: Matthew Brost <matthew.brost@...el.com>
>>>>>>>>>>>>>>>>>>> Signed-off-by: Balbir Singh <balbirs@...dia.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)

If you hit a partially-mapped folio, instead of migrating, you would 
actually want to split and then migrate I assume.

-- 
Cheers,

David / dhildenb

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