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Message-ID: <6442f975-6363-4969-a0bf-55d06eec9528@nvidia.com>
Date: Tue, 5 Aug 2025 20:36:48 +1000
From: Balbir Singh <balbirs@...dia.com>
To: Mika Penttilä <mpenttil@...hat.com>,
Zi Yan <ziy@...dia.com>
Cc: David Hildenbrand <david@...hat.com>, 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 8/5/25 20:35, Mika Penttilä wrote:
>
> On 8/5/25 13:27, Balbir Singh wrote:
>
>> On 8/5/25 14:24, Mika Penttilä wrote:
>>> Hi,
>>>
>>> On 8/5/25 07:10, Balbir Singh wrote:
>>>> On 8/5/25 09:26, Mika Penttilä wrote:
>>>>> Hi,
>>>>>
>>>>> On 8/5/25 01:46, Balbir Singh wrote:
>>>>>> On 8/2/25 22:13, Mika Penttilä wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> On 8/2/25 13:37, Balbir Singh wrote:
>>>>>>>> FYI:
>>>>>>>>
>>>>>>>> I have the following patch on top of my series that seems to make it work
>>>>>>>> without requiring the helper to split device private folios
>>>>>>>>
>>>>>>> I think this looks much better!
>>>>>>>
>>>>>> Thanks!
>>>>>>
>>>>>>>> Signed-off-by: Balbir Singh <balbirs@...dia.com>
>>>>>>>> ---
>>>>>>>> include/linux/huge_mm.h | 1 -
>>>>>>>> lib/test_hmm.c | 11 +++++-
>>>>>>>> mm/huge_memory.c | 76 ++++-------------------------------------
>>>>>>>> mm/migrate_device.c | 51 +++++++++++++++++++++++++++
>>>>>>>> 4 files changed, 67 insertions(+), 72 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>>>>>>>> index 19e7e3b7c2b7..52d8b435950b 100644
>>>>>>>> --- a/include/linux/huge_mm.h
>>>>>>>> +++ b/include/linux/huge_mm.h
>>>>>>>> @@ -343,7 +343,6 @@ unsigned long thp_get_unmapped_area_vmflags(struct file *filp, unsigned long add
>>>>>>>> vm_flags_t vm_flags);
>>>>>>>>
>>>>>>>> bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins);
>>>>>>>> -int split_device_private_folio(struct folio *folio);
>>>>>>>> int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>>>>>>>> unsigned int new_order, bool unmapped);
>>>>>>>> int min_order_for_split(struct folio *folio);
>>>>>>>> diff --git a/lib/test_hmm.c b/lib/test_hmm.c
>>>>>>>> index 341ae2af44ec..444477785882 100644
>>>>>>>> --- a/lib/test_hmm.c
>>>>>>>> +++ b/lib/test_hmm.c
>>>>>>>> @@ -1625,13 +1625,22 @@ static vm_fault_t dmirror_devmem_fault(struct vm_fault *vmf)
>>>>>>>> * the mirror but here we use it to hold the page for the simulated
>>>>>>>> * device memory and that page holds the pointer to the mirror.
>>>>>>>> */
>>>>>>>> - rpage = vmf->page->zone_device_data;
>>>>>>>> + rpage = folio_page(page_folio(vmf->page), 0)->zone_device_data;
>>>>>>>> dmirror = rpage->zone_device_data;
>>>>>>>>
>>>>>>>> /* FIXME demonstrate how we can adjust migrate range */
>>>>>>>> order = folio_order(page_folio(vmf->page));
>>>>>>>> nr = 1 << order;
>>>>>>>>
>>>>>>>> + /*
>>>>>>>> + * When folios are partially mapped, we can't rely on the folio
>>>>>>>> + * order of vmf->page as the folio might not be fully split yet
>>>>>>>> + */
>>>>>>>> + if (vmf->pte) {
>>>>>>>> + order = 0;
>>>>>>>> + nr = 1;
>>>>>>>> + }
>>>>>>>> +
>>>>>>>> /*
>>>>>>>> * Consider a per-cpu cache of src and dst pfns, but with
>>>>>>>> * large number of cpus that might not scale well.
>>>>>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>>>>>> index 1fc1efa219c8..863393dec1f1 100644
>>>>>>>> --- a/mm/huge_memory.c
>>>>>>>> +++ b/mm/huge_memory.c
>>>>>>>> @@ -72,10 +72,6 @@ static unsigned long deferred_split_count(struct shrinker *shrink,
>>>>>>>> struct shrink_control *sc);
>>>>>>>> static unsigned long deferred_split_scan(struct shrinker *shrink,
>>>>>>>> struct shrink_control *sc);
>>>>>>>> -static int __split_unmapped_folio(struct folio *folio, int new_order,
>>>>>>>> - struct page *split_at, struct xa_state *xas,
>>>>>>>> - struct address_space *mapping, bool uniform_split);
>>>>>>>> -
>>>>>>>> static bool split_underused_thp = true;
>>>>>>>>
>>>>>>>> static atomic_t huge_zero_refcount;
>>>>>>>> @@ -2924,51 +2920,6 @@ static void __split_huge_zero_page_pmd(struct vm_area_struct *vma,
>>>>>>>> pmd_populate(mm, pmd, pgtable);
>>>>>>>> }
>>>>>>>>
>>>>>>>> -/**
>>>>>>>> - * 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));
>>>>>>>> - ret = __split_unmapped_folio(folio, 0, &folio->page, NULL, NULL, true);
>>>>>>>> - VM_WARN_ON(ret);
>>>>>>>> - for (new_folio = folio_next(folio); new_folio != end_folio;
>>>>>>>> - new_folio = folio_next(new_folio)) {
>>>>>>>> - zone_device_private_split_cb(folio, new_folio);
>>>>>>>> - folio_ref_unfreeze(new_folio, 1 + folio_expected_ref_count(
>>>>>>>> - new_folio));
>>>>>>>> - }
>>>>>>>> -
>>>>>>>> - /*
>>>>>>>> - * Mark the end of the folio split for device private THP
>>>>>>>> - * split
>>>>>>>> - */
>>>>>>>> - zone_device_private_split_cb(folio, NULL);
>>>>>>>> - folio_ref_unfreeze(folio, 1 + folio_expected_ref_count(folio));
>>>>>>>> - return ret;
>>>>>>>> -}
>>>>>>>> -
>>>>>>>> static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>>>>>>>> unsigned long haddr, bool freeze)
>>>>>>>> {
>>>>>>>> @@ -3064,30 +3015,15 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>>>>>>>> freeze = false;
>>>>>>>> if (!freeze) {
>>>>>>>> rmap_t rmap_flags = RMAP_NONE;
>>>>>>>> - unsigned long addr = haddr;
>>>>>>>> - struct folio *new_folio;
>>>>>>>> - struct folio *end_folio = folio_next(folio);
>>>>>>>>
>>>>>>>> if (anon_exclusive)
>>>>>>>> rmap_flags |= RMAP_EXCLUSIVE;
>>>>>>>>
>>>>>>>> - folio_lock(folio);
>>>>>>>> - folio_get(folio);
>>>>>>>> -
>>>>>>>> - split_device_private_folio(folio);
>>>>>>>> -
>>>>>>>> - for (new_folio = folio_next(folio);
>>>>>>>> - new_folio != end_folio;
>>>>>>>> - new_folio = folio_next(new_folio)) {
>>>>>>>> - addr += PAGE_SIZE;
>>>>>>>> - folio_unlock(new_folio);
>>>>>>>> - folio_add_anon_rmap_ptes(new_folio,
>>>>>>>> - &new_folio->page, 1,
>>>>>>>> - vma, addr, rmap_flags);
>>>>>>>> - }
>>>>>>>> - folio_unlock(folio);
>>>>>>>> - folio_add_anon_rmap_ptes(folio, &folio->page,
>>>>>>>> - 1, vma, haddr, rmap_flags);
>>>>>>>> + folio_ref_add(folio, HPAGE_PMD_NR - 1);
>>>>>>>> + if (anon_exclusive)
>>>>>>>> + rmap_flags |= RMAP_EXCLUSIVE;
>>>>>>>> + folio_add_anon_rmap_ptes(folio, page, HPAGE_PMD_NR,
>>>>>>>> + vma, haddr, rmap_flags);
>>>>>>>> }
>>>>>>>> }
>>>>>>>>
>>>>>>>> @@ -4065,7 +4001,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>>>>>>>> if (nr_shmem_dropped)
>>>>>>>> shmem_uncharge(mapping->host, nr_shmem_dropped);
>>>>>>>>
>>>>>>>> - if (!ret && is_anon)
>>>>>>>> + if (!ret && is_anon && !folio_is_device_private(folio))
>>>>>>>> remap_flags = RMP_USE_SHARED_ZEROPAGE;
>>>>>>>>
>>>>>>>> remap_page(folio, 1 << order, remap_flags);
>>>>>>>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
>>>>>>>> index 49962ea19109..4264c0290d08 100644
>>>>>>>> --- a/mm/migrate_device.c
>>>>>>>> +++ b/mm/migrate_device.c
>>>>>>>> @@ -248,6 +248,8 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>>>>>>>> * page table entry. Other special swap entries are not
>>>>>>>> * migratable, and we ignore regular swapped page.
>>>>>>>> */
>>>>>>>> + struct folio *folio;
>>>>>>>> +
>>>>>>>> entry = pte_to_swp_entry(pte);
>>>>>>>> if (!is_device_private_entry(entry))
>>>>>>>> goto next;
>>>>>>>> @@ -259,6 +261,55 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>>>>>>>> pgmap->owner != migrate->pgmap_owner)
>>>>>>>> goto next;
>>>>>>>>
>>>>>>>> + folio = page_folio(page);
>>>>>>>> + if (folio_test_large(folio)) {
>>>>>>>> + struct folio *new_folio;
>>>>>>>> + struct folio *new_fault_folio;
>>>>>>>> +
>>>>>>>> + /*
>>>>>>>> + * The reason for finding pmd present with a
>>>>>>>> + * device private pte and a large folio for the
>>>>>>>> + * pte is partial unmaps. Split the folio now
>>>>>>>> + * for the migration to be handled correctly
>>>>>>>> + */
>>>>>>>> + pte_unmap_unlock(ptep, ptl);
>>>>>>>> +
>>>>>>>> + folio_get(folio);
>>>>>>>> + if (folio != fault_folio)
>>>>>>>> + folio_lock(folio);
>>>>>>>> + if (split_folio(folio)) {
>>>>>>>> + if (folio != fault_folio)
>>>>>>>> + folio_unlock(folio);
>>>>>>>> + ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
>>>>>>>> + goto next;
>>>>>>>> + }
>>>>>>>> +
>>>>>>> The nouveau migrate_to_ram handler needs adjustment also if split happens.
>>>>>>>
>>>>>> test_hmm needs adjustment because of the way the backup folios are setup.
>>>>> nouveau should check the folio order after the possible split happens.
>>>>>
>>>> You mean the folio_split callback?
>>> no, nouveau_dmem_migrate_to_ram():
>>> ..
>>> sfolio = page_folio(vmf->page);
>>> order = folio_order(sfolio);
>>> ...
>>> migrate_vma_setup()
>>> ..
>>> if sfolio is split order still reflects the pre-split order
>>>
>> Will fix, good catch!
>>
>>>>>>>> + /*
>>>>>>>> + * After the split, get back the extra reference
>>>>>>>> + * on the fault_page, this reference is checked during
>>>>>>>> + * folio_migrate_mapping()
>>>>>>>> + */
>>>>>>>> + if (migrate->fault_page) {
>>>>>>>> + new_fault_folio = page_folio(migrate->fault_page);
>>>>>>>> + folio_get(new_fault_folio);
>>>>>>>> + }
>>>>>>>> +
>>>>>>>> + new_folio = page_folio(page);
>>>>>>>> + pfn = page_to_pfn(page);
>>>>>>>> +
>>>>>>>> + /*
>>>>>>>> + * Ensure the lock is held on the correct
>>>>>>>> + * folio after the split
>>>>>>>> + */
>>>>>>>> + if (folio != new_folio) {
>>>>>>>> + folio_unlock(folio);
>>>>>>>> + folio_lock(new_folio);
>>>>>>>> + }
>>>>>>> Maybe careful not to unlock fault_page ?
>>>>>>>
>>>>>> split_page will unlock everything but the original folio, the code takes the lock
>>>>>> on the folio corresponding to the new folio
>>>>> I mean do_swap_page() unlocks folio of fault_page and expects it to remain locked.
>>>>>
>>>> Not sure I follow what you're trying to elaborate on here
>>> do_swap_page:
>>> ..
>>> if (trylock_page(vmf->page)) {
>>> ret = pgmap->ops->migrate_to_ram(vmf);
>>> <- vmf->page should be locked here even after split
>>> unlock_page(vmf->page);
>>>
>> Yep, the split will unlock all tail folios, leaving the just head folio locked
>> and this the change, the lock we need to hold is the folio lock associated with
>> fault_page, pte entry and not unlock when the cause is a fault. The code seems
>> to do the right thing there, let me double check
>
> Yes the fault case is ok. But if migrate not for a fault, we should not leave any page locked
>
migrate_vma_finalize() handles this
Balbir
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