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Message-ID: <b25b3610-5755-4b7e-9a9f-d7f1dc3e4bdc@nvidia.com>
Date: Sat, 5 Jul 2025 10:26:05 +1000
From: Balbir Singh <balbirs@...dia.com>
To: Mika Penttilä <mpenttil@...hat.com>, linux-mm@...ck.org
Cc: akpm@...ux-foundation.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>,
David Hildenbrand <david@...hat.com>, 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>, Zi Yan <ziy@...dia.com>,
Kefeng Wang <wangkefeng.wang@...wei.com>, Jane Chu <jane.chu@...cle.com>,
Alistair Popple <apopple@...dia.com>, Donet Tom <donettom@...ux.ibm.com>
Subject: Re: [v1 resend 08/12] mm/thp: add split during migration support
On 7/4/25 16:43, Mika Penttilä wrote:
>
> On 7/4/25 08:17, Mika Penttilä wrote:
>> On 7/4/25 02:35, Balbir Singh wrote:
>>> Support splitting pages during THP zone device migration as needed.
>>> The common case that arises is that after setup, during migrate
>>> the destination might not be able to allocate MIGRATE_PFN_COMPOUND
>>> pages.
>>>
>>> Add a new routine migrate_vma_split_pages() to support the splitting
>>> of already isolated pages. The pages being migrated are already unmapped
>>> and marked for migration during setup (via unmap). folio_split() and
>>> __split_unmapped_folio() take additional isolated arguments, to avoid
>>> unmapping and remaping these pages and unlocking/putting the 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>
>>>
>>> Signed-off-by: Balbir Singh <balbirs@...dia.com>
>>> ---
>>> include/linux/huge_mm.h | 11 ++++++--
>>> mm/huge_memory.c | 54 ++++++++++++++++++++-----------------
>>> mm/migrate_device.c | 59 ++++++++++++++++++++++++++++++++---------
>>> 3 files changed, 85 insertions(+), 39 deletions(-)
>>>
>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>>> index 65a1bdf29bb9..5f55a754e57c 100644
>>> --- a/include/linux/huge_mm.h
>>> +++ b/include/linux/huge_mm.h
>>> @@ -343,8 +343,8 @@ 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_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>>> - unsigned int new_order);
>>> +int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>>> + unsigned int new_order, bool isolated);
>>> int min_order_for_split(struct folio *folio);
>>> int split_folio_to_list(struct folio *folio, struct list_head *list);
>>> bool uniform_split_supported(struct folio *folio, unsigned int new_order,
>>> @@ -353,6 +353,13 @@ bool non_uniform_split_supported(struct folio *folio, unsigned int new_order,
>>> bool warns);
>>> int folio_split(struct folio *folio, unsigned int new_order, struct page *page,
>>> struct list_head *list);
>>> +
>>> +static inline int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>>> + unsigned int new_order)
>>> +{
>>> + return __split_huge_page_to_list_to_order(page, list, new_order, false);
>>> +}
>>> +
>>> /*
>>> * try_folio_split - try to split a @folio at @page using non uniform split.
>>> * @folio: folio to be split
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index d55e36ae0c39..e00ddfed22fa 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -3424,15 +3424,6 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
>>> new_folio->mapping = folio->mapping;
>>> new_folio->index = folio->index + i;
>>>
>>> - /*
>>> - * page->private should not be set in tail pages. Fix up and warn once
>>> - * if private is unexpectedly set.
>>> - */
>>> - if (unlikely(new_folio->private)) {
>>> - VM_WARN_ON_ONCE_PAGE(true, new_head);
>>> - new_folio->private = NULL;
>>> - }
>>> -
>>> if (folio_test_swapcache(folio))
>>> new_folio->swap.val = folio->swap.val + i;
>>>
>>> @@ -3519,7 +3510,7 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>>> struct page *split_at, struct page *lock_at,
>>> struct list_head *list, pgoff_t end,
>>> struct xa_state *xas, struct address_space *mapping,
>>> - bool uniform_split)
>>> + bool uniform_split, bool isolated)
>>> {
>>> struct lruvec *lruvec;
>>> struct address_space *swap_cache = NULL;
>>> @@ -3643,8 +3634,9 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>>> percpu_ref_get_many(&release->pgmap->ref,
>>> (1 << new_order) - 1);
>>>
>>> - lru_add_split_folio(origin_folio, release, lruvec,
>>> - list);
>>> + if (!isolated)
>>> + lru_add_split_folio(origin_folio, release,
>>> + lruvec, list);
>>>
>>> /* Some pages can be beyond EOF: drop them from cache */
>>> if (release->index >= end) {
>>> @@ -3697,6 +3689,12 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>>> if (nr_dropped)
>>> shmem_uncharge(mapping->host, nr_dropped);
>>>
>>> + /*
>>> + * Don't remap and unlock isolated folios
>>> + */
>>> + if (isolated)
>>> + return ret;
>>> +
>>> remap_page(origin_folio, 1 << order,
>>> folio_test_anon(origin_folio) ?
>>> RMP_USE_SHARED_ZEROPAGE : 0);
>>> @@ -3790,6 +3788,7 @@ bool uniform_split_supported(struct folio *folio, unsigned int new_order,
>>> * @lock_at: a page within @folio to be left locked to caller
>>> * @list: after-split folios will be put on it if non NULL
>>> * @uniform_split: perform uniform split or not (non-uniform split)
>>> + * @isolated: The pages are already unmapped
>>> *
>>> * It calls __split_unmapped_folio() to perform uniform and non-uniform split.
>>> * It is in charge of checking whether the split is supported or not and
>>> @@ -3800,7 +3799,7 @@ bool uniform_split_supported(struct folio *folio, unsigned int new_order,
>>> */
>>> static int __folio_split(struct folio *folio, unsigned int new_order,
>>> struct page *split_at, struct page *lock_at,
>>> - struct list_head *list, bool uniform_split)
>>> + struct list_head *list, bool uniform_split, bool isolated)
>>> {
>>> struct deferred_split *ds_queue = get_deferred_split_queue(folio);
>>> XA_STATE(xas, &folio->mapping->i_pages, folio->index);
>>> @@ -3846,14 +3845,16 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>>> * is taken to serialise against parallel split or collapse
>>> * operations.
>>> */
>>> - anon_vma = folio_get_anon_vma(folio);
>>> - if (!anon_vma) {
>>> - ret = -EBUSY;
>>> - goto out;
>>> + if (!isolated) {
>>> + anon_vma = folio_get_anon_vma(folio);
>>> + if (!anon_vma) {
>>> + ret = -EBUSY;
>>> + goto out;
>>> + }
>>> + anon_vma_lock_write(anon_vma);
>>> }
>>> end = -1;
>>> mapping = NULL;
>>> - anon_vma_lock_write(anon_vma);
>>> } else {
>>> unsigned int min_order;
>>> gfp_t gfp;
>>> @@ -3920,7 +3921,8 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>>> goto out_unlock;
>>> }
>>>
>>> - unmap_folio(folio);
>>> + if (!isolated)
>>> + unmap_folio(folio);
>>>
>>> /* block interrupt reentry in xa_lock and spinlock */
>>> local_irq_disable();
>>> @@ -3973,14 +3975,15 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>>>
>>> ret = __split_unmapped_folio(folio, new_order,
>>> split_at, lock_at, list, end, &xas, mapping,
>>> - uniform_split);
>>> + uniform_split, isolated);
>>> } else {
>>> spin_unlock(&ds_queue->split_queue_lock);
>>> fail:
>>> if (mapping)
>>> xas_unlock(&xas);
>>> local_irq_enable();
>>> - remap_page(folio, folio_nr_pages(folio), 0);
>>> + if (!isolated)
>>> + remap_page(folio, folio_nr_pages(folio), 0);
>>> ret = -EAGAIN;
>>> }
>>>
>>> @@ -4046,12 +4049,13 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
>>> * Returns -EINVAL when trying to split to an order that is incompatible
>>> * with the folio. Splitting to order 0 is compatible with all folios.
>>> */
>>> -int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>>> - unsigned int new_order)
>>> +int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
>>> + unsigned int new_order, bool isolated)
>>> {
>>> struct folio *folio = page_folio(page);
>>>
>>> - return __folio_split(folio, new_order, &folio->page, page, list, true);
>>> + return __folio_split(folio, new_order, &folio->page, page, list, true,
>>> + isolated);
>>> }
>>>
>>> /*
>>> @@ -4080,7 +4084,7 @@ int folio_split(struct folio *folio, unsigned int new_order,
>>> struct page *split_at, struct list_head *list)
>>> {
>>> return __folio_split(folio, new_order, split_at, &folio->page, list,
>>> - false);
>>> + false, false);
>>> }
>>>
>>> int min_order_for_split(struct folio *folio)
>>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
>>> index 41d0bd787969..acd2f03b178d 100644
>>> --- a/mm/migrate_device.c
>>> +++ b/mm/migrate_device.c
>>> @@ -813,6 +813,24 @@ static int migrate_vma_insert_huge_pmd_page(struct migrate_vma *migrate,
>>> src[i] &= ~MIGRATE_PFN_MIGRATE;
>>> return 0;
>>> }
>>> +
>>> +static void migrate_vma_split_pages(struct migrate_vma *migrate,
>>> + unsigned long idx, unsigned long addr,
>>> + struct folio *folio)
>>> +{
>>> + unsigned long i;
>>> + unsigned long pfn;
>>> + unsigned long flags;
>>> +
>>> + folio_get(folio);
>>> + split_huge_pmd_address(migrate->vma, addr, true);
>>> + __split_huge_page_to_list_to_order(folio_page(folio, 0), NULL, 0, true);
>> We already have reference to folio, why is folio_get() needed ?
>>
>> Splitting the page splits pmd for anon folios, why is there split_huge_pmd_address() ?
>
> Oh I see
> + if (!isolated)
> + unmap_folio(folio);
>
> which explains the explicit split_huge_pmd_address(migrate->vma, addr, true);
>
> Still, why the folio_get(folio);?
>
>
That is for split_huge_pmd_address, when called with freeze=true, it drops the
ref count on the page
if (freeze)
put_page(page);
Balbir
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