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Message-ID: <e1889eb8-d2d9-4d97-b9ae-e50158442945@redhat.com>
Date: Fri, 4 Jul 2025 08:17:33 +0300
From: Mika Penttilä <mpenttil@...hat.com>
To: Balbir Singh <balbirs@...dia.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 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() ?
> + migrate->src[idx] &= ~MIGRATE_PFN_COMPOUND;
> + flags = migrate->src[idx] & ((1UL << MIGRATE_PFN_SHIFT) - 1);
> + pfn = migrate->src[idx] >> MIGRATE_PFN_SHIFT;
> + for (i = 1; i < HPAGE_PMD_NR; i++)
> + migrate->src[i+idx] = migrate_pfn(pfn + i) | flags;
> +}
> #else /* !CONFIG_ARCH_ENABLE_THP_MIGRATION */
> static int migrate_vma_insert_huge_pmd_page(struct migrate_vma *migrate,
> unsigned long addr,
> @@ -822,6 +840,11 @@ static int migrate_vma_insert_huge_pmd_page(struct migrate_vma *migrate,
> {
> return 0;
> }
> +
> +static void migrate_vma_split_pages(struct migrate_vma *migrate,
> + unsigned long idx, unsigned long addr,
> + struct folio *folio)
> +{}
> #endif
>
> /*
> @@ -971,8 +994,9 @@ static void __migrate_device_pages(unsigned long *src_pfns,
> struct migrate_vma *migrate)
> {
> struct mmu_notifier_range range;
> - unsigned long i;
> + unsigned long i, j;
> bool notified = false;
> + unsigned long addr;
>
> for (i = 0; i < npages; ) {
> struct page *newpage = migrate_pfn_to_page(dst_pfns[i]);
> @@ -1014,12 +1038,16 @@ static void __migrate_device_pages(unsigned long *src_pfns,
> (!(dst_pfns[i] & MIGRATE_PFN_COMPOUND))) {
> nr = HPAGE_PMD_NR;
> src_pfns[i] &= ~MIGRATE_PFN_COMPOUND;
> - src_pfns[i] &= ~MIGRATE_PFN_MIGRATE;
> - goto next;
> + } else {
> + nr = 1;
> }
>
> - migrate_vma_insert_page(migrate, addr, &dst_pfns[i],
> - &src_pfns[i]);
> + for (j = 0; j < nr && i + j < npages; j++) {
> + src_pfns[i+j] |= MIGRATE_PFN_MIGRATE;
> + migrate_vma_insert_page(migrate,
> + addr + j * PAGE_SIZE,
> + &dst_pfns[i+j], &src_pfns[i+j]);
> + }
> goto next;
> }
>
> @@ -1041,7 +1069,9 @@ static void __migrate_device_pages(unsigned long *src_pfns,
> MIGRATE_PFN_COMPOUND);
> goto next;
> }
> - src_pfns[i] &= ~MIGRATE_PFN_MIGRATE;
> + nr = 1 << folio_order(folio);
> + addr = migrate->start + i * PAGE_SIZE;
> + migrate_vma_split_pages(migrate, i, addr, folio);
> } else if ((src_pfns[i] & MIGRATE_PFN_MIGRATE) &&
> (dst_pfns[i] & MIGRATE_PFN_COMPOUND) &&
> !(src_pfns[i] & MIGRATE_PFN_COMPOUND)) {
> @@ -1076,12 +1106,17 @@ static void __migrate_device_pages(unsigned long *src_pfns,
> BUG_ON(folio_test_writeback(folio));
>
> if (migrate && migrate->fault_page == page)
> - extra_cnt = 1;
> - r = folio_migrate_mapping(mapping, newfolio, folio, extra_cnt);
> - if (r != MIGRATEPAGE_SUCCESS)
> - src_pfns[i] &= ~MIGRATE_PFN_MIGRATE;
> - else
> - folio_migrate_flags(newfolio, folio);
> + extra_cnt++;
> + for (j = 0; j < nr && i + j < npages; j++) {
> + folio = page_folio(migrate_pfn_to_page(src_pfns[i+j]));
> + newfolio = page_folio(migrate_pfn_to_page(dst_pfns[i+j]));
> +
> + r = folio_migrate_mapping(mapping, newfolio, folio, extra_cnt);
> + if (r != MIGRATEPAGE_SUCCESS)
> + src_pfns[i+j] &= ~MIGRATE_PFN_MIGRATE;
> + else
> + folio_migrate_flags(newfolio, folio);
> + }
> next:
> i += nr;
> }
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