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Message-ID: <ab46303e-a891-4f48-8a86-964155a1073d@nvidia.com>
Date: Sat, 2 Aug 2025 20:37:44 +1000
From: Balbir Singh <balbirs@...dia.com>
To: Zi Yan <ziy@...dia.com>, Mika Penttilä
<mpenttil@...hat.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
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
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;
+ }
+
+ /*
+ * 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);
+ }
+ folio_put(folio);
+ addr = start;
+ goto again;
+ }
+
mpfn = migrate_pfn(page_to_pfn(page)) |
MIGRATE_PFN_MIGRATE;
if (is_writable_device_private_entry(entry))
--
2.50.1
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