[PATCH v3 0/6] Fix device page migration in low memory fallback

[PATCH v3 0/6] Fix device page migration in low memory fallback

[Matthew Brost]

LLMs made my breakfast, lunch, and dinner. Not really. They served as an
assistive tool while I performed the debugging, testing, and analysis
needed to isolate the root cause in core MM while fixing a known DRM SVM
issue involving THP allocation failures in the CPU fault-to-device page
migration path.

When a CPU faults on a device private PMD and the driver cannot allocate
a compound destination folio, the source THP has to be split. That path
is broken: the CPU fault reference makes the split always fail, and it
demotes the PMD only in the faulting VMA, leaving any other VMA mapping
the folio pointing a huge PMD at an order-0 page.
The latter is memory corruption, previously masked by the former.

The DRM side had its own problems in the same fallback: there was no
order-0 fallback at all despite a TODO saying one was needed, the error
path computed folio_order() after put_page(), and once the destination
is demoted to order-0 the source page array has to be populated per
page rather than per folio head, or the copy stops after one page.

Validation was performed using xe_exec_system_allocator. The issue was
initially discovered on systems configured with an artificially
constrained memory footprint (mem=8G), where failures occurred
intermittently. Error injection was then introduced to reliably
reproduce the failure condition, enabling thorough validation of the
fix. Results were confirmed through pass/fail A/B testing.

Matt

v2::
 - Add assert in 'Fix folio allocation fallback and use-after-put' for
   THP placement invariant which Sashiko hallucinated as a bug [1]
 - Add 'Clear MIGRATE_PFN_MIGRATE on all sub-folios of a split THP'
   (Sashiko)
 - Fix checkpatch issues (CI)
 - Swap cache issue flagged by Sashiko [1] not fixed as this code doesn't
   appear reachable (i.e., dead code). Can address in a follow up if
   needed
v3:
 - Noticed Arvind's patch [2] fixes some of core issues attempting to be
   fixed to in v1, v2. Verified Arvind's patch works as expected and
   including it in this rev for Intel's CI which is based on drm-tip and
   does not have Arvind's patch. Dropping unnecessary fixes from v1, v2
   as a result.
 - Include 'mm/migrate_device: Do not write past the end of the src_pfns
   array' unrelated fix flagged by Sashiko in v2
 - Include 'dma-unmap pages before handling migration errors'
   unreleated fix flagged by Sashiko in v2

[1] https://sashiko.dev/#/patchset/20260805113338.3742178-1-matthew.brost%40intel.com
[2] https://patchew.org/linux/20260728062832.1107127-1-arvind.yadav@intel.com/

Arvind Yadav (1):
  mm/migrate_device: Clear stale mapping after freeing swapcache

Matthew Brost (5):
  mm/migrate_device: Do not write past the end of the src_pfns array
  mm/migrate_device: Fix THP splitting of a CPU faulted device private
    folio
  drm/pagemap: dma-unmap pages before handling migration errors
  drm/pagemap: Fix folio allocation fallback and use-after-put
  drm/pagemap: Add fault injection for higher-order RAM folio allocation

 drivers/gpu/drm/drm_pagemap.c | 176 +++++++++++++++++++++++++++-------
 mm/migrate_device.c           | 115 +++++++++++++++++++---
 2 files changed, 248 insertions(+), 43 deletions(-)

-- 
2.34.1

[PATCH v3 1/6] mm/migrate_device: Clear stale mapping after freeing swapcache

[Matthew Brost]

From: Arvind Yadav <arvind.yadav@intel.com>

__migrate_device_pages() reads the folio mapping before calling
folio_free_swap(). When folio_free_swap() succeeds, the folio is removed
from the swap cache, but the saved mapping still points to swap_space.

Passing the stale mapping to folio_migrate_mapping() makes it use the
mapped-folio path for a folio that is no longer in swapcache. It can
then operate on swap_space.i_pages with invalid reference accounting,
eventually triggering a folio reference count BUG.

After a successful split, nr still contains the number of pages in the
original large folio, although each resulting page is now a separate
order-0 folio. Reset nr to 1 so each split folio is processed separately,
including its own swapcache removal and mapping lookup.

Refresh the saved mapping after folio_free_swap() so the current folio
state is used during migration.

v2:
- Refresh the mapping using folio_mapping(), as suggested by Zi Yan.

v3:
- Reset nr to 1 after a successful split so each resulting folio is
  processed independently, as suggested by Zi Yan.

v4:
- Re-read each source folio's mapping immediately before
  folio_migrate_mapping(), as suggested by Balbir Singh.
- Add a warning to validate the post-split order-0 invariant,
  as suggested by Balbir Singh.

Fixes: df263d9a7dff ("mm/migrate_device: try to handle swapcache pages")
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: Ying Huang <ying.huang@linux.alibaba.com>
Cc: Alistair Popple <apopple@nvidia.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Balbir Singh <balbirs@nvidia.com>
Signed-off-by: Arvind Yadav <arvind.yadav@intel.com>
---
 mm/migrate_device.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 908d2d4ec43a..162d29b2807a 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -1183,6 +1183,13 @@ static void __migrate_device_pages(unsigned long *src_pfns,
 							 MIGRATE_PFN_COMPOUND);
 					goto next;
 				}
+
+				/*
+				 * reset nr so that only first after-split folio
+				 * is processed below
+				 */
+				VM_WARN_ON_ONCE(folio_test_large(folio));
+				nr = 1;
 			} else if ((src_pfns[i] & MIGRATE_PFN_MIGRATE) &&
 				(dst_pfns[i] & MIGRATE_PFN_COMPOUND) &&
 				!(src_pfns[i] & MIGRATE_PFN_COMPOUND)) {
@@ -1222,6 +1229,12 @@ static void __migrate_device_pages(unsigned long *src_pfns,
 			folio = page_folio(migrate_pfn_to_page(src_pfns[i+j]));
 			newfolio = page_folio(migrate_pfn_to_page(dst_pfns[i+j]));
 
+			/*
+			 * folio_free_swap() removed the folio from the swap
+			 * cache. Refresh the saved mapping before migration.
+			 */
+			mapping = folio_mapping(folio);
+
 			r = folio_migrate_mapping(mapping, newfolio, folio, extra_cnt);
 			if (r)
 				src_pfns[i+j] &= ~MIGRATE_PFN_MIGRATE;
-- 
2.34.1

[PATCH v3 2/6] mm/migrate_device: Do not write past the end of the src_pfns array

[Matthew Brost]

migrate_device_range() and migrate_device_pfns() zero the tail entries
of a large folio without checking them against @npages:

	for (j = 1; j < nr; j++)
		src_pfns[i+j] = 0;

@nr comes from the folio, not from the array, so a folio that extends
past the end of the range being migrated writes beyond src_pfns[].
Callers size that array for @npages entries, so this corrupts whatever
follows it.

Bound the loop by @npages. The subsequent "i += j - 1" still terminates
the outer loop correctly: on a bounded exit j is @npages - i, leaving i
at @npages after the increment.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Fixes: a30b48bf1b24 ("mm/migrate_device: implement THP migration of zone device pages")
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Nico Pache <nico.pache@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: Ying Huang <ying.huang@linux.alibaba.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@gmail.com>
Cc: Simona Vetter <simona@ffwll.ch>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Francois Dugast <francois.dugast@intel.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org
Assisted-by: GitHub_Copilot:claude-opus-5
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
---
 mm/migrate_device.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 162d29b2807a..ae9027421b80 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -1415,7 +1415,7 @@ int migrate_device_range(unsigned long *src_pfns, unsigned long start,
 		nr = folio_nr_pages(folio);
 		if (nr > 1) {
 			src_pfns[i] |= MIGRATE_PFN_COMPOUND;
-			for (j = 1; j < nr; j++)
+			for (j = 1; j < nr && (i + j) < npages; j++)
 				src_pfns[i+j] = 0;
 			i += j - 1;
 			pfn += j - 1;
@@ -1449,7 +1449,7 @@ int migrate_device_pfns(unsigned long *src_pfns, unsigned long npages)
 		nr = folio_nr_pages(folio);
 		if (nr > 1) {
 			src_pfns[i] |= MIGRATE_PFN_COMPOUND;
-			for (j = 1; j < nr; j++)
+			for (j = 1; j < nr && (i + j) < npages; j++)
 				src_pfns[i+j] = 0;
 			i += j - 1;
 		}
-- 
2.34.1

[PATCH v3 3/6] mm/migrate_device: Fix THP splitting of a CPU faulted device private folio

[Matthew Brost]

When a CPU faults on a device private PMD and the device driver can only
allocate order-0 destination folios, __migrate_device_pages() has to
split the source THP via migrate_vma_split_unmapped_folio(). That path
is broken in two independent ways when the fault is what triggered the
migration.

First, the split never succeeds. At the point folio_split_unmapped() is
called the folio carries two references beyond the ones it is
entitled to:

  1 - taken by do_huge_pmd_device_private() for the duration of the
      ->migrate_to_ram() callback
  2 - taken by migrate_vma_collect_huge_pmd() when the folio was
      collected

(the mapping reference having been dropped by set_pmd_migration_entry()).

folio_split_unmapped() requires folio_expected_ref_count(folio) ==
folio_ref_count(folio) - 1, i.e. it tolerates exactly one caller
reference. With both of the above held the check sees 2 against an
expected 0 and returns -EAGAIN, so the migration is abandoned and the
CPU fault makes no progress.

The PTE-based split path does not have this problem:
migrate_vma_split_folio() is called before any collect reference is
taken and explicitly skips folio_get() for the fault folio, so the fault
reference is the single caller reference the split expects.

Fix it by dropping the fault reference across the split and re-taking it
afterwards. do_huge_pmd_device_private() derives the fault page from the
PMD entry, so it is always the head page of the folio and always ends up
in the head folio of an uniform split to order 0; re-taking the
reference on the folio therefore puts it back exactly where
do_huge_pmd_device_private() will release it. The folio cannot be freed
while the reference is dropped because the collect reference is still
held.

Second, the folio is split globally but the page tables were demoted
only locally:

	split_huge_pmd_address(migrate->vma, addr, true);
	ret = folio_split_unmapped(folio, 0);

migrate_device_unmap() unmaps via try_to_migrate(folio, 0), deliberately
without TTU_SPLIT_HUGE_PMD, so every VMA that PMD maps the folio is left
holding a PMD sized migration entry. A folio that was PMD mapped in more
than one VMA -- after fork(), for example -- therefore keeps huge
migration entries in all the other VMAs while only migrate->vma is
demoted.

folio_split_unmapped() does not notice: the folio is fully unmapped, so
it only looks at the refcount and happily splits to order 0. The other
VMAs are then left pointing a huge PMD at an order-0 folio, and
migrate_vma_finalize() -> remove_migration_ptes() walks into it:

  page dumped because: VM_BUG_ON_FOLIO(folio_test_hugetlb(folio) ||
                                       !folio_test_pmd_mappable(folio))
  kernel BUG at mm/migrate.c:368!
  RIP: 0010:remove_migration_pte+0x56a/0x9b0
  Call Trace:
   rmap_walk_anon+0xfc/0x260
   remove_migration_ptes+0x79/0xb0
   __migrate_device_finalize+0x113/0x290
   __drm_pagemap_migrate_to_ram+0x278/0x360 [drm_gpusvm_helper]
   drm_pagemap_migrate_to_ram+0x5c/0x80 [drm_gpusvm_helper]
   do_huge_pmd_device_private+0x160/0x280

Without CONFIG_DEBUG_VM the VM_BUG_ON_FOLIO() is compiled out and
remove_migration_pmd() installs a huge PMD pointing at an order-0 page
instead, along with add_mm_counter(mm, MM_ANONPAGES, HPAGE_PMD_NR). The
victim mm then maps 2MB of address space onto a single 4K page, which
shows up later as bad rss-counter state, leaked page tables and page
allocator freelist corruption in unrelated processes.

Note this second problem was latent before the refcount fix above: the
split always failed, and the failed attempt left migrate->vma demoted,
so the retried fault took the PTE path, where __folio_split() unmaps
with TTU_SPLIT_HUGE_PMD and demotes every VMA.

Fix it by walking the rmap and demoting every PMD sized migration entry
mapping the folio before splitting it. Demote with freeze = false: entry
creation in __split_huge_pmd_locked() is dispatched on
pmd_is_migration_entry(), not on freeze, so a migration PMD becomes PTE
sized migration entries either way, and freeze only controls a trailing
put_page(). With freeze = false there is no refcount change at all,
which makes the demotion idempotent across N VMAs.

rmap_walk_control.anon_lock is deliberately left unset:
folio_lock_anon_vma_read() depends on folio_mapped(), and the folio is
already fully unmapped here. This mirrors remove_migration_ptes().

Finally, refuse the split for a folio that is not anonymous. The rmap
walk would otherwise reach a file backed VMA, where
split_huge_pmd_address() zaps the PMD instead of demoting it.

Fixes: 4265d67e405a ("mm/migrate_device: add THP splitting during migration")
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Nico Pache <nico.pache@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: Ying Huang <ying.huang@linux.alibaba.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@gmail.com>
Cc: Simona Vetter <simona@ffwll.ch>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Francois Dugast <francois.dugast@intel.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org
Assisted-by: GitHub_Copilot:claude-opus-5
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
---
 mm/migrate_device.c | 98 ++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 89 insertions(+), 9 deletions(-)

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index ae9027421b80..ae17bd516d24 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -899,22 +899,104 @@ static int migrate_vma_insert_huge_pmd_page(struct migrate_vma *migrate,
 	return 0;
 }
 
+static bool migrate_vma_split_pmd_one(struct folio *folio,
+				      struct vm_area_struct *vma,
+				      unsigned long addr, void *arg)
+{
+	DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, addr, PVMW_SYNC | PVMW_MIGRATION);
+
+	while (page_vma_mapped_walk(&pvmw)) {
+		if (pvmw.pte)
+			continue;
+
+		addr = pvmw.address;
+		page_vma_mapped_walk_done(&pvmw);
+
+		/*
+		 * Demote with freeze = false: the PMD already holds a
+		 * migration entry, so __split_huge_pmd_locked() creates PTE
+		 * sized migration entries from it and leaves the refcount
+		 * alone. There is at most one PMD mapping @folio per VMA, so
+		 * stop the walk here.
+		 */
+		split_huge_pmd_address(vma, addr, false);
+		break;
+	}
+
+	return true;
+}
+
+/*
+ * Demote every PMD sized migration entry that maps @folio to PTE sized ones.
+ *
+ * migrate_device_unmap() unmaps with try_to_migrate(folio, 0), i.e. without
+ * TTU_SPLIT_HUGE_PMD, so a folio that was PMD mapped in several VMAs -- after
+ * fork(), for instance -- ends up with a PMD sized migration entry in every one
+ * of them. folio_split_unmapped() below does not care, it only looks at the
+ * refcount, so splitting the folio without demoting all of those first would
+ * leave the other VMAs pointing a huge PMD at what is now an order-0 folio.
+ * remove_migration_ptes() trips over that in migrate_vma_finalize().
+ */
+static void migrate_vma_split_pmd_mappings(struct folio *folio)
+{
+	struct rmap_walk_control rwc = {
+		.rmap_one = migrate_vma_split_pmd_one,
+	};
+
+	/*
+	 * Do not pass .anon_lock: folio_lock_anon_vma_read() requires
+	 * folio_mapped(), and @folio is already fully unmapped here.
+	 */
+	rmap_walk(folio, &rwc);
+}
+
 static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate,
-					    unsigned long idx, unsigned long addr,
+					    unsigned long idx,
 					    struct folio *folio)
 {
 	unsigned long i;
 	unsigned long pfn;
 	unsigned long flags;
+	bool fault_folio;
 	int ret = 0;
 
 	/*
-	 * take a reference, since split_huge_pmd_address() with freeze = true
-	 * drops a reference at the end.
+	 * migrate_vma_split_pmd_mappings() walks the rmap, and
+	 * split_huge_pmd_address() zaps rather than demotes a PMD in a VMA that
+	 * is not anonymous. migrate_vma_collect_huge_pmd() does not check the
+	 * VMA type, so a file THP can reach here; the rest of the migrate_vma()
+	 * machinery only supports anonymous memory anyway.
 	 */
-	folio_get(folio);
-	split_huge_pmd_address(migrate->vma, addr, true);
+	if (!folio_test_anon(folio))
+		return -EINVAL;
+
+	/*
+	 * A CPU fault on a device private PMD holds an extra reference on the
+	 * folio, taken by do_huge_pmd_device_private(). folio_split_unmapped()
+	 * only tolerates a single caller reference, so the split would always
+	 * fail with -EAGAIN while this fault reference is held.
+	 *
+	 * do_huge_pmd_device_private() derives the fault page from the PMD
+	 * entry, so it is always the head page of @folio, and therefore always
+	 * ends up in the head folio after an uniform split to order 0. Drop
+	 * the reference across the split and re-take it on the head folio
+	 * afterwards, leaving the reference exactly where it is expected to be
+	 * released.
+	 *
+	 * The folio cannot go away while the reference is dropped: the
+	 * reference taken by migrate_vma_collect_huge_pmd() is still held.
+	 */
+	fault_folio = migrate->fault_page &&
+		page_folio(migrate->fault_page) == folio;
+
+	migrate_vma_split_pmd_mappings(folio);
+
+	if (fault_folio)
+		folio_put(folio);
 	ret = folio_split_unmapped(folio, 0);
+	if (fault_folio)
+		folio_get(folio);
+
 	if (ret)
 		return ret;
 	migrate->src[idx] &= ~MIGRATE_PFN_COMPOUND;
@@ -935,7 +1017,7 @@ static int migrate_vma_insert_huge_pmd_page(struct migrate_vma *migrate,
 }
 
 static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate,
-					    unsigned long idx, unsigned long addr,
+					    unsigned long idx,
 					    struct folio *folio)
 {
 	return 0;
@@ -1103,7 +1185,6 @@ static void __migrate_device_pages(unsigned long *src_pfns,
 	struct mmu_notifier_range range;
 	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]);
@@ -1177,8 +1258,7 @@ static void __migrate_device_pages(unsigned long *src_pfns,
 					goto next;
 				}
 				nr = 1 << folio_order(folio);
-				addr = migrate->start + i * PAGE_SIZE;
-				if (migrate_vma_split_unmapped_folio(migrate, i, addr, folio)) {
+				if (migrate_vma_split_unmapped_folio(migrate, i, folio)) {
 					src_pfns[i] &= ~(MIGRATE_PFN_MIGRATE |
 							 MIGRATE_PFN_COMPOUND);
 					goto next;
-- 
2.34.1

[PATCH v3 4/6] drm/pagemap: dma-unmap pages before handling migration errors

[Matthew Brost]

drm_pagemap_migrate_unmap_pages() relies on the pages array to determine
which pages require DMA unmapping. However,
drm_pagemap_migration_unlock_put_pages() clears the array as part of its
cleanup, leaving drm_pagemap_migrate_unmap_pages() with no valid page
information if it is called afterward.

Call drm_pagemap_migrate_unmap_pages() before
drm_pagemap_migration_unlock_put_pages() so the pages array remains
valid during DMA unmapping.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Fixes: f86ad0ed620c ("drm/gpusvm, drm/pagemap: Move migration functionality to drm_pagemap")
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Nico Pache <nico.pache@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: Ying Huang <ying.huang@linux.alibaba.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@gmail.com>
Cc: Simona Vetter <simona@ffwll.ch>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Francois Dugast <francois.dugast@intel.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
---
 drivers/gpu/drm/drm_pagemap.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/drm_pagemap.c b/drivers/gpu/drm/drm_pagemap.c
index 892b325fa99b..aef1fcea663d 100644
--- a/drivers/gpu/drm/drm_pagemap.c
+++ b/drivers/gpu/drm/drm_pagemap.c
@@ -1175,12 +1175,12 @@ int drm_pagemap_evict_to_ram(struct drm_pagemap_devmem *devmem_allocation)
 		goto err_finalize;
 
 err_finalize:
+	drm_pagemap_migrate_unmap_pages(devmem_allocation->dev, pagemap_addr, dst, npages,
+					DMA_FROM_DEVICE, &state);
 	if (err)
 		drm_pagemap_migration_unlock_put_pages(npages, dst);
 	migrate_device_pages(src, dst, npages);
 	migrate_device_finalize(src, dst, npages);
-	drm_pagemap_migrate_unmap_pages(devmem_allocation->dev, pagemap_addr, dst, npages,
-					DMA_FROM_DEVICE, &state);
 
 err_free:
 	kvfree(buf);
@@ -1305,14 +1305,14 @@ static int __drm_pagemap_migrate_to_ram(struct vm_area_struct *vas,
 		goto err_finalize;
 
 err_finalize:
-	if (err)
-		drm_pagemap_migration_unlock_put_pages(npages, migrate.dst);
-	migrate_vma_pages(&migrate);
-	migrate_vma_finalize(&migrate);
 	if (dev)
 		drm_pagemap_migrate_unmap_pages(dev, pagemap_addr, migrate.dst,
 						npages, DMA_FROM_DEVICE,
 						&state);
+	if (err)
+		drm_pagemap_migration_unlock_put_pages(npages, migrate.dst);
+	migrate_vma_pages(&migrate);
+	migrate_vma_finalize(&migrate);
 err_free:
 	kvfree(buf);
 err_out:
-- 
2.34.1

[PATCH v3 5/6] drm/pagemap: Fix folio allocation fallback and use-after-put

[Matthew Brost]

drm_pagemap_migrate_populate_ram_pfn() had two issues when populating
RAM PFNs with higher-order folios:

1. The higher-order vma_alloc_folio()/folio_alloc() calls did not pass
   __GFP_NOWARN, so a THP allocation failure under memory pressure
   would spam the kernel log, and there was no fallback path despite a
   TODO comment stating one was needed. Add __GFP_NOWARN to the
   higher-order allocation and, on failure, fall back to order-0
   allocations for the entire range originally covered by the failed
   higher-order allocation, leaving MIGRATE_PFN_COMPOUND unset for
   those PFNs.

2. In the free_pages error path, order was computed via
   folio_order(page_folio(page)) *after* put_page(page) had already
   dropped the reference, resulting in a use-after-free/put when that
   was the last reference on the page. Compute order before releasing
   the page.

Introducing the fallback in 1. also requires the source page array
handed to ->copy_to_ram() to be built differently. Both callers only
populated the entry at the head of each source folio, relying on the
copy callback to derive the rest of the folio from the order recorded
in the matching drm_pagemap_addr. Once the destination has been demoted
to order-0 folios the drm_pagemap_addr entries are per-page, so a source
page is needed for every one of them; leaving them NULL makes the copy
callback stop after the first page and the remainder of the range is
never copied.

The source folio is only split later, by migrate_vma_pages() /
migrate_device_pages(), so its order cannot be used to detect the
demotion - test the destination for MIGRATE_PFN_COMPOUND instead. Factor
the array population out into drm_pagemap_migrate_populate_src_pages()
and use it from both drm_pagemap_evict_to_ram() and
__drm_pagemap_migrate_to_ram().

Fixes: ddeda6136038 ("drm/pagemap: Allocate folios when possible")
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Nico Pache <nico.pache@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: Ying Huang <ying.huang@linux.alibaba.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@gmail.com>
Cc: Simona Vetter <simona@ffwll.ch>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Francois Dugast <francois.dugast@intel.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org
Assisted-by: GitHub_Copilot:claude-opus-5
Signed-off-by: Matthew Brost <matthew.brost@intel.com>

---
 v2: Add THP-mid-PMD invariant (Sashiko)
---
 drivers/gpu/drm/drm_pagemap.c | 128 +++++++++++++++++++++++++++-------
 1 file changed, 103 insertions(+), 25 deletions(-)

diff --git a/drivers/gpu/drm/drm_pagemap.c b/drivers/gpu/drm/drm_pagemap.c
index aef1fcea663d..51c6f12e4256 100644
--- a/drivers/gpu/drm/drm_pagemap.c
+++ b/drivers/gpu/drm/drm_pagemap.c
@@ -383,6 +383,58 @@ drm_pagemap_migrate_map_system_pages(struct device *dev,
 	return 0;
 }
 
+/**
+ * drm_pagemap_migrate_populate_src_pages() - Populate the source page array
+ * @pages: Array of source pages to populate
+ * @src_mpfn: Source array of migrate PFNs
+ * @dst_mpfn: Destination array of migrate PFNs
+ * @npages: Number of pages in the arrays
+ *
+ * Populate @pages with the device pages the copy callback is to read from.
+ *
+ * Entries are normally only populated at the head of each source folio, with
+ * the copy callback deriving the rest of the folio from the order recorded in
+ * the corresponding drm_pagemap_addr. That does not work where
+ * drm_pagemap_migrate_populate_ram_pfn() had to demote a higher-order source
+ * folio to order-0 destination folios: the drm_pagemap_addr entries are then
+ * per-page, and the copy callback needs a source page for each of them.
+ * Populate every entry for those ranges.
+ *
+ * Note that the source folio itself is only split later, by
+ * migrate_vma_pages() / migrate_device_pages(), so its order cannot be used to
+ * detect the demotion - the destination has to be inspected instead.
+ */
+static void drm_pagemap_migrate_populate_src_pages(struct page **pages,
+						   unsigned long *src_mpfn,
+						   unsigned long *dst_mpfn,
+						   unsigned long npages)
+{
+	unsigned long i;
+
+	for (i = 0; i < npages;) {
+		struct page *page = migrate_pfn_to_page(src_mpfn[i]);
+		unsigned int order = 0;
+		unsigned long j, nr;
+
+		if (!page) {
+			i++;
+			continue;
+		}
+
+		order = folio_order(page_folio(page));
+		nr = NR_PAGES(order);
+
+		if (order && !(dst_mpfn[i] & MIGRATE_PFN_COMPOUND)) {
+			for (j = 0; j < nr && i + j < npages; j++)
+				pages[i + j] = folio_page(page_folio(page), j);
+		} else {
+			pages[i] = page;
+		}
+
+		i += nr;
+	}
+}
+
 /**
  * drm_pagemap_migrate_unmap_pages() - Unmap pages previously mapped for GPU SVM migration
  * @dev: The device for which the pages were mapped
@@ -875,6 +927,7 @@ static int drm_pagemap_migrate_populate_ram_pfn(struct vm_area_struct *vas,
 		struct page *page = NULL, *src_page;
 		struct folio *folio;
 		unsigned int order = 0;
+		gfp_t gfp = GFP_HIGHUSER;
 
 		if (!(src_mpfn[i] & MIGRATE_PFN_MIGRATE))
 			goto next;
@@ -891,11 +944,51 @@ static int drm_pagemap_migrate_populate_ram_pfn(struct vm_area_struct *vas,
 
 		order = folio_order(page_folio(src_page));
 
-		/* TODO: Support fallback to single pages if THP allocation fails */
+		/*
+		 * A large source folio is always collected whole, at its head
+		 * page, PMD aligned and flagged MIGRATE_PFN_COMPOUND: anything
+		 * else is split before it reaches us, either by
+		 * migrate_vma_collect_pmd() or, for the eviction path, by
+		 * migrate_device_pfns(). Both the order-0 fallback below and
+		 * drm_pagemap_migrate_populate_src_pages() rely on that, as
+		 * they index the folio from @i.
+		 */
+		WARN_ON_ONCE(order &&
+			     (src_page != folio_page(page_folio(src_page), 0) ||
+			      !(src_mpfn[i] & MIGRATE_PFN_COMPOUND)));
+
+		if (order)
+			gfp |= __GFP_NOWARN;
+
 		if (vas)
-			folio = vma_alloc_folio(GFP_HIGHUSER, order, vas, addr);
+			folio = vma_alloc_folio(gfp, order, vas, addr);
 		else
-			folio = folio_alloc(GFP_HIGHUSER, order);
+			folio = folio_alloc(gfp, order);
+
+		if (!folio && order) {
+			/*
+			 * Higher-order allocation failed, fall back to
+			 * order-0 allocations for the entire range covered
+			 * by the original higher-order allocation, without
+			 * setting MIGRATE_PFN_COMPOUND, until we move past
+			 * that range.
+			 */
+			unsigned long nr = NR_PAGES(order);
+			unsigned long j;
+
+			gfp &= ~__GFP_NOWARN;
+			for (j = 0; j < nr && i < npages; j++, i++, addr += PAGE_SIZE) {
+				folio = vas ?
+					vma_alloc_folio(gfp, 0, vas, addr) :
+					folio_alloc(gfp, 0);
+				if (!folio)
+					goto free_pages;
+
+				page = folio_page(folio, 0);
+				mpfn[i] = migrate_pfn(page_to_pfn(page));
+			}
+			continue;
+		}
 
 		if (!folio)
 			goto free_pages;
@@ -940,11 +1033,11 @@ static int drm_pagemap_migrate_populate_ram_pfn(struct vm_area_struct *vas,
 		if (!page)
 			goto next_put;
 
+		order = folio_order(page_folio(page));
+
 		put_page(page);
 		mpfn[i] = 0;
 
-		order = folio_order(page_folio(page));
-
 next_put:
 		i += NR_PAGES(order);
 	}
@@ -1120,7 +1213,7 @@ int drm_pagemap_evict_to_ram(struct drm_pagemap_devmem *devmem_allocation)
 	unsigned long *src, *dst;
 	struct drm_pagemap_addr *pagemap_addr;
 	void *buf;
-	int i, err = 0;
+	int err = 0;
 	unsigned int retry_count = 2;
 
 	npages = devmem_allocation->size >> PAGE_SHIFT;
@@ -1160,15 +1253,7 @@ int drm_pagemap_evict_to_ram(struct drm_pagemap_devmem *devmem_allocation)
 	if (err)
 		goto err_finalize;
 
-	for (i = 0; i < npages;) {
-		unsigned int order = 0;
-
-		pages[i] = migrate_pfn_to_page(src[i]);
-		if (pages[i])
-			order = folio_order(page_folio(pages[i]));
-
-		i += NR_PAGES(order);
-	}
+	drm_pagemap_migrate_populate_src_pages(pages, src, dst, npages);
 
 	err = ops->copy_to_ram(pages, pagemap_addr, npages, NULL);
 	if (err)
@@ -1235,7 +1320,7 @@ static int __drm_pagemap_migrate_to_ram(struct vm_area_struct *vas,
 	struct drm_pagemap_addr *pagemap_addr;
 	unsigned long start, end;
 	void *buf;
-	int i, err = 0;
+	int err = 0;
 
 	zdd = drm_pagemap_page_zone_device_data(page);
 	if (time_before64(get_jiffies_64(), zdd->devmem_allocation->timeslice_expiration))
@@ -1290,15 +1375,8 @@ static int __drm_pagemap_migrate_to_ram(struct vm_area_struct *vas,
 	if (err)
 		goto err_finalize;
 
-	for (i = 0; i < npages;) {
-		unsigned int order = 0;
-
-		pages[i] = migrate_pfn_to_page(migrate.src[i]);
-		if (pages[i])
-			order = folio_order(page_folio(pages[i]));
-
-		i += NR_PAGES(order);
-	}
+	drm_pagemap_migrate_populate_src_pages(pages, migrate.src, migrate.dst,
+					       npages);
 
 	err = ops->copy_to_ram(pages, pagemap_addr, npages, NULL);
 	if (err)
-- 
2.34.1

[PATCH v3 6/6] drm/pagemap: Add fault injection for higher-order RAM folio allocation

[Matthew Brost]

Migrating a device-private THP back to system memory has two distinct
paths in __migrate_device_pages(): the fast path where both source and
destination carry MIGRATE_PFN_COMPOUND, and the fallback path where the
destination could only be satisfied with order-0 folios and the source
THP therefore has to be split via migrate_vma_split_unmapped_folio().

The fallback path only triggers under genuine memory pressure, which
makes it both rare and awkward to reproduce, yet it is the path where
the interesting refcounting happens (the CPU fault holds an extra
reference on the device folio taken by do_huge_pmd_device_private()).

Add a fault_attr, modelled on backup_fault_inject in ttm_pool.c, that
forces the higher-order allocation in
drm_pagemap_migrate_populate_ram_pfn() to fail so the existing order-0
fallback is taken deterministically.

The attribute is exposed at /sys/kernel/debug/drm_pagemap_fault_inject
and requires CONFIG_FAULT_INJECTION_DEBUG_FS. With
CONFIG_FAULT_INJECTION disabled the helper compiles out to a constant
false and the injection has no cost.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Nico Pache <nico.pache@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: Ying Huang <ying.huang@linux.alibaba.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@gmail.com>
Cc: Simona Vetter <simona@ffwll.ch>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Francois Dugast <francois.dugast@intel.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
Assisted-by: GitHub_Copilot:claude-opus-5
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
---
 drivers/gpu/drm/drm_pagemap.c | 36 ++++++++++++++++++++++++++++++++++-
 1 file changed, 35 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_pagemap.c b/drivers/gpu/drm/drm_pagemap.c
index 51c6f12e4256..6ae8c9aa36cc 100644
--- a/drivers/gpu/drm/drm_pagemap.c
+++ b/drivers/gpu/drm/drm_pagemap.c
@@ -3,6 +3,7 @@
  * Copyright © 2024-2025 Intel Corporation
  */
 
+#include <linux/debugfs.h>
 #include <linux/dma-fence.h>
 #include <linux/dma-mapping.h>
 #include <linux/migrate.h>
@@ -12,6 +13,27 @@
 #include <drm/drm_pagemap_util.h>
 #include <drm/drm_print.h>
 
+#ifdef CONFIG_FAULT_INJECTION
+#include <linux/fault-inject.h>
+static DECLARE_FAULT_ATTR(migrate_to_ram_fault_inject);
+
+/*
+ * Force a higher-order destination folio allocation to fail in
+ * drm_pagemap_migrate_populate_ram_pfn(), exercising the order-0 fallback
+ * (and, in turn, the THP split path in __migrate_device_pages()) without
+ * having to drive the system into actual memory pressure.
+ */
+static bool drm_pagemap_fault_inject_folio(void)
+{
+	return should_fail(&migrate_to_ram_fault_inject, 1);
+}
+#else
+static bool drm_pagemap_fault_inject_folio(void)
+{
+	return false;
+}
+#endif
+
 /**
  * DOC: Overview
  *
@@ -960,7 +982,9 @@ static int drm_pagemap_migrate_populate_ram_pfn(struct vm_area_struct *vas,
 		if (order)
 			gfp |= __GFP_NOWARN;
 
-		if (vas)
+		if (order && drm_pagemap_fault_inject_folio())
+			folio = NULL;
+		else if (vas)
 			folio = vma_alloc_folio(gfp, order, vas, addr);
 		else
 			folio = folio_alloc(gfp, order);
@@ -1554,6 +1578,16 @@ void drm_pagemap_destroy(struct drm_pagemap *dpagemap, bool is_atomic_or_reclaim
 		kfree(dpagemap);
 }
 
+static int __init drm_pagemap_module_init(void)
+{
+#if defined(CONFIG_DEBUG_FS) && defined(CONFIG_FAULT_INJECTION)
+	fault_create_debugfs_attr("drm_pagemap_fault_inject", NULL,
+				  &migrate_to_ram_fault_inject);
+#endif
+	return 0;
+}
+module_init(drm_pagemap_module_init);
+
 static void drm_pagemap_exit(void)
 {
 	flush_work(&drm_pagemap_work);
-- 
2.34.1

Re: [PATCH v3 2/6] mm/migrate_device: Do not write past the end of the src_pfns array

[Balbir Singh]

On 8/6/26 9:10 AM, Matthew Brost wrote:
> migrate_device_range() and migrate_device_pfns() zero the tail entries
> of a large folio without checking them against @npages:
> 
> 	for (j = 1; j < nr; j++)
> 		src_pfns[i+j] = 0;
> 
> @nr comes from the folio, not from the array, so a folio that extends
> past the end of the range being migrated writes beyond src_pfns[].
> Callers size that array for @npages entries, so this corrupts whatever
> follows it.
> 
> Bound the loop by @npages. The subsequent "i += j - 1" still terminates
> the outer loop correctly: on a bounded exit j is @npages - i, leaving i
> at @npages after the increment.
> 
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Fixes: a30b48bf1b24 ("mm/migrate_device: implement THP migration of zone device pages")
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@kernel.org>
> Cc: Lorenzo Stoakes <ljs@kernel.org>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Liam R. Howlett <liam@infradead.org>
> Cc: Nico Pache <nico.pache@linux.dev>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Dev Jain <dev.jain@arm.com>
> Cc: Barry Song <baohua@kernel.org>
> Cc: Lance Yang <lance.yang@linux.dev>
> Cc: Usama Arif <usama.arif@linux.dev>
> Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
> Cc: Rakie Kim <rakie.kim@sk.com>
> Cc: Byungchul Park <byungchul@sk.com>
> Cc: Gregory Price <gourry@gourry.net>
> Cc: Ying Huang <ying.huang@linux.alibaba.com>
> Cc: Alistair Popple <apopple@nvidia.com>
> Cc: Balbir Singh <balbirs@nvidia.com>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Cc: Maxime Ripard <mripard@kernel.org>
> Cc: Thomas Zimmermann <tzimmermann@suse.de>
> Cc: David Airlie <airlied@gmail.com>
> Cc: Simona Vetter <simona@ffwll.ch>
> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> Cc: Francois Dugast <francois.dugast@intel.com>
> Cc: dri-devel@lists.freedesktop.org
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> Cc: stable@vger.kernel.org
> Assisted-by: GitHub_Copilot:claude-opus-5
> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> ---
>  mm/migrate_device.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index 162d29b2807a..ae9027421b80 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -1415,7 +1415,7 @@ int migrate_device_range(unsigned long *src_pfns, unsigned long start,
>  		nr = folio_nr_pages(folio);
>  		if (nr > 1) {
>  			src_pfns[i] |= MIGRATE_PFN_COMPOUND;
> -			for (j = 1; j < nr; j++)
> +			for (j = 1; j < nr && (i + j) < npages; j++)
>  				src_pfns[i+j] = 0;

I have a similar patch lined up in my clean ups (that I am yet to send out), but
the patch was more along the lines of

  		nr = folio_nr_pages(folio);
+		if (nr > npages - i) {
+			migrate_device_folio_unlock(folio);
+			src_pfns[i] = 0;
+			continue;
+		}
+

This prevents partial selection, migrate_device_unmap() does take npages as an
argument. We could change the increment of i here to skip past the entire folio.

I am OK with this change as well

>  			i += j - 1;
>  			pfn += j - 1;
> @@ -1449,7 +1449,7 @@ int migrate_device_pfns(unsigned long *src_pfns, unsigned long npages)
>  		nr = folio_nr_pages(folio);
>  		if (nr > 1) {
>  			src_pfns[i] |= MIGRATE_PFN_COMPOUND;
> -			for (j = 1; j < nr; j++)
> +			for (j = 1; j < nr && (i + j) < npages; j++)
>  				src_pfns[i+j] = 0;
>  			i += j - 1;
>  		}

Reviewed-by: Balbir Singh <balbirs@nvidia.com>

Re: [PATCH v3 3/6] mm/migrate_device: Fix THP splitting of a CPU faulted device private folio

[Balbir Singh]

On 8/6/26 9:10 AM, Matthew Brost wrote:
> When a CPU faults on a device private PMD and the device driver can only
> allocate order-0 destination folios, __migrate_device_pages() has to
> split the source THP via migrate_vma_split_unmapped_folio(). That path
> is broken in two independent ways when the fault is what triggered the
> migration.
> 
> First, the split never succeeds. At the point folio_split_unmapped() is
> called the folio carries two references beyond the ones it is
> entitled to:
> 
>   1 - taken by do_huge_pmd_device_private() for the duration of the
>       ->migrate_to_ram() callback
>   2 - taken by migrate_vma_collect_huge_pmd() when the folio was
>       collected
> 
> (the mapping reference having been dropped by set_pmd_migration_entry()).
> 
> folio_split_unmapped() requires folio_expected_ref_count(folio) ==
> folio_ref_count(folio) - 1, i.e. it tolerates exactly one caller
> reference. With both of the above held the check sees 2 against an
> expected 0 and returns -EAGAIN, so the migration is abandoned and the
> CPU fault makes no progress.
> 
> The PTE-based split path does not have this problem:
> migrate_vma_split_folio() is called before any collect reference is
> taken and explicitly skips folio_get() for the fault folio, so the fault
> reference is the single caller reference the split expects.
> 
> Fix it by dropping the fault reference across the split and re-taking it
> afterwards. do_huge_pmd_device_private() derives the fault page from the
> PMD entry, so it is always the head page of the folio and always ends up
> in the head folio of an uniform split to order 0; re-taking the
> reference on the folio therefore puts it back exactly where
> do_huge_pmd_device_private() will release it. The folio cannot be freed
> while the reference is dropped because the collect reference is still
> held.
> 
> Second, the folio is split globally but the page tables were demoted
> only locally:
> 
> 	split_huge_pmd_address(migrate->vma, addr, true);
> 	ret = folio_split_unmapped(folio, 0);
> 
> migrate_device_unmap() unmaps via try_to_migrate(folio, 0), deliberately
> without TTU_SPLIT_HUGE_PMD, so every VMA that PMD maps the folio is left
> holding a PMD sized migration entry. A folio that was PMD mapped in more
> than one VMA -- after fork(), for example -- therefore keeps huge
> migration entries in all the other VMAs while only migrate->vma is
> demoted.
> 
> folio_split_unmapped() does not notice: the folio is fully unmapped, so
> it only looks at the refcount and happily splits to order 0. The other
> VMAs are then left pointing a huge PMD at an order-0 folio, and
> migrate_vma_finalize() -> remove_migration_ptes() walks into it:
> 
>   page dumped because: VM_BUG_ON_FOLIO(folio_test_hugetlb(folio) ||
>                                        !folio_test_pmd_mappable(folio))
>   kernel BUG at mm/migrate.c:368!
>   RIP: 0010:remove_migration_pte+0x56a/0x9b0
>   Call Trace:
>    rmap_walk_anon+0xfc/0x260
>    remove_migration_ptes+0x79/0xb0
>    __migrate_device_finalize+0x113/0x290
>    __drm_pagemap_migrate_to_ram+0x278/0x360 [drm_gpusvm_helper]
>    drm_pagemap_migrate_to_ram+0x5c/0x80 [drm_gpusvm_helper]
>    do_huge_pmd_device_private+0x160/0x280
> 
> Without CONFIG_DEBUG_VM the VM_BUG_ON_FOLIO() is compiled out and
> remove_migration_pmd() installs a huge PMD pointing at an order-0 page
> instead, along with add_mm_counter(mm, MM_ANONPAGES, HPAGE_PMD_NR). The
> victim mm then maps 2MB of address space onto a single 4K page, which
> shows up later as bad rss-counter state, leaked page tables and page
> allocator freelist corruption in unrelated processes.
> 
> Note this second problem was latent before the refcount fix above: the
> split always failed, and the failed attempt left migrate->vma demoted,
> so the retried fault took the PTE path, where __folio_split() unmaps
> with TTU_SPLIT_HUGE_PMD and demotes every VMA.
>

The refcount fix to enable migration of PMD's in fault context. IOW, if we need
PMD migration in the context of a CPU fault, then the proposed patch is the
right fix, otherwise we split and migrate and that has no crashes/impact?


> Fix it by walking the rmap and demoting every PMD sized migration entry
> mapping the folio before splitting it. Demote with freeze = false: entry
> creation in __split_huge_pmd_locked() is dispatched on
> pmd_is_migration_entry(), not on freeze, so a migration PMD becomes PTE
> sized migration entries either way, and freeze only controls a trailing
> put_page(). With freeze = false there is no refcount change at all,
> which makes the demotion idempotent across N VMAs.
> 
> rmap_walk_control.anon_lock is deliberately left unset:
> folio_lock_anon_vma_read() depends on folio_mapped(), and the folio is
> already fully unmapped here. This mirrors remove_migration_ptes().
> 
> Finally, refuse the split for a folio that is not anonymous. The rmap
> walk would otherwise reach a file backed VMA, where
> split_huge_pmd_address() zaps the PMD instead of demoting it.
> 
> Fixes: 4265d67e405a ("mm/migrate_device: add THP splitting during migration")
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@kernel.org>
> Cc: Lorenzo Stoakes <ljs@kernel.org>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Liam R. Howlett <liam@infradead.org>
> Cc: Nico Pache <nico.pache@linux.dev>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Dev Jain <dev.jain@arm.com>
> Cc: Barry Song <baohua@kernel.org>
> Cc: Lance Yang <lance.yang@linux.dev>
> Cc: Usama Arif <usama.arif@linux.dev>
> Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
> Cc: Rakie Kim <rakie.kim@sk.com>
> Cc: Byungchul Park <byungchul@sk.com>
> Cc: Gregory Price <gourry@gourry.net>
> Cc: Ying Huang <ying.huang@linux.alibaba.com>
> Cc: Alistair Popple <apopple@nvidia.com>
> Cc: Balbir Singh <balbirs@nvidia.com>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Cc: Maxime Ripard <mripard@kernel.org>
> Cc: Thomas Zimmermann <tzimmermann@suse.de>
> Cc: David Airlie <airlied@gmail.com>
> Cc: Simona Vetter <simona@ffwll.ch>
> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> Cc: Francois Dugast <francois.dugast@intel.com>
> Cc: dri-devel@lists.freedesktop.org
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> Cc: stable@vger.kernel.org
> Assisted-by: GitHub_Copilot:claude-opus-5
> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> ---
>  mm/migrate_device.c | 98 ++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 89 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index ae9027421b80..ae17bd516d24 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -899,22 +899,104 @@ static int migrate_vma_insert_huge_pmd_page(struct migrate_vma *migrate,
>  	return 0;
>  }
>  
> +static bool migrate_vma_split_pmd_one(struct folio *folio,
> +				      struct vm_area_struct *vma,
> +				      unsigned long addr, void *arg)
> +{
> +	DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, addr, PVMW_SYNC | PVMW_MIGRATION);
> +
> +	while (page_vma_mapped_walk(&pvmw)) {
> +		if (pvmw.pte)
> +			continue;
> +
> +		addr = pvmw.address;
> +		page_vma_mapped_walk_done(&pvmw);
> +
> +		/*
> +		 * Demote with freeze = false: the PMD already holds a
> +		 * migration entry, so __split_huge_pmd_locked() creates PTE
> +		 * sized migration entries from it and leaves the refcount
> +		 * alone. There is at most one PMD mapping @folio per VMA, so
> +		 * stop the walk here.
> +		 */
> +		split_huge_pmd_address(vma, addr, false);
> +		break;
> +	}
> +
> +	return true;
> +}
> +
> +/*
> + * Demote every PMD sized migration entry that maps @folio to PTE sized ones.
> + *
> + * migrate_device_unmap() unmaps with try_to_migrate(folio, 0), i.e. without
> + * TTU_SPLIT_HUGE_PMD, so a folio that was PMD mapped in several VMAs -- after
> + * fork(), for instance -- ends up with a PMD sized migration entry in every one
> + * of them. folio_split_unmapped() below does not care, it only looks at the
> + * refcount, so splitting the folio without demoting all of those first would
> + * leave the other VMAs pointing a huge PMD at what is now an order-0 folio.
> + * remove_migration_ptes() trips over that in migrate_vma_finalize().
> + */
> +static void migrate_vma_split_pmd_mappings(struct folio *folio)
> +{
> +	struct rmap_walk_control rwc = {
> +		.rmap_one = migrate_vma_split_pmd_one,
> +	};
> +
> +	/*
> +	 * Do not pass .anon_lock: folio_lock_anon_vma_read() requires
> +	 * folio_mapped(), and @folio is already fully unmapped here.
> +	 */
> +	rmap_walk(folio, &rwc);
> +}
> +
>  static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate,
> -					    unsigned long idx, unsigned long addr,
> +					    unsigned long idx,
>  					    struct folio *folio)
>  {
>  	unsigned long i;
>  	unsigned long pfn;
>  	unsigned long flags;
> +	bool fault_folio;
>  	int ret = 0;
>  
>  	/*
> -	 * take a reference, since split_huge_pmd_address() with freeze = true
> -	 * drops a reference at the end.
> +	 * migrate_vma_split_pmd_mappings() walks the rmap, and
> +	 * split_huge_pmd_address() zaps rather than demotes a PMD in a VMA that
> +	 * is not anonymous. migrate_vma_collect_huge_pmd() does not check the
> +	 * VMA type, so a file THP can reach here; the rest of the migrate_vma()
> +	 * machinery only supports anonymous memory anyway.
>  	 */
> -	folio_get(folio);
> -	split_huge_pmd_address(migrate->vma, addr, true);
> +	if (!folio_test_anon(folio))
> +		return -EINVAL;
> +
> +	/*
> +	 * A CPU fault on a device private PMD holds an extra reference on the
> +	 * folio, taken by do_huge_pmd_device_private(). folio_split_unmapped()
> +	 * only tolerates a single caller reference, so the split would always
> +	 * fail with -EAGAIN while this fault reference is held.
> +	 *
> +	 * do_huge_pmd_device_private() derives the fault page from the PMD
> +	 * entry, so it is always the head page of @folio, and therefore always
> +	 * ends up in the head folio after an uniform split to order 0. Drop
> +	 * the reference across the split and re-take it on the head folio
> +	 * afterwards, leaving the reference exactly where it is expected to be
> +	 * released.
> +	 *
> +	 * The folio cannot go away while the reference is dropped: the
> +	 * reference taken by migrate_vma_collect_huge_pmd() is still held.
> +	 */
> +	fault_folio = migrate->fault_page &&
> +		page_folio(migrate->fault_page) == folio;
> +
> +	migrate_vma_split_pmd_mappings(folio);
> +
> +	if (fault_folio)
> +		folio_put(folio);
>  	ret = folio_split_unmapped(folio, 0);
> +	if (fault_folio)
> +		folio_get(folio);
> +
>  	if (ret)
>  		return ret;
>  	migrate->src[idx] &= ~MIGRATE_PFN_COMPOUND;
> @@ -935,7 +1017,7 @@ static int migrate_vma_insert_huge_pmd_page(struct migrate_vma *migrate,
>  }
>  
>  static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate,
> -					    unsigned long idx, unsigned long addr,
> +					    unsigned long idx,
>  					    struct folio *folio)
>  {
>  	return 0;
> @@ -1103,7 +1185,6 @@ static void __migrate_device_pages(unsigned long *src_pfns,
>  	struct mmu_notifier_range range;
>  	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]);
> @@ -1177,8 +1258,7 @@ static void __migrate_device_pages(unsigned long *src_pfns,
>  					goto next;
>  				}
>  				nr = 1 << folio_order(folio);
> -				addr = migrate->start + i * PAGE_SIZE;
> -				if (migrate_vma_split_unmapped_folio(migrate, i, addr, folio)) {
> +				if (migrate_vma_split_unmapped_folio(migrate, i, folio)) {
>  					src_pfns[i] &= ~(MIGRATE_PFN_MIGRATE |
>  							 MIGRATE_PFN_COMPOUND);
>  					goto next;


Reviewed-by: Balbir Singh <balbirs@nvidia.com>