Re: [PATCH] fixup: mm/migrate_device: handle partially mapped folios during

["Mika Penttilä" <mpenttil@redhat.com>]

Hi,

On 11/20/25 19:05, Matthew Brost wrote:

> Splitting a partially mapped folio caused a regression in the Intel Xe
> SVM test suite in the mremap section, resulting in the following stack
> trace:
>
>  NFO: task kworker/u65:2:1642 blocked for more than 30 seconds.
> [  212.624286]       Tainted: G S      W           6.18.0-rc6-xe+ #1719
> [  212.638288] Workqueue: xe_page_fault_work_queue xe_pagefault_queue_work [xe]
> [  212.638323] Call Trace:
> [  212.638324]  <TASK>
> [  212.638325]  __schedule+0x4b0/0x990
> [  212.638330]  schedule+0x22/0xd0
> [  212.638331]  io_schedule+0x41/0x60
> [  212.638333]  migration_entry_wait_on_locked+0x1d8/0x2d0
> [  212.638336]  ? __pfx_wake_page_function+0x10/0x10
> [  212.638339]  migration_entry_wait+0xd2/0xe0
> [  212.638341]  hmm_vma_walk_pmd+0x7c9/0x8d0
> [  212.638343]  walk_pgd_range+0x51d/0xa40
> [  212.638345]  __walk_page_range+0x75/0x1e0
> [  212.638347]  walk_page_range_mm+0x138/0x1f0
> [  212.638349]  hmm_range_fault+0x59/0xa0
> [  212.638351]  drm_gpusvm_get_pages+0x194/0x7b0 [drm_gpusvm_helper]
> [  212.638354]  drm_gpusvm_range_get_pages+0x2d/0x40 [drm_gpusvm_helper]
> [  212.638355]  __xe_svm_handle_pagefault+0x259/0x900 [xe]
> [  212.638375]  ? update_load_avg+0x7f/0x6c0
> [  212.638377]  ? update_curr+0x13d/0x170
> [  212.638379]  xe_svm_handle_pagefault+0x37/0x90 [xe]
> [  212.638396]  xe_pagefault_queue_work+0x2da/0x3c0 [xe]
> [  212.638420]  process_one_work+0x16e/0x2e0
> [  212.638422]  worker_thread+0x284/0x410
> [  212.638423]  ? __pfx_worker_thread+0x10/0x10
> [  212.638425]  kthread+0xec/0x210
> [  212.638427]  ? __pfx_kthread+0x10/0x10
> [  212.638428]  ? __pfx_kthread+0x10/0x10
> [  212.638430]  ret_from_fork+0xbd/0x100
> [  212.638433]  ? __pfx_kthread+0x10/0x10
> [  212.638434]  ret_from_fork_asm+0x1a/0x30
> [  212.638436]  </TASK>
>
> The issue appears to be that migration PTEs are not properly removed
> after a split.
>
> This change refactors the code to perform the split in a slightly
> different manner while retaining the original patch’s intent. With this
> update, the Intel Xe SVM test suite fully passes.
>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Zi Yan <ziy@nvidia.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>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
> Cc: Nico Pache <npache@redhat.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Dev Jain <dev.jain@arm.com>
> Cc: Barry Song <baohua@kernel.org>
> Cc: Lyude Paul <lyude@redhat.com>
> Cc: Danilo Krummrich <dakr@kernel.org>
> Cc: David Airlie <airlied@gmail.com>
> Cc: Simona Vetter <simona@ffwll.ch>
> Cc: Ralph Campbell <rcampbell@nvidia.com>
> Cc: Mika Penttilä <mpenttil@redhat.com>
> Cc: Francois Dugast <francois.dugast@intel.com>
> Cc: Balbir Singh <balbirs@nvidia.com>
> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
>
> ---
> This fixup should be squashed into the patch "mm/migrate_device: handle
> partially mapped folios during" in mm/mm-unstable
>
> I replaced the original patch with a local patch I authored a while back
> that solves the same problem but uses a different code structure. The
> failing test case—only available on an Xe driver—passes with this patch.
> I can attempt to fix up the original patch within its structure if
> that’s preferred.
> ---
>  mm/migrate_device.c | 42 ++++++++++++++++++++++++------------------
>  1 file changed, 24 insertions(+), 18 deletions(-)
>
> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> index fa42d2ebd024..69e88f4a2563 100644
> --- a/mm/migrate_device.c
> +++ b/mm/migrate_device.c
> @@ -254,6 +254,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>  	spinlock_t *ptl;
>  	struct folio *fault_folio = migrate->fault_page ?
>  		page_folio(migrate->fault_page) : NULL;
> +	struct folio *split_folio = NULL;
>  	pte_t *ptep;
>  
>  again:
> @@ -266,10 +267,11 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>  			return 0;
>  	}
>  
> -	ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
> +	ptep = pte_offset_map_lock(mm, pmdp, start, &ptl);
>  	if (!ptep)
>  		goto again;
>  	arch_enter_lazy_mmu_mode();
> +	ptep += (addr - start) / PAGE_SIZE;
>  
>  	for (; addr < end; addr += PAGE_SIZE, ptep++) {
>  		struct dev_pagemap *pgmap;
> @@ -347,22 +349,6 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>  					pgmap->owner != migrate->pgmap_owner)
>  					goto next;
>  			}
> -			folio = page ? page_folio(page) : NULL;
> -			if (folio && folio_test_large(folio)) {
> -				int ret;
> -
> -				pte_unmap_unlock(ptep, ptl);
> -				ret = migrate_vma_split_folio(folio,
> -							  migrate->fault_page);
> -
> -				if (ret) {
> -					ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
> -					goto next;
> -				}
> -
> -				addr = start;
> -				goto again;
> -			}
>  			mpfn = migrate_pfn(pfn) | MIGRATE_PFN_MIGRATE;
>  			mpfn |= pte_write(pte) ? MIGRATE_PFN_WRITE : 0;
>  		}
> @@ -400,6 +386,11 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>  			bool anon_exclusive;
>  			pte_t swp_pte;
>  
> +			if (folio_order(folio)) {
> +				split_folio = folio;
> +				goto split;
> +			}
> +
>  			flush_cache_page(vma, addr, pte_pfn(pte));
>  			anon_exclusive = folio_test_anon(folio) &&
>  					  PageAnonExclusive(page);
> @@ -478,8 +469,23 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>  	if (unmapped)
>  		flush_tlb_range(walk->vma, start, end);
>  
> +split:
>  	arch_leave_lazy_mmu_mode();
> -	pte_unmap_unlock(ptep - 1, ptl);
> +	pte_unmap_unlock(ptep - 1 + !!split_folio, ptl);
> +
> +	if (split_folio) {
> +		int ret;
> +
> +		ret = split_folio(split_folio);
> +		if (fault_folio != split_folio)
> +			folio_unlock(split_folio);

I think wrong folio is left locked in case of fault_folio != NULL. Look how
migrate_vma_split_folio() handles it.

> +		folio_put(split_folio);
> +		if (ret)
> +			return migrate_vma_collect_skip(addr, end, walk);
> +
> +		split_folio = NULL;
> +		goto again;
> +	}
>  
>  	return 0;
>  }

How is this making a difference, I suppose it's only the 
migrate_vma_collect_skip() after failed split? 
Why are you just removing the other migrate_vma_split_folio() call? 

--Mika