On 9/23/25 12:23, Zi Yan wrote: > On 16 Sep 2025, at 8:21, Balbir Singh wrote: > >> Extend migrate_vma_collect_pmd() to handle partially mapped large folios >> that require splitting before migration can proceed. >> >> During PTE walk in the collection phase, if a large folio is only >> partially mapped in the migration range, it must be split to ensure the >> folio is correctly migrated. >> >> Signed-off-by: Balbir Singh <[email protected]> >> Cc: David Hildenbrand <[email protected]> >> Cc: Zi Yan <[email protected]> >> Cc: Joshua Hahn <[email protected]> >> Cc: Rakie Kim <[email protected]> >> Cc: Byungchul Park <[email protected]> >> Cc: Gregory Price <[email protected]> >> Cc: Ying Huang <[email protected]> >> Cc: Alistair Popple <[email protected]> >> Cc: Oscar Salvador <[email protected]> >> Cc: Lorenzo Stoakes <[email protected]> >> Cc: Baolin Wang <[email protected]> >> Cc: "Liam R. Howlett" <[email protected]> >> Cc: Nico Pache <[email protected]> >> Cc: Ryan Roberts <[email protected]> >> Cc: Dev Jain <[email protected]> >> Cc: Barry Song <[email protected]> >> Cc: Lyude Paul <[email protected]> >> Cc: Danilo Krummrich <[email protected]> >> Cc: David Airlie <[email protected]> >> Cc: Simona Vetter <[email protected]> >> Cc: Ralph Campbell <[email protected]> >> Cc: Mika Penttilä <[email protected]> >> Cc: Matthew Brost <[email protected]> >> Cc: Francois Dugast <[email protected]> >> --- >> mm/migrate_device.c | 82 +++++++++++++++++++++++++++++++++++++++++++++ >> 1 file changed, 82 insertions(+) >> >> diff --git a/mm/migrate_device.c b/mm/migrate_device.c >> index abd9f6850db6..70c0601f70ea 100644 >> --- a/mm/migrate_device.c >> +++ b/mm/migrate_device.c >> @@ -54,6 +54,53 @@ static int migrate_vma_collect_hole(unsigned long start, >> return 0; >> } >> >> +/** >> + * migrate_vma_split_folio() - Helper function to split a THP folio >> + * @folio: the folio to split >> + * @fault_page: struct page associated with the fault if any >> + * >> + * Returns 0 on success >> + */ >> +static int migrate_vma_split_folio(struct folio *folio, >> + struct page *fault_page) >> +{ >> + int ret; >> + struct folio *fault_folio = fault_page ? page_folio(fault_page) : NULL; >> + struct folio *new_fault_folio = NULL; >> + >> + if (folio != fault_folio) { >> + folio_get(folio); >> + folio_lock(folio); >> + } >> + >> + ret = split_folio(folio); >> + if (ret) { >> + if (folio != fault_folio) { >> + folio_unlock(folio); >> + folio_put(folio); >> + } >> + return ret; >> + } >> + >> + new_fault_folio = fault_page ? page_folio(fault_page) : NULL; >> + >> + /* >> + * Ensure the lock is held on the correct >> + * folio after the split >> + */ >> + if (!new_fault_folio) { >> + folio_unlock(folio); >> + folio_put(folio); >> + } else if (folio != new_fault_folio) { >> + folio_get(new_fault_folio); >> + folio_lock(new_fault_folio); >> + folio_unlock(folio); >> + folio_put(folio); >> + } >> + >> + return 0; >> +} >> + >> static int migrate_vma_collect_pmd(pmd_t *pmdp, >> unsigned long start, >> unsigned long end, >> @@ -136,6 +183,8 @@ 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; >> @@ -147,6 +196,23 @@ 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)) { >> + 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; >> + } > > This does not look right to me. > > The folio here is device private, but migrate_vma_split_folio() > calls split_folio(), which cannot handle device private folios yet. > Your change to split_folio() is in Patch 10 and should be moved > before this patch. >
Patch 10 is to split the folio in the middle of migration (when we have converted the entries to migration entries). This patch relies on the changes in patch 4. I agree the names are confusing, I'll reword the functions Thanks for the review Balbir
