Page migration of a VM_LOCKED page tends to fail, because when the old page is unmapped, it is put on the mlock pagevec with raised refcount, which then fails the freeze. At first I thought this would be fixed by a local mlock_page_drain() at the upper rmap_walk() level - which would have nicely batched all the munlocks of that page; but tests show that the task can too easily move to another cpu, leaving pagevec residue behind which fails the migration. So try_to_migrate_one() drain the local pagevec after page_remove_rmap() from a VM_LOCKED vma; and do the same in try_to_unmap_one(), whose TTU_IGNORE_MLOCK users would want the same treatment; and do the same in remove_migration_pte() - not important when successfully inserting a new page, but necessary when hoping to retry after failure. Any new pagevec runs the risk of adding a new way of stranding, and we might discover other corners where mlock_page_drain() or lru_add_drain() would now help. If the mlock pagevec raises doubts, we can easily add a sysctl to tune its length to 1, which reverts to synchronous operation. Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> --- mm/migrate.c | 2 ++ mm/rmap.c | 4 ++++ 2 files changed, 6 insertions(+) diff --git a/mm/migrate.c b/mm/migrate.c index f4bcf1541b62..e7d0b68d5dcb 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -251,6 +251,8 @@ static bool remove_migration_pte(struct page *page, struct vm_area_struct *vma, page_add_file_rmap(new, vma, false); set_pte_at(vma->vm_mm, pvmw.address, pvmw.pte, pte); } + if (vma->vm_flags & VM_LOCKED) + mlock_page_drain(smp_processor_id()); /* No need to invalidate - it was non-present before */ update_mmu_cache(vma, pvmw.address, pvmw.pte); diff --git a/mm/rmap.c b/mm/rmap.c index 5442a5c97a85..714bfdc72c7b 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -1656,6 +1656,8 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma, * See Documentation/vm/mmu_notifier.rst */ page_remove_rmap(subpage, vma, PageHuge(page)); + if (vma->vm_flags & VM_LOCKED) + mlock_page_drain(smp_processor_id()); put_page(page); } @@ -1930,6 +1932,8 @@ static bool try_to_migrate_one(struct page *page, struct vm_area_struct *vma, * See Documentation/vm/mmu_notifier.rst */ page_remove_rmap(subpage, vma, PageHuge(page)); + if (vma->vm_flags & VM_LOCKED) + mlock_page_drain(smp_processor_id()); put_page(page); } -- 2.34.1
