From d9770fcc1c0c5b3e77dfac83b47defa3981fa7cd Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Wed, 7 Jul 2021 13:08:53 -0700 Subject: [PATCH] mm/rmap: fix old bug: munlocking THP missed other mlocks The kernel recovers in due course from missing Mlocked pages: but there was no point in calling page_mlock() (formerly known as try_to_munlock()) on a THP, because nothing got done even when it was found to be mapped in another VM_LOCKED vma. It's true that we need to be careful: Mlocked accounting of pte-mapped THPs is too difficult (so consistently avoided); but Mlocked accounting of only-pmd-mapped THPs is supposed to work, even when multiple mappings are mlocked and munlocked or munmapped. Refine the tests. There is already a VM_BUG_ON_PAGE(PageDoubleMap) in page_mlock(), so page_mlock_one() does not even have to worry about that complication. (I said the kernel recovers: but would page reclaim be likely to split THP before rediscovering that it's VM_LOCKED? I've not followed that up) Fixes: 9a73f61bdb8a ("thp, mlock: do not mlock PTE-mapped file huge pages") Signed-off-by: Hugh Dickins Reviewed-by: Shakeel Butt Acked-by: Kirill A. Shutemov Link: https://lore.kernel.org/lkml/cfa154c-d595-406-eb7d-eb9df730f944@google.com/ Cc: Andrew Morton Cc: Alistair Popple Cc: Jason Gunthorpe Cc: Ralph Campbell Cc: Christoph Hellwig Cc: Yang Shi Signed-off-by: Linus Torvalds --- mm/rmap.c | 13 ++++++++----- 1 file changed, 8 insertions(+), 5 deletions(-) diff --git a/mm/rmap.c b/mm/rmap.c index 746013e282c3..0e83c3be8568 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -1442,8 +1442,9 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma, */ if (!(flags & TTU_IGNORE_MLOCK)) { if (vma->vm_flags & VM_LOCKED) { - /* PTE-mapped THP are never mlocked */ - if (!PageTransCompound(page)) { + /* PTE-mapped THP are never marked as mlocked */ + if (!PageTransCompound(page) || + (PageHead(page) && !PageDoubleMap(page))) { /* * Holding pte lock, we do *not* need * mmap_lock here @@ -1984,9 +1985,11 @@ static bool page_mlock_one(struct page *page, struct vm_area_struct *vma, * munlock_vma_pages_range(). */ if (vma->vm_flags & VM_LOCKED) { - /* PTE-mapped THP are never mlocked */ - if (!PageTransCompound(page)) - mlock_vma_page(page); + /* + * PTE-mapped THP are never marked as mlocked, but + * this function is never called when PageDoubleMap(). + */ + mlock_vma_page(page); page_vma_mapped_walk_done(&pvmw); }