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 <>
Cc: Andrew Morton <>
Cc: Alistair Popple <>
Cc: Jason Gunthorpe <>
Cc: Ralph Campbell <>
Cc: Christoph Hellwig <>
Cc: Yang Shi <>
Signed-off-by: Linus Torvalds <>
1 file changed