Re: [PATCH 2/4] mm/rmap: fix old bug: munlocking THP missed other mlocks

[Shakeel Butt]

On Wed, Jul 7, 2021 at 1:08 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
>
> 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.)

I think, yes, page reclaim will split the THP prematurely in this case.

>
> Fixes: 9a73f61bdb8a ("thp, mlock: do not mlock PTE-mapped file huge pages")
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>

Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>