Re: [PATCH mm-unstable v17 06/14] mm/khugepaged: generalize collapse_huge_page for mTHP collapse

[Usama Arif]

On Mon, 11 May 2026 12:58:06 -0600 Nico Pache <npache@xxxxxxxxxx> wrote:

> Pass an order and offset to collapse_huge_page to support collapsing anon
> memory to arbitrary orders within a PMD. order indicates what mTHP size we
> are attempting to collapse to, and offset indicates were in the PMD to
> start the collapse attempt.
> 
> For non-PMD collapse we must leave the anon VMA write locked until after
> we collapse the mTHP-- in the PMD case all the pages are isolated, but in
> the mTHP case this is not true, and we must keep the lock to prevent
> access/changes to the page tables. This can happen if the rmap walkers hit
> a pmd_none while the PMD entry is currently unavailable due to being
> temporarily removed during the collapse phase.
> 
> Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
> ---
>  mm/khugepaged.c | 93 +++++++++++++++++++++++++++++--------------------
>  1 file changed, 55 insertions(+), 38 deletions(-)
> 

The patch did 2 things:

Make it work with any order and not just PMD order.

Keeps anon_vma_write held across the copy and install for non-PMD orders,
as mTHP leaves the out-of-range PTEs mapped while the PMD is temporarily none.
rmap walkers cannot reach here until PMD is isntalled.

Acked-by: Usama Arif <usama.arif@xxxxxxxxx>