Re: [PATCH v4 05/21] mm: switch the rmap lock held option off in compat layer

Vlastimil Babka (SUSE)
2026-03-24T14:26:42Z

On 3/20/26 23:39, Lorenzo Stoakes (Oracle) wrote:
> In the mmap_prepare compatibility layer, we don't need to hold the rmap
> lock, as we are being called from an .mmap handler.
>
> The .mmap_prepare hook, when invoked in the VMA logic, is called prior to
> the VMA being instantiated, but the completion hook is called after the VMA
> is linked into the maple tree, meaning rmap walkers can reach it.
>
> The mmap hook does not link the VMA into the tree, so this cannot happen.
>
> Therefore it's safe to simply disable this in the mmap_prepare
> compatibility layer.
>
> Also update VMA tests code to reflect current compatibility layer state.
>
> Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>

Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

a typo fix below, Andrew can fix locally?

> ---
> mm/util.c | 6 ++++-
> tools/testing/vma/include/dup.h | 42 +++++++++++++++++----------------
> 2 files changed, 27 insertions(+), 21 deletions(-)
>
> diff --git a/mm/util.c b/mm/util.c
> index a2cfa0d77c35..182f0f5cc400 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -1204,6 +1204,7 @@ int compat_vma_mmap(struct file *file, struct vm_area_struct *vma)
>
> .action.type = MMAP_NOTHING, /* Default */
> };
> + struct mmap_action *action = &desc.action;
> int err;
>
> err = vfs_mmap_prepare(file, &desc);
> @@ -1214,8 +1215,11 @@ int compat_vma_mmap(struct file *file, struct vm_area_struct *vma)
> if (err)
> return err;
>
> + /* being invoked from .mmmap means we don't have to enforce this. */

.mmap

> + action->hide_from_rmap_until_complete = false;
> +
> set_vma_from_desc(vma, &desc);
> - err = mmap_action_complete(vma, &desc.action);
> + err = mmap_action_complete(vma, action);
> if (err) {
> const size_t len = vma_pages(vma) << PAGE_SHIFT;
>