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

Lorenzo Stoakes (Oracle)
2026-03-24T16:35:25Z

On Tue, Mar 24, 2026 at 03:26:28PM +0100, Vlastimil Babka (SUSE) wrote:
> 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

mmmmm map! ;)

Andrew - could you fixup in place? Thanks.

>
> > + 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;
> >