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

[Lorenzo Stoakes (Oracle)]

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