Re: [PATCH 21/30] mm/vma: add and use vma_[add/sub]_pgoff()

[Lorenzo Stoakes]

On Thu, Jul 02, 2026 at 12:20:10PM +0100, Pedro Falcato wrote:
> On Mon, Jun 29, 2026 at 01:23:32PM +0100, Lorenzo Stoakes wrote:
> > Add helpers for adding or subtracting to a VMA's page offset, exposed
> > internally for VMA users within mm in mm/vma.h.
> >
> > This is to lay the foundations for tracking anonymous page offset for
> > MAP_PRIVATE file-backed mappings, where adding and subtracting from this
> > value must be reflected in both the file and anonymous offsets.
> >
> > These are used on VMA split and downward stack expansion.
> >
> > No functional change intended.
> >
> > Signed-off-by: Lorenzo Stoakes <ljs@xxxxxxxxxx>
> > ---
> >  mm/nommu.c                      |  6 ++++--
> >  mm/vma.c                        |  6 +++---
> >  mm/vma.h                        | 12 ++++++++++++
> >  tools/testing/vma/include/dup.h | 13 ++++++++++++-
> >  4 files changed, 31 insertions(+), 6 deletions(-)
> >
> > diff --git a/mm/nommu.c b/mm/nommu.c
> > index 7333d855e974..c7fafcd87c14 100644
> > --- a/mm/nommu.c
> > +++ b/mm/nommu.c
> > @@ -41,6 +41,7 @@
> >  #include <asm/tlbflush.h>
> >  #include <asm/mmu_context.h>
> >  #include "internal.h"
> > +#include "vma.h"
> >
> >  unsigned long highest_memmap_pfn;
> >  int heap_stack_gap = 0;
> > @@ -1338,7 +1339,8 @@ static int split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
> >  		region->vm_top = region->vm_end = new->vm_end = addr;
> >  	} else {
> >  		region->vm_start = new->vm_start = addr;
> > -		region->vm_pgoff = new->vm_pgoff += npages;
> > +		vma_add_pgoff(new, npages);
> > +		region->vm_pgoff = vma_start_pgoff(new);
> >  	}
> >
> >  	vma_iter_config(vmi, new->vm_start, new->vm_end);
> > @@ -1355,7 +1357,7 @@ static int split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
> >  	delete_nommu_region(vma->vm_region);
> >  	if (new_below) {
> >  		vma->vm_region->vm_start = vma->vm_start = addr;
> > -		vma->vm_pgoff += npages;
> > +		vma_add_pgoff(vma, npages);
> >  		vma->vm_region->vm_pgoff = vma_start_pgoff(vma);
> >  	} else {
> >  		vma->vm_region->vm_end = vma->vm_end = addr;
> > diff --git a/mm/vma.c b/mm/vma.c
> > index 185d07397ca6..cb7222e20c93 100644
> > --- a/mm/vma.c
> > +++ b/mm/vma.c
> > @@ -517,7 +517,7 @@ __split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
> >  		new->vm_end = addr;
> >  	} else {
> >  		new->vm_start = addr;
> > -		new->vm_pgoff += linear_page_delta(vma, addr);
> > +		vma_add_pgoff(new, linear_page_delta(vma, addr));
> >  	}
> >
> >  	err = -ENOMEM;
> > @@ -556,7 +556,7 @@ __split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
> >
> >  	if (new_below) {
> >  		vma->vm_start = addr;
> > -		vma->vm_pgoff += (addr - new->vm_start) >> PAGE_SHIFT;
> > +		vma_add_pgoff(vma, (addr - new->vm_start) >> PAGE_SHIFT);
> >  	} else {
> >  		vma->vm_end = addr;
> >  	}
> > @@ -3305,7 +3305,7 @@ int expand_downwards(struct vm_area_struct *vma, unsigned long address)
> >  				vm_stat_account(mm, vma->vm_flags, grow);
> >  				anon_vma_interval_tree_pre_update_vma(vma);
> >  				vma->vm_start = address;
> > -				vma->vm_pgoff -= grow;
> > +				vma_sub_pgoff(vma, grow);
> >  				/* Overwrite old entry in mtree. */
> >  				vma_iter_store_overwrite(&vmi, vma);
> >  				anon_vma_interval_tree_post_update_vma(vma);
> > diff --git a/mm/vma.h b/mm/vma.h
> > index 2342516ce00e..47fe35e5307e 100644
> > --- a/mm/vma.h
> > +++ b/mm/vma.h
> > @@ -247,6 +247,18 @@ static inline pgoff_t vmg_end_pgoff(const struct vma_merge_struct *vmg)
> >  	return vmg_start_pgoff(vmg) + vmg_pages(vmg);
> >  }
> >
> > +static inline void vma_add_pgoff(struct vm_area_struct *vma, pgoff_t delta)
> > +{
> > +	vma_assert_can_modify(vma);
> > +	vma->vm_pgoff += delta;
> > +}
> > +
> > +static inline void vma_sub_pgoff(struct vm_area_struct *vma, pgoff_t delta)
> > +{
> > +	vma_assert_can_modify(vma);
> > +	vma->vm_pgoff -= delta;
> > +}
> > +
> >  #define VMG_STATE(name, mm_, vmi_, start_, end_, vma_flags_, pgoff_)	\
> >  	struct vma_merge_struct name = {				\
> >  		.mm = mm_,						\
> > diff --git a/tools/testing/vma/include/dup.h b/tools/testing/vma/include/dup.h
> > index 7ed165c8d9bc..41fea90a344d 100644
> > --- a/tools/testing/vma/include/dup.h
> > +++ b/tools/testing/vma/include/dup.h
> > @@ -1163,6 +1163,11 @@ static inline struct vm_area_struct *vma_next(struct vma_iterator *vmi)
> >  	return mas_find(&vmi->mas, ULONG_MAX);
> >  }
> >
> > +static inline bool vma_is_attached(struct vm_area_struct *vma)
> > +{
> > +	return refcount_read(&vma->vm_refcnt);
> > +}
> > +
> >  /*
> >   * WARNING: to avoid racing with vma_mark_attached()/vma_mark_detached(), these
> >   * assertions should be made either under mmap_write_lock or when the object
> > @@ -1170,7 +1175,13 @@ static inline struct vm_area_struct *vma_next(struct vma_iterator *vmi)
> >   */
> >  static inline void vma_assert_attached(struct vm_area_struct *vma)
> >  {
> > -	WARN_ON_ONCE(!refcount_read(&vma->vm_refcnt));
> > +	WARN_ON_ONCE(!vma_is_attached(vma));
> > +}
> > +
> > +static inline void vma_assert_can_modify(struct vm_area_struct *vma)
> > +{
> > +	if (vma_is_attached(vma))
> > +		vma_assert_write_locked(vma);
> >  }
>
> These hunks in dup.h look lost. Should perhaps be on the previous patch
> (adding the helpers).

Yeah, it's because the VMA code starts actually using them in a way that
otherwise breaks the tests at this point, but you're right, functionally it'd be
nicer to add them at the point they're introduced.

Will fix that up on respin!

>
> Anyway, Obviously Correct(tm).
>
> Reviewed-by: Pedro Falcato <pfalcato@xxxxxxx>

Thanks!

>
> --
> Pedro

Cheers, Lorenzo