Re: [RFC PATCH 4/6] mm/memory: Add tree limit to free_pgtables()

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* Lorenzo Stoakes <lorenzo.stoakes@oracle.com> [250819 15:14]:
> On Fri, Aug 15, 2025 at 03:10:29PM -0400, Liam R. Howlett wrote:
> > The ceiling and tree search limit need to be different arguments for the
> > future change in the failed fork attempt.
> >
> > No functional changes intended.
> >
> > Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
> 
> (Obv. in addition to comment about broken VMA tests :P)
> 
> I guess intent is that if we discover any page tables beyond tree_max then
> we ought to just wipe them all out so, in effect, we don't consider
> mappings at or past tree_max to be valid?

Actually... there are some archs that map outside the vma and they are
valid.. I think mips? and I think lower, but yeah.. it's needed.  This
is why prev->vm_end and next->vm_start are used as page table limits,
afaik.  This is a serious annoyance because it frequently adds walks
that are infrequently necessary to the vma tree.

> 
> I feel like we need a comment to this effect as this is confusing as it is.
> 
> Could we add a kerneldoc comment for free_pgtables() spelling this out?

I'll add a note here, but confusion will probably increase.  I'll add a
note about the tree max as well.

> 
> > ---
> >  mm/internal.h | 4 +++-
> >  mm/memory.c   | 7 ++++---
> >  mm/mmap.c     | 2 +-
> >  mm/vma.c      | 3 ++-
> >  4 files changed, 10 insertions(+), 6 deletions(-)
> >
> > diff --git a/mm/internal.h b/mm/internal.h
> > index 45b725c3dc030..f9a278ac76d83 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -444,7 +444,9 @@ void folio_activate(struct folio *folio);
> >
> >  void free_pgtables(struct mmu_gather *tlb, struct ma_state *mas,
> >  		   struct vm_area_struct *start_vma, unsigned long floor,
> > -		   unsigned long ceiling, bool mm_wr_locked);
> > +		   unsigned long ceiling, unsigned long tree_max,
> > +		   bool mm_wr_locked);
> > +
> >  void pmd_install(struct mm_struct *mm, pmd_t *pmd, pgtable_t *pte);
> >
> >  struct zap_details;
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 0ba4f6b718471..3346514562bba 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -371,7 +371,8 @@ void free_pgd_range(struct mmu_gather *tlb,
> >
> >  void free_pgtables(struct mmu_gather *tlb, struct ma_state *mas,
> >  		   struct vm_area_struct *vma, unsigned long floor,
> > -		   unsigned long ceiling, bool mm_wr_locked)
> > +		   unsigned long ceiling, unsigned long tree_max,
> > +		   bool mm_wr_locked)
> >  {
> >  	struct unlink_vma_file_batch vb;
> >
> > @@ -385,7 +386,7 @@ void free_pgtables(struct mmu_gather *tlb, struct ma_state *mas,
> >  		 * Note: USER_PGTABLES_CEILING may be passed as ceiling and may
> >  		 * be 0.  This will underflow and is okay.
> >  		 */
> > -		next = mas_find(mas, ceiling - 1);
> > +		next = mas_find(mas, tree_max - 1);
> 
> Do we need to put some sort of sanity checks in to make sure tree_max <= ceiling
> (though this 0 case is a pain... so I guess tree_max - 1 <= ceiling - 1?)

Sure!

> 
> >  		if (unlikely(xa_is_zero(next)))
> >  			next = NULL;
> >
> > @@ -405,7 +406,7 @@ void free_pgtables(struct mmu_gather *tlb, struct ma_state *mas,
> >  		 */
> >  		while (next && next->vm_start <= vma->vm_end + PMD_SIZE) {
> >  			vma = next;
> > -			next = mas_find(mas, ceiling - 1);
> > +			next = mas_find(mas, tree_max - 1);
> >  			if (unlikely(xa_is_zero(next)))
> >  				next = NULL;
> >  			if (mm_wr_locked)
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 0995a48b46d59..eba2bc81bc749 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -1311,7 +1311,7 @@ void exit_mmap(struct mm_struct *mm)
> >  	mt_clear_in_rcu(&mm->mm_mt);
> >  	vma_iter_set(&vmi, vma->vm_end);
> >  	free_pgtables(&tlb, &vmi.mas, vma, FIRST_USER_ADDRESS,
> > -		      USER_PGTABLES_CEILING, true);
> > +		      USER_PGTABLES_CEILING, USER_PGTABLES_CEILING, true);
> >  	tlb_finish_mmu(&tlb);
> >
> >  	/*
> > diff --git a/mm/vma.c b/mm/vma.c
> > index fd270345c25d3..aa75ca8618609 100644
> > --- a/mm/vma.c
> > +++ b/mm/vma.c
> > @@ -486,6 +486,7 @@ void unmap_region(struct ma_state *mas, struct vm_area_struct *vma,
> >  		   /* mm_wr_locked = */ true);
> >  	mas_set(mas, vma->vm_end);
> >  	free_pgtables(&tlb, mas, vma, prev ? prev->vm_end : FIRST_USER_ADDRESS,
> > +		      next ? next->vm_start : USER_PGTABLES_CEILING,
> >  		      next ? next->vm_start : USER_PGTABLES_CEILING,
> >  		      /* mm_wr_locked = */ true);
> >  	tlb_finish_mmu(&tlb);
> > @@ -1232,7 +1233,7 @@ static inline void vms_clear_ptes(struct vma_munmap_struct *vms,
> >  	mas_set(mas_detach, 1);
> >  	/* start and end may be different if there is no prev or next vma. */
> >  	free_pgtables(&tlb, mas_detach, vms->vma, vms->unmap_start,
> > -		      vms->unmap_end, mm_wr_locked);
> > +		      vms->unmap_end, vms->unmap_end, mm_wr_locked);
> >  	tlb_finish_mmu(&tlb);
> >  	vms->clear_ptes = false;
> >  }
> > --
> > 2.47.2
> >