Re: [PATCH v1 4/9] mm/memory: Add tree limit to free_pgtables()

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Tue, Sep 09, 2025 at 03:09:40PM -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.
>
> Add some documentation around free_pgtables() and the limits in an
> attempt to clarify the floor and ceiling use as well as the new
> tree_max.
>
> Test code also updated.
>
> No functional changes intended.
>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>

LGTM other than the nits below, so with those addressed feel free to add:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
>  mm/internal.h                    |  4 +++-
>  mm/memory.c                      | 28 +++++++++++++++++++++++++---
>  mm/mmap.c                        |  2 +-
>  mm/vma.c                         |  3 ++-
>  tools/testing/vma/vma_internal.h |  3 ++-
>  5 files changed, 33 insertions(+), 7 deletions(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index 63e3ec8d63be7..d295252407fee 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 3e0404bd57a02..24716b3713f66 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -369,12 +369,34 @@ void free_pgd_range(struct mmu_gather *tlb,
>  	} while (pgd++, addr = next, addr != end);
>  }
>
> +/*

NIT: /** right? This looks like a kernel-doc to me!

> + * free_pgtables() - Free a range of page tables
> + * @tlb: The mmu gather
> + * @mas: The maple state
> + * @vma: The first vma
> + * @floor: The lowest page table address
> + * @ceiling: The highest page table address
> + * @tree_max: The highest tree search address
> + * @mm_wr_locked: boolean indicating if the mm is write locked
> + *
> + * Note: Floor and ceiling are provided to indicate the absolute range of the
> + * page tables that should be removed.  This can differ from the vma mappings on
> + * some archs that may have mappings that need to be removed outside the vmas.
> + * Note that the prev->vm_end and next->vm_start are often used.

Great write-up though you are missing some horrified noises re: the arches doing
that, I guess the reader has to play them back in their head... ;)

> + *
> + * The tree_max differs from the ceiling when a dup_mmap() failed and the tree
> + * has unrelated data to the mm_struct being torn down.
> + */

Ohhh nice nice thanks for adding this comment!

>  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;
>
> +	/* underflow can happen and is fine */

I am taking this as a sign that underflow is in fact fine _everywhere_
including in internal plumbing! :P

> +	WARN_ON_ONCE(tree_max - 1 > ceiling - 1);

Hmm, so if tree_max == 1, and ceilling == 0, we're ok with that (would
resolve to tree_max = 0 and ceiling == ULONG_MAX), I guess relating to the
'interpret 0 as everything' semantics I think we have for ceiling?

I guess it's because these are exclusive.

So perhaps worth updating comment to:

/*
 * these values are exclusive bounds, with 0 being interpreted as the
 * entire range, so underflow is fine.
 */

or similar, just to really underline that...

> +
>  	tlb_free_vmas(tlb);
>
>  	do {
> @@ -385,7 +407,7 @@ 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);
>  		if (unlikely(xa_is_zero(next)))
>  			next = NULL;
>
> @@ -405,7 +427,7 @@ 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 a290448a53bb2..0f4808f135fe6 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);

NIT: Might be nice, while we're here, to add your (very nice) convention of
prefacing boolean params with the param name, so here:

..., /* mm_wr_locked= */ true);

>  	tlb_finish_mmu(&tlb);
>
>  	/*
> diff --git a/mm/vma.c b/mm/vma.c
> index a648e0555c873..1bae142bbc0f1 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;
>  }
> diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h
> index 07167446dcf42..823d379e1fac2 100644
> --- a/tools/testing/vma/vma_internal.h
> +++ b/tools/testing/vma/vma_internal.h
> @@ -900,7 +900,8 @@ static inline void unmap_vmas(struct mmu_gather *tlb, struct ma_state *mas,
>
>  static inline 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)
>  {
>  	(void)tlb;
>  	(void)mas;
> --
> 2.47.2
>