Re: [PATCH v9 04/17] mm: introduce vma_iter_store_attached() to use with attached vmas

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

On Fri, Jan 10, 2025 at 08:25:51PM -0800, Suren Baghdasaryan wrote:
> vma_iter_store() functions can be used both when adding a new vma and
> when updating an existing one. However for existing ones we do not need
> to mark them attached as they are already marked that way. Introduce
> vma_iter_store_attached() to be used with already attached vmas.

OK I guess the intent of this is to reinstate the previously existing
asserts, only explicitly checking those places where we attach.

I'm a little concerned that by doing this, somebody might simply invoke
this function without realising the implications.

Can we have something functional like

vma_iter_store_new() and vma_iter_store_overwrite()

?

I don't like us just leaving vma_iter_store() quietly making an assumption
that a caller doesn't necessarily realise.

Also it's more greppable this way.

I had a look through callers and it does seem you've snagged them all
correctly.

>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  include/linux/mm.h | 12 ++++++++++++
>  mm/vma.c           |  8 ++++----
>  mm/vma.h           | 11 +++++++++--
>  3 files changed, 25 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 2b322871da87..2f805f1a0176 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -821,6 +821,16 @@ static inline void vma_assert_locked(struct vm_area_struct *vma)
>  		vma_assert_write_locked(vma);
>  }
>
> +static inline void vma_assert_attached(struct vm_area_struct *vma)
> +{
> +	VM_BUG_ON_VMA(vma->detached, vma);
> +}
> +
> +static inline void vma_assert_detached(struct vm_area_struct *vma)
> +{
> +	VM_BUG_ON_VMA(!vma->detached, vma);
> +}
> +
>  static inline void vma_mark_attached(struct vm_area_struct *vma)
>  {
>  	vma->detached = false;
> @@ -866,6 +876,8 @@ static inline void vma_end_read(struct vm_area_struct *vma) {}
>  static inline void vma_start_write(struct vm_area_struct *vma) {}
>  static inline void vma_assert_write_locked(struct vm_area_struct *vma)
>  		{ mmap_assert_write_locked(vma->vm_mm); }
> +static inline void vma_assert_attached(struct vm_area_struct *vma) {}
> +static inline void vma_assert_detached(struct vm_area_struct *vma) {}
>  static inline void vma_mark_attached(struct vm_area_struct *vma) {}
>  static inline void vma_mark_detached(struct vm_area_struct *vma) {}
>
> diff --git a/mm/vma.c b/mm/vma.c
> index d603494e69d7..b9cf552e120c 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -660,14 +660,14 @@ static int commit_merge(struct vma_merge_struct *vmg,
>  	vma_set_range(vmg->vma, vmg->start, vmg->end, vmg->pgoff);
>
>  	if (expanded)
> -		vma_iter_store(vmg->vmi, vmg->vma);
> +		vma_iter_store_attached(vmg->vmi, vmg->vma);
>
>  	if (adj_start) {
>  		adjust->vm_start += adj_start;
>  		adjust->vm_pgoff += PHYS_PFN(adj_start);
>  		if (adj_start < 0) {
>  			WARN_ON(expanded);
> -			vma_iter_store(vmg->vmi, adjust);
> +			vma_iter_store_attached(vmg->vmi, adjust);
>  		}
>  	}

I kind of feel this whole function (that yes, I added :>) though derived
from existing logic) needs rework, as it's necessarily rather confusing.

But hey, that's on me :)

But this does look right... OK see this as a note-to-self...

>
> @@ -2845,7 +2845,7 @@ int expand_upwards(struct vm_area_struct *vma, unsigned long address)
>  				anon_vma_interval_tree_pre_update_vma(vma);
>  				vma->vm_end = address;
>  				/* Overwrite old entry in mtree. */
> -				vma_iter_store(&vmi, vma);
> +				vma_iter_store_attached(&vmi, vma);
>  				anon_vma_interval_tree_post_update_vma(vma);
>
>  				perf_event_mmap(vma);
> @@ -2925,7 +2925,7 @@ int expand_downwards(struct vm_area_struct *vma, unsigned long address)
>  				vma->vm_start = address;
>  				vma->vm_pgoff -= grow;
>  				/* Overwrite old entry in mtree. */
> -				vma_iter_store(&vmi, vma);
> +				vma_iter_store_attached(&vmi, vma);
>  				anon_vma_interval_tree_post_update_vma(vma);
>
>  				perf_event_mmap(vma);
> diff --git a/mm/vma.h b/mm/vma.h
> index 2a2668de8d2c..63dd38d5230c 100644
> --- a/mm/vma.h
> +++ b/mm/vma.h
> @@ -365,9 +365,10 @@ static inline struct vm_area_struct *vma_iter_load(struct vma_iterator *vmi)
>  }
>
>  /* Store a VMA with preallocated memory */
> -static inline void vma_iter_store(struct vma_iterator *vmi,
> -				  struct vm_area_struct *vma)
> +static inline void vma_iter_store_attached(struct vma_iterator *vmi,
> +					   struct vm_area_struct *vma)
>  {
> +	vma_assert_attached(vma);
>
>  #if defined(CONFIG_DEBUG_VM_MAPLE_TREE)
>  	if (MAS_WARN_ON(&vmi->mas, vmi->mas.status != ma_start &&
> @@ -390,7 +391,13 @@ static inline void vma_iter_store(struct vma_iterator *vmi,
>
>  	__mas_set_range(&vmi->mas, vma->vm_start, vma->vm_end - 1);
>  	mas_store_prealloc(&vmi->mas, vma);
> +}
> +
> +static inline void vma_iter_store(struct vma_iterator *vmi,
> +				  struct vm_area_struct *vma)
> +{
>  	vma_mark_attached(vma);
> +	vma_iter_store_attached(vmi, vma);
>  }
>

See comment at top, and we need some comments here to explain why we're
going to pains to do this.

What about mm/nommu.c? I guess these cases are always new VMAs.

We probably definitely need to check this series in a nommu setup, have you
done this? As I can see this breaking things. Then again I suppose you'd
have expected bots to moan by now...

>  static inline unsigned long vma_iter_addr(struct vma_iterator *vmi)
> --
> 2.47.1.613.gc27f4b7a9f-goog
>