Re: [PATCH 7/9] mm/madvise: let madvise_{dontneed,free}_single_vma() caller batches tlb flushes

[Lorenzo Stoakes]

Super super UBER nitty but... pretty sure the subject here should be <= 75
chars right? :P

On Mon, Mar 10, 2025 at 10:23:16AM -0700, SeongJae Park wrote:
> Update madvise_dontneed_single_vma() and madvise_free_single_vma()
> functions so that the caller can pass an mmu_gather object that should
> be initialized and will be finished outside, for batched tlb flushes.
> Also modify their internal code to support such usage by skipping the
> initialization and finishing of self-allocated mmu_gather object if it
> received a valid mmu_gather object.
>
> Signed-off-by: SeongJae Park <sj@xxxxxxxxxx>
> ---
>  mm/internal.h |  3 +++
>  mm/madvise.c  | 37 +++++++++++++++++++++++++------------
>  mm/memory.c   | 16 +++++++++++++---
>  3 files changed, 41 insertions(+), 15 deletions(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index 0caa64dc2cb7..ce7fb2383f65 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -438,6 +438,9 @@ void unmap_page_range(struct mmu_gather *tlb,
>  			     struct vm_area_struct *vma,
>  			     unsigned long addr, unsigned long end,
>  			     struct zap_details *details);
> +void unmap_vma_single(struct mmu_gather *tlb, struct vm_area_struct *vma,
> +		      unsigned long addr, unsigned long size,
> +		      struct zap_details *details);
>  int folio_unmap_invalidate(struct address_space *mapping, struct folio *folio,
>  			   gfp_t gfp);
>
> diff --git a/mm/madvise.c b/mm/madvise.c
> index ba2a78795207..d7ea71c6422c 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -794,12 +794,19 @@ static const struct mm_walk_ops madvise_free_walk_ops = {
>  	.walk_lock		= PGWALK_RDLOCK,
>  };
>
> -static int madvise_free_single_vma(struct vm_area_struct *vma,
> -			unsigned long start_addr, unsigned long end_addr)
> +static int madvise_free_single_vma(
> +		struct mmu_gather *caller_tlb, struct vm_area_struct *vma,

I find this interface horrible, and super confusing. It's not clear at all
what's going on here.

Why not use your new helper struct to add a field you can thread through
here?

> +		unsigned long start_addr, unsigned long end_addr)
>  {
>  	struct mm_struct *mm = vma->vm_mm;
>  	struct mmu_notifier_range range;
> -	struct mmu_gather tlb;
> +	struct mmu_gather self_tlb;
> +	struct mmu_gather *tlb;
> +
> +	if (caller_tlb)
> +		tlb = caller_tlb;
> +	else
> +		tlb = &self_tlb;
>
>  	/* MADV_FREE works for only anon vma at the moment */
>  	if (!vma_is_anonymous(vma))
> @@ -815,16 +822,18 @@ static int madvise_free_single_vma(struct vm_area_struct *vma,
>  				range.start, range.end);
>
>  	lru_add_drain();
> -	tlb_gather_mmu(&tlb, mm);
> +	if (!caller_tlb)
> +		tlb_gather_mmu(tlb, mm);

Yeah really don't like this.

Ideally we'd abstract the mmu_gather struct to the helper struct (which I
see you do in a subsequent patch anyway) would be ideal if you could find a
way to make that work.

But if not, then:

if (behavior->batched_tlb)
	tlb_gather_mmu(&tlb, mm);

etc. etc.

Would work better.

>  	update_hiwater_rss(mm);
>
>  	mmu_notifier_invalidate_range_start(&range);
> -	tlb_start_vma(&tlb, vma);
> +	tlb_start_vma(tlb, vma);

Also not a fan of making tlb refer to a pointer now when before it
didn't... I mean that's more of a nit and maybe unavoidable, but still!

I mean yeah ok this is probably unavoidable, ignore.

>  	walk_page_range(vma->vm_mm, range.start, range.end,
> -			&madvise_free_walk_ops, &tlb);
> -	tlb_end_vma(&tlb, vma);
> +			&madvise_free_walk_ops, tlb);
> +	tlb_end_vma(tlb, vma);
>  	mmu_notifier_invalidate_range_end(&range);
> -	tlb_finish_mmu(&tlb);
> +	if (!caller_tlb)
> +		tlb_finish_mmu(tlb);
>
>  	return 0;
>  }
> @@ -848,7 +857,8 @@ static int madvise_free_single_vma(struct vm_area_struct *vma,
>   * An interface that causes the system to free clean pages and flush
>   * dirty pages is already available as msync(MS_INVALIDATE).
>   */
> -static long madvise_dontneed_single_vma(struct vm_area_struct *vma,
> +static long madvise_dontneed_single_vma(struct mmu_gather *tlb,
> +					struct vm_area_struct *vma,
>  					unsigned long start, unsigned long end)
>  {
>  	struct zap_details details = {
> @@ -856,7 +866,10 @@ static long madvise_dontneed_single_vma(struct vm_area_struct *vma,
>  		.even_cows = true,
>  	};
>
> -	zap_page_range_single(vma, start, end - start, &details);
> +	if (!tlb)
> +		zap_page_range_single(vma, start, end - start, &details);

Please don't put the negation case first, it's confusing. Swap them!


> +	else
> +		unmap_vma_single(tlb, vma, start, end - start, &details);
>  	return 0;
>  }
>
> @@ -951,9 +964,9 @@ static long madvise_dontneed_free(struct vm_area_struct *vma,
>  	}
>
>  	if (behavior == MADV_DONTNEED || behavior == MADV_DONTNEED_LOCKED)
> -		return madvise_dontneed_single_vma(vma, start, end);
> +		return madvise_dontneed_single_vma(NULL, vma, start, end);
>  	else if (behavior == MADV_FREE)
> -		return madvise_free_single_vma(vma, start, end);
> +		return madvise_free_single_vma(NULL, vma, start, end);

Not to labour the point, but this is also horrid, passing a mystery NULL
parameter first...

>  	else
>  		return -EINVAL;
>  }
> diff --git a/mm/memory.c b/mm/memory.c
> index 88c478e2ed1a..3256b9713cbd 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1995,9 +1995,19 @@ void unmap_vmas(struct mmu_gather *tlb, struct ma_state *mas,
>  	mmu_notifier_invalidate_range_end(&range);
>  }
>
> -static void unmap_vma_single(struct mmu_gather *tlb,
> -		struct vm_area_struct *vma, unsigned long address,
> -		unsigned long size, struct zap_details *details)
> +/**
> + * unmap_vma_single - remove user pages in a given range
> + * @tlb: pointer to the caller's struct mmu_gather
> + * @vma: vm_area_struct holding the applicable pages
> + * @address: starting address of the pages
> + * @size: number of bytes to remove
> + * @details: details of shared cache invalidation
> + *
> + * @tlb shouldn't be NULL.  The range must fit into one VMA.

Can we add some VM_WARN_ON[_ONCE]()'s for these conditions please?

Thanks for documenting!

> + */
> +void unmap_vma_single(struct mmu_gather *tlb, struct vm_area_struct *vma,
> +		      unsigned long address, unsigned long size,
> +		      struct zap_details *details)
>  {
>  	const unsigned long end = address + size;
>  	struct mmu_notifier_range range;
> --
> 2.39.5