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

Lorenzo Stoakes
2025-03-11T13:07:49Z

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@kernel.org>
> ---
> 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