Re: [PATCH] khugepaged: Optimize __collapse_huge_page_copy_succeeded() for large folios by PTE batching

[Dev Jain]

On 18/06/25 10:56 pm, Lorenzo Stoakes wrote:
> On Wed, Jun 18, 2025 at 03:56:07PM +0530, Dev Jain wrote:
> > Use PTE batching to optimize __collapse_huge_page_copy_succeeded().
> >
> > On arm64, suppose khugepaged is scanning a pte-mapped 2MB THP for collapse.
> > Then, calling ptep_clear() for every pte will cause a TLB flush for every
> > contpte block. Instead, clear_full_ptes() does a
> > contpte_try_unfold_partial() which will flush the TLB only for the (if any)
> > starting and ending contpte block, if they partially overlap with the range
> > khugepaged is looking at.
> >
> > For all arches, there should be a benefit due to batching atomic operations
> > on mapcounts due to folio_remove_rmap_ptes().
> >
> > No issues were observed with mm-selftests.
> >
> > Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
> > ---
> >   mm/khugepaged.c | 31 +++++++++++++++++++++++--------
> >   1 file changed, 23 insertions(+), 8 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index d45d08b521f6..649ccb2670f8 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -700,12 +700,14 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
> >   						spinlock_t *ptl,
> >   						struct list_head *compound_pagelist)
> >   {
> > +	unsigned long end = address + HPAGE_PMD_SIZE;
> I assume we always enter here with aligned address...

Yes.

> >   	struct folio *src, *tmp;
> > -	pte_t *_pte;
> > +	pte_t *_pte = pte;
> >   	pte_t pteval;
> > +	int nr_ptes;
> >
> > -	for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
> > -	     _pte++, address += PAGE_SIZE) {
> > +	do {
> > +		nr_ptes = 1;
> >   		pteval = ptep_get(_pte);
> >   		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> >   			add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
> > @@ -719,23 +721,36 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
> >   				ksm_might_unmap_zero_page(vma->vm_mm, pteval);
> >   			}
> >   		} else {
> Existing code but hate this level of indentation.
>
> The code before was (barely) sort of ok-ish, but now it's realyl out of hand.
>
> On the other hand, I look at __collapse_huge_page_isolate() and want to cry so I
> guess this maybe is something that needs addressing outside of this patch.

Trust me I have already cried a lot before while doing the khugepaged mTHP stuff :)

> > +			const fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
> > +			int max_nr_ptes;
> > +			bool is_large;
> > +
> >   			struct page *src_page = pte_page(pteval);
> >
> >   			src = page_folio(src_page);
> > -			if (!folio_test_large(src))
> > +			is_large = folio_test_large(src);
> > +			if (!is_large)
> >   				release_pte_folio(src);
> Hm, in this case right, release_pte_folio() does a folio_unlock().
>
> Where does a large folio get unlocked?
>
> I mean this must have been existing code because I don't see where this
> happens previously either.
>
> > +
> > +			max_nr_ptes = (end - address) >> PAGE_SHIFT;
> > +			if (is_large && max_nr_ptes != 1)
> Is it really that harmful if max_nr_ptes == 1? Doesn't folio_pte_batch()
> figure it out?

Yup it will figure that out, was just following the pattern of zap_present_ptes
and copy_present_ptes. Will drop this.

> > +				nr_ptes = folio_pte_batch(src, address, _pte,
> > +							  pteval, max_nr_ptes,
> > +							  flags, NULL, NULL, NULL);
> > +
> It'd be nice(r) if this was:
>
> if (folio_test_large(src))
> 	nr_ptes = folio_pte_batch(src, address, _pte,
> 		pteval, max_nr_ptes,
> 		flags, NULL, NULL, NULL);
> else
> 	release_pte_folio(src);
>
> But even that is horrid because of the asymmetry.
>
> >   			/*
> >   			 * ptl mostly unnecessary, but preempt has to
> >   			 * be disabled to update the per-cpu stats
> >   			 * inside folio_remove_rmap_pte().
> >   			 */
> >   			spin_lock(ptl);
> > -			ptep_clear(vma->vm_mm, address, _pte);
> > -			folio_remove_rmap_pte(src, src_page, vma);
> > +			clear_full_ptes(vma->vm_mm, address, _pte, nr_ptes, false);
> Be nice to use 'Liam's convention' of sticking `/* full = */ false)` on the
> end here so we know what the false refers to.

Sounds good, although in the other mail David mentioned a way to elide this
so I will prefer that.

> > +			folio_remove_rmap_ptes(src, src_page, nr_ptes, vma);
> Kinda neat that folio_remove_map_pte() is jus ta define onto this with
> nr_ptes == 1 :)
>
> >   			spin_unlock(ptl);
> > -			free_folio_and_swap_cache(src);
> > +			free_swap_cache(src);
> > +			folio_put_refs(src, nr_ptes);
> >   		}
> > -	}
> > +	} while (_pte += nr_ptes, address += nr_ptes * PAGE_SIZE, address != end);
> >
> >   	list_for_each_entry_safe(src, tmp, compound_pagelist, lru) {
> >   		list_del(&src->lru);
> > --
> > 2.30.2
> I can't see much wrong with this though, just 'yuck' at existing code
> really :)