Re: [PATCH 1/3] khugepaged: Optimize __collapse_huge_page_copy_succeeded() by PTE batching

[Dev Jain]

On 25/06/25 5:44 pm, Lorenzo Stoakes wrote:
> You forgot the v2 here :) this breaks b4 shazam...
>
> I managed to do this on the cover letter (but not patches) of a series
> before. So you're in good company... ;)
>
> On Wed, Jun 25, 2025 at 11:28:04AM +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>
> Overall looking way way better! Just some nits below.
>
> > ---
> >   mm/khugepaged.c | 27 +++++++++++++++++++++------
> >   1 file changed, 21 insertions(+), 6 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index d45d08b521f6..3944b112d452 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -700,12 +700,15 @@ 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;
> >   	struct folio *src, *tmp;
> > -	pte_t *_pte;
> >   	pte_t pteval;
> > +	pte_t *_pte;
> > +	int nr_ptes;
> >
> > -	for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
> > -	     _pte++, address += PAGE_SIZE) {
> > +	for (_pte = pte; _pte < pte + HPAGE_PMD_NR; _pte += nr_ptes,
> > +	     address += nr_ptes * PAGE_SIZE) {
> Thanks this is much better.
>
> > +		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);
> > @@ -719,21 +722,33 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
> >   				ksm_might_unmap_zero_page(vma->vm_mm, pteval);
> >   			}
> >   		} else {
> > +			const fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
> > +			int max_nr_ptes;
> > +
> >   			struct page *src_page = pte_page(pteval);
> >
> >   			src = page_folio(src_page);
> >   			if (!folio_test_large(src))
> >   				release_pte_folio(src);
> > +
> > +			max_nr_ptes = (end - address) >> PAGE_SHIFT;
> > +			if (folio_test_large(src))
> > +				nr_ptes = folio_pte_batch(src, address, _pte,
> > +							  pteval, max_nr_ptes,
> > +							  flags, NULL, NULL, NULL);
> Nit, but we only use max_nr_ptes here so could declare and set here, e.g.:
>
> 			if (folio_test_large(src)) {
> 				int max_nr_ptes = (end - address) >> PAGE_SHIFT;
>
> 				nr_ptes = folio_pte_batch(src, address, _pte,
> 							  pteval, max_nr_ptes,
> 							  flags, NULL, NULL, NULL);
> 			}
>
> BTW I think David raised it, but is there a way to wrap folio_pte_batch() to not
> have to NULL, NULL, NULL here? :)
>
>
> oh and if we do this, we can also combine this line with above so:
>
> 			if (folio_test_large(src)) {
> 				int max_nr_ptes = (end - address) >> PAGE_SHIFT;
>
> 				nr_ptes = folio_pte_batch(src, address, _pte,
> 							  pteval, max_nr_ptes,
> 							  flags, NULL, NULL, NULL);
> 			} else {
>    				release_pte_folio(src);
> 			}
>
> Which is neater.

Okay.

> > +
> >   			/*
> >   			 * 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,
> > +					/* full = */ false);
> > +			folio_remove_rmap_ptes(src, src_page, nr_ptes, vma);
> >   			spin_unlock(ptl);
> > -			free_folio_and_swap_cache(src);
> > +			free_swap_cache(src);
> > +			folio_put_refs(src, nr_ptes);
> >   		}
> >   	}
> >
> > --
> > 2.30.2