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

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

On Thu, Jun 19, 2025 at 08:52:51AM +0530, Dev Jain wrote:
>
> 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@arm.com>
> > > ---
> > >   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.

OK cool would be weird otherwise :)

>
> >
> > >   	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 :)

Seems we all cry together about this code ;)

>
> >
> >
> > > +			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.

Thanks

>
> >
> > > +				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.

OK

>
> >
> > > +			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 :)