Re: [PATCH v2 2/2] mm: Optimize mremap() by PTE batching

[Dev Jain]

On 08/05/25 7:30 am, Zi Yan wrote:
> On 7 May 2025, at 2:02, Dev Jain wrote:
>
> > To use PTE batching, we want to determine whether the folio mapped by
> > the PTE is large, thus requiring the use of vm_normal_folio(). We want
> > to avoid the cost of vm_normal_folio() if the code path doesn't already
> > require the folio. For arm64, pte_batch_hint() does the job. To generalize
> > this hint, add a helper which will determine whether two consecutive PTEs
> > point to consecutive PFNs, in which case there is a high probability that
> > the underlying folio is large.
> > Next, use folio_pte_batch() to optimize move_ptes(). On arm64, if the ptes
> > are painted with the contig bit, then ptep_get() will iterate through all 16
> > entries to collect a/d bits. Hence this optimization will result in a 16x
> > reduction in the number of ptep_get() calls. Next, ptep_get_and_clear()
> > will eventually call contpte_try_unfold() on every contig block, thus
> > flushing the TLB for the complete large folio range. Instead, use
> > get_and_clear_full_ptes() so as to elide TLBIs on each contig block, and only
> > do them on the starting and ending contig block.
> >
> > Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
> > ---
> >   include/linux/pgtable.h | 29 +++++++++++++++++++++++++++++
> >   mm/mremap.c             | 37 ++++++++++++++++++++++++++++++-------
> >   2 files changed, 59 insertions(+), 7 deletions(-)
> >
> > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> > index b50447ef1c92..38dab1f562ed 100644
> > --- a/include/linux/pgtable.h
> > +++ b/include/linux/pgtable.h
> > @@ -369,6 +369,35 @@ static inline pgd_t pgdp_get(pgd_t *pgdp)
> >   }
> >   #endif
> >
> > +/**
> > + * maybe_contiguous_pte_pfns - Hint whether the page mapped by the pte belongs
> > + * to a large folio.
> > + * @ptep: Pointer to the page table entry.
> > + * @pte: The page table entry.
> > + *
> > + * This helper is invoked when the caller wants to batch over a set of ptes
> > + * mapping a large folio, but the concerned code path does not already have
> > + * the folio. We want to avoid the cost of vm_normal_folio() only to find that
> > + * the underlying folio was small; i.e keep the small folio case as fast as
> > + * possible.
> > + *
> > + * The caller must ensure that ptep + 1 exists.
>
> ptep points to an entry in a PTE page. As long as it is not pointing
> to the last entry, ptep+1 should always exist. With PTRS_PER_PTE and
> sizeof(pte_t), you can check ptep address to figure out whether it
> is the last entry of a PTE page, right? Let me know if I misunderstand
> anything.

Sounds correct to me.

> --
> Best Regards,
> Yan, Zi