Re: [PATCH v2 13/15] mm/memory: optimize fork() with PTE-mapped THP

[David Hildenbrand <david@redhat.com>]

On 25.01.24 20:32, David Hildenbrand wrote:
> Let's implement PTE batching when consecutive (present) PTEs map
> consecutive pages of the same large folio, and all other PTE bits besides
> the PFNs are equal.
> 
> We will optimize folio_pte_batch() separately, to ignore selected
> PTE bits. This patch is based on work by Ryan Roberts.
> 
> Use __always_inline for __copy_present_ptes() and keep the handling for
> single PTEs completely separate from the multi-PTE case: we really want
> the compiler to optimize for the single-PTE case with small folios, to
> not degrade performance.
> 
> Note that PTE batching will never exceed a single page table and will
> always stay within VMA boundaries.
> 
> Further, processing PTE-mapped THP that maybe pinned and have
> PageAnonExclusive set on at least one subpage should work as expected,
> but there is room for improvement: We will repeatedly (1) detect a PTE
> batch (2) detect that we have to copy a page (3) fall back and allocate a
> single page to copy a single page. For now we won't care as pinned pages
> are a corner case, and we should rather look into maintaining only a
> single PageAnonExclusive bit for large folios.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>   include/linux/pgtable.h |  31 +++++++++++
>   mm/memory.c             | 112 +++++++++++++++++++++++++++++++++-------
>   2 files changed, 124 insertions(+), 19 deletions(-)
> 
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 351cd9dc7194f..891ed246978a4 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -650,6 +650,37 @@ static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long addres
>   }
>   #endif
>   
> +#ifndef wrprotect_ptes
> +/**
> + * wrprotect_ptes - Write-protect consecutive pages that are mapped to a
> + *		    contiguous range of addresses.
> + * @mm: Address space to map the pages into.
> + * @addr: Address the first page is mapped at.
> + * @ptep: Page table pointer for the first entry.
> + * @nr: Number of pages to write-protect.
> + *
> + * May be overridden by the architecture; otherwise, implemented as a simple
> + * loop over ptep_set_wrprotect().
> + *
> + * Note that PTE bits in the PTE range besides the PFN can differ. For example,
> + * some PTEs might already be write-protected.
> + *
> + * Context: The caller holds the page table lock.  The pages all belong
> + * to the same folio.  The PTEs are all in the same PMD.
> + */

After writing documentation for another two such functions, I'll change this to:

/**
  * wrprotect_ptes - Write-protect PTEs that map consecutive pages of the same
  *		    folio.
  * @mm: Address space the pages are mapped into.
  * @addr: Address the first page is mapped at.
  * @ptep: Page table pointer for the first entry.
  * @nr: Number of entries to write-protect.
  *
  * May be overridden by the architecture; otherwise, implemented as a simple
  * loop over ptep_set_wrprotect().
  *
  * Note that PTE bits in the PTE range besides the PFN can differ. For example,
  * some PTEs might be write-protected.
  *
  * Context: The caller holds the page table lock.  The PTEs map consecutive
  * pages that belong to the same folio.  The PTEs are all in the same PMD.
  */

-- 
Cheers,

David / dhildenb