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

David Hildenbrand
2024-01-26T19:11:25Z

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