Re: [PATCH] mm: Optimise nth_page for contiguous memmap

["Zi Yan" <ziy@nvidia.com>]

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On 13 Apr 2021, at 15:46, Matthew Wilcox (Oracle) wrote:

> If the memmap is virtually contiguous (either because we're using
> a virtually mapped memmap or because we don't support a discontig
> memmap at all), then we can implement nth_page() by simple addition.
> Contrary to popular belief, the compiler is not able to optimise this
> itself for a vmemmap configuration.  This reduces one example user (sg.c)
> by four instructions:
>
>         struct page *page = nth_page(rsv_schp->pages[k], offset >> PAGE_SHIFT);
>
> before:
>    49 8b 45 70             mov    0x70(%r13),%rax
>    48 63 c9                movslq %ecx,%rcx
>    48 c1 eb 0c             shr    $0xc,%rbx
>    48 8b 04 c8             mov    (%rax,%rcx,8),%rax
>    48 2b 05 00 00 00 00    sub    0x0(%rip),%rax
>            R_X86_64_PC32      vmemmap_base-0x4
>    48 c1 f8 06             sar    $0x6,%rax
>    48 01 d8                add    %rbx,%rax
>    48 c1 e0 06             shl    $0x6,%rax
>    48 03 05 00 00 00 00    add    0x0(%rip),%rax
>            R_X86_64_PC32      vmemmap_base-0x4
>
> after:
>    49 8b 45 70             mov    0x70(%r13),%rax
>    48 63 c9                movslq %ecx,%rcx
>    48 c1 eb 0c             shr    $0xc,%rbx
>    48 c1 e3 06             shl    $0x6,%rbx
>    48 03 1c c8             add    (%rax,%rcx,8),%rbx
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
>  include/linux/mm.h | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 25b9041f9925..2327f99b121f 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -234,7 +234,11 @@ int overcommit_policy_handler(struct ctl_table *, int, void *, size_t *,
>  int __add_to_page_cache_locked(struct page *page, struct address_space *mapping,
>  		pgoff_t index, gfp_t gfp, void **shadowp);
>
> +#if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
>  #define nth_page(page,n) pfn_to_page(page_to_pfn((page)) + (n))
> +#else
> +#define nth_page(page,n) ((page) + (n))
> +#endif
>
>  /* to align the pointer to the (next) page boundary */
>  #define PAGE_ALIGN(addr) ALIGN(addr, PAGE_SIZE)
> -- 
> 2.30.2

LGTM. Thanks.

Reviewed-by: Zi Yan <ziy@nvidia.com>

—
Best Regards,
Yan Zi

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