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SubjectRe: [PATCH] mm: Optimise nth_page for contiguous memmap
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On 13.04.21 21:46, 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

For sparsemem we could optimize within a single memory section. But not
sure if it's worth the trouble.

Reviewed-by: David Hildenbrand <david@redhat.com>

--
Thanks,

David / dhildenb

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