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) <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
---
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
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) <[email protected]>
> Reviewed-by: Christoph Hellwig <[email protected]>
> ---
> 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 <[email protected]>
--
Thanks,
David / dhildenb
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) <[email protected]>
> Reviewed-by: Christoph Hellwig <[email protected]>
> ---
> 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 <[email protected]>
—
Best Regards,
Yan Zi
On Wed, Apr 14, 2021 at 05:24:42PM +0200, David Hildenbrand wrote:
> On 13.04.21 21:46, Matthew Wilcox (Oracle) wrote:
> > +#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.
Not only is it not worth the trouble, I suspect it's more expensive to
test-and-branch than just unconditionally call pfn_to_page() and
page_to_pfn(). That said, I haven't measured.
SPARSEMEM_VMEMMAP is default Y, and enabled by arm64, ia64, powerpc,
riscv, s390, sparc and x86. I mean ... do we care any more?
> Reviewed-by: David Hildenbrand <[email protected]>
>
> --
> Thanks,
>
> David / dhildenb
>