From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
Subject: mm: optimise nth_page for contiguous memmap
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
Link: https://lkml.kernel.org/r/20210413194625.1472345-1-willy@xxxxxxxxxxxxx
Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: FUJITA Tomonori <fujita.tomonori@xxxxxxxxxxxxx>
Cc: Douglas Gilbert <dougg@xxxxxxxxxx>
Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/mm.h | 4 ++++
1 file changed, 4 insertions(+)
--- a/include/linux/mm.h~mm-optimise-nth_page-for-contiguous-memmap
+++ a/include/linux/mm.h
@@ -234,7 +234,11 @@ int overcommit_policy_handler(struct ctl
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)
_
