From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
To: Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Tejun Heo <tj@kernel.org>,
FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>,
Douglas Gilbert <dougg@torque.net>,
Chris Wilson <chris@chris-wilson.co.uk>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>,
Christoph Hellwig <hch@lst.de>
Subject: [PATCH] mm: Optimise nth_page for contiguous memmap
Date: Tue, 13 Apr 2021 20:46:25 +0100 [thread overview]
Message-ID: <20210413194625.1472345-1-willy@infradead.org> (raw)
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
next reply other threads:[~2021-04-13 19:46 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-13 19:46 Matthew Wilcox (Oracle) [this message]
2021-04-14 15:24 ` [PATCH] mm: Optimise nth_page for contiguous memmap David Hildenbrand
2021-04-14 18:51 ` Matthew Wilcox
2021-04-14 15:27 ` Zi Yan
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