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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
Message-Id: <20210413194625.1472345-1-willy@infradead.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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