+ mm-optimise-nth_page-for-contiguous-memmap.patch added to -mm tree

[akpm@linux-foundation.org]

The patch titled
     Subject: mm: optimise nth_page for contiguous memmap
has been added to the -mm tree.  Its filename is
     mm-optimise-nth_page-for-contiguous-memmap.patch

This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/mm-optimise-nth_page-for-contiguous-memmap.patch
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/mm-optimise-nth_page-for-contiguous-memmap.patch

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------------------------------------------------------
From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
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@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Douglas Gilbert <dougg@torque.net>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 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)
_

Patches currently in -mm which might be from willy@infradead.org are

mm-make-__dump_page-static.patch
mm-debug-factor-pagepoisoned-out-of-__dump_page.patch
mm-page_owner-constify-dump_page_owner.patch
mm-make-compound_head-const-preserving.patch
mm-constify-get_pfnblock_flags_mask-and-get_pfnblock_migratetype.patch
mm-constify-page_count-and-page_ref_count.patch
mm-optimise-nth_page-for-contiguous-memmap.patch