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[91.12.100.112]) by smtp.gmail.com with ESMTPSA id x2sm15399098wrg.31.2021.04.14.08.24.42 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 14 Apr 2021 08:24:43 -0700 (PDT) Subject: Re: [PATCH] mm: Optimise nth_page for contiguous memmap To: "Matthew Wilcox (Oracle)" , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Tejun Heo , FUJITA Tomonori , Douglas Gilbert , Chris Wilson Cc: Christoph Hellwig References: <20210413194625.1472345-1-willy@infradead.org> From: David Hildenbrand Organization: Red Hat Message-ID: <67922c5b-0a7b-4eab-9fee-455acf555ebf@redhat.com> Date: Wed, 14 Apr 2021 17:24:42 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: <20210413194625.1472345-1-willy@infradead.org> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=david@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Stat-Signature: gzdq3csn439goanws13f6kihji91f69h X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 0C6F82000254 Received-SPF: none (redhat.com>: No applicable sender policy available) receiver=imf28; identity=mailfrom; envelope-from=""; helo=us-smtp-delivery-124.mimecast.com; client-ip=216.205.24.124 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1618413887-44665 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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) > Reviewed-by: Christoph Hellwig > --- > 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 -- Thanks, David / dhildenb