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[91.12.105.184]) by smtp.gmail.com with ESMTPSA id x7sm11903755eds.67.2021.04.19.02.38.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 19 Apr 2021 02:38:42 -0700 (PDT) Subject: Re: [PATCH] secretmem: optimize page_is_secretmem() To: Mike Rapoport Cc: Andrew Morton , Alexander Viro , Andy Lutomirski , Arnd Bergmann , Borislav Petkov , Catalin Marinas , Christopher Lameter , Dan Williams , Dave Hansen , Elena Reshetova , "H. Peter Anvin" , Ingo Molnar , James Bottomley , "Kirill A. Shutemov" , Matthew Wilcox , Matthew Garrett , Mark Rutland , Michal Hocko , Mike Rapoport , Michael Kerrisk , Palmer Dabbelt , Paul Walmsley , Peter Zijlstra , "Rafael J. Wysocki" , Rick Edgecombe , Roman Gushchin , Shakeel Butt , Shuah Khan , Thomas Gleixner , Tycho Andersen , Will Deacon , Yury Norov , linux-api@vger.kernel.org, linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-nvdimm@lists.01.org, linux-riscv@lists.infradead.org, x86@kernel.org, kernel test robot References: <20210419084218.7466-1-rppt@kernel.org> <3b30ac54-8a92-5f54-28f0-f110a40700c7@redhat.com> From: David Hildenbrand Organization: Red Hat Message-ID: Date: Mon, 19 Apr 2021 11:38:40 +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: 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 X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: A4040E00011C X-Stat-Signature: pphk9qc3dft4jkh4n15go8rzbgxkok9t Received-SPF: none (redhat.com>: No applicable sender policy available) receiver=imf30; 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: 1618825112-950189 Content-Transfer-Encoding: quoted-printable 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 19.04.21 11:36, Mike Rapoport wrote: > On Mon, Apr 19, 2021 at 11:15:02AM +0200, David Hildenbrand wrote: >> On 19.04.21 10:42, Mike Rapoport wrote: >>> From: Mike Rapoport >>> >>> Kernel test robot reported -4.2% regression of will-it-scale.per_thre= ad_ops >>> due to commit "mm: introduce memfd_secret system call to create "secr= et" >>> memory areas". >>> >>> The perf profile of the test indicated that the regression is caused = by >>> page_is_secretmem() called from gup_pte_range() (inlined by gup_pgd_r= ange): >>> >>> 27.76 +2.5 30.23 perf-profile.children.cycles-pp.gup_pgd_r= ange >>> 0.00 +3.2 3.19 =C2=B1 2% perf-profile.children.cycles-pp.page= _mapping >>> 0.00 +3.7 3.66 =C2=B1 2% perf-profile.children.cycles-pp.page= _is_secretmem >>> >>> Further analysis showed that the slow down happens because neither >>> page_is_secretmem() nor page_mapping() are not inline and moreover, >>> multiple page flags checks in page_mapping() involve calling >>> compound_head() several times for the same page. >>> >>> Make page_is_secretmem() inline and replace page_mapping() with page = flag >>> checks that do not imply page-to-head conversion. >>> >>> Reported-by: kernel test robot >>> Signed-off-by: Mike Rapoport >>> --- >>> >>> @Andrew, >>> The patch is vs v5.12-rc7-mmots-2021-04-15-16-28, I'd appreciate if i= t would >>> be added as a fixup to the memfd_secret series. >>> >>> include/linux/secretmem.h | 26 +++++++++++++++++++++++++- >>> mm/secretmem.c | 12 +----------- >>> 2 files changed, 26 insertions(+), 12 deletions(-) >>> >>> diff --git a/include/linux/secretmem.h b/include/linux/secretmem.h >>> index 907a6734059c..b842b38cbeb1 100644 >>> --- a/include/linux/secretmem.h >>> +++ b/include/linux/secretmem.h >>> @@ -4,8 +4,32 @@ >>> #ifdef CONFIG_SECRETMEM >>> +extern const struct address_space_operations secretmem_aops; >>> + >>> +static inline bool page_is_secretmem(struct page *page) >>> +{ >>> + struct address_space *mapping; >>> + >>> + /* >>> + * Using page_mapping() is quite slow because of the actual call >>> + * instruction and repeated compound_head(page) inside the >>> + * page_mapping() function. >>> + * We know that secretmem pages are not compound and LRU so we can >>> + * save a couple of cycles here. >>> + */ >>> + if (PageCompound(page) || !PageLRU(page)) >>> + return false; >> >> I'd assume secretmem pages are rare in basically every setup out there= . So >> maybe throwing in a couple of likely()/unlikely() might make sense. >=20 > I'd say we could do unlikely(page_is_secretmem()) at call sites. Here I= can > hardly estimate which pages are going to be checked. > =20 >>> + >>> + mapping =3D (struct address_space *) >>> + ((unsigned long)page->mapping & ~PAGE_MAPPING_FLAGS); >>> + >> >> Not sure if open-coding page_mapping is really a good idea here -- or = even >> necessary after the fast path above is in place. Anyhow, just my 2 cen= ts. >=20 > Well, most if the -4.2% of the performance regression kbuild reported w= ere > due to repeated compount_head(page) in page_mapping(). So the whole poi= nt > of this patch is to avoid calling page_mapping(). I would have thought the fast path "(PageCompound(page) ||=20 !PageLRU(page))" would already avoid calling page_mapping() in many cases= . --=20 Thanks, David / dhildenb