linux-kernel - Re: [PATCH] secretmem: optimize page_is

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Message-ID: <YH1PE4oWeicpJT9g@kernel.org>
Date:   Mon, 19 Apr 2021 12:36:19 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christopher Lameter <cl@...ux.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>,
        Matthew Garrett <mjg59@...f.ucam.org>,
        Mark Rutland <mark.rutland@....com>,
        Michal Hocko <mhocko@...e.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Roman Gushchin <guro@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tycho Andersen <tycho@...ho.ws>, Will Deacon <will@...nel.org>,
        Yury Norov <yury.norov@...il.com>, linux-api@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-riscv@...ts.infradead.org,
        x86@...nel.org, kernel test robot <oliver.sang@...el.com>
Subject: Re: [PATCH] secretmem: optimize page_is_secretmem()

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 <rppt@...ux.ibm.com>
> > 
> > Kernel test robot reported -4.2% regression of will-it-scale.per_thread_ops
> > due to commit "mm: introduce memfd_secret system call to create "secret"
> > 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_range):
> > 
> >   27.76  +2.5  30.23       perf-profile.children.cycles-pp.gup_pgd_range
> >    0.00  +3.2   3.19 ± 2%  perf-profile.children.cycles-pp.page_mapping
> >    0.00  +3.7   3.66 ± 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 <oliver.sang@...el.com>
> > Signed-off-by: Mike Rapoport <rppt@...ux.ibm.com>
> > ---
> > 
> > @Andrew,
> > The patch is vs v5.12-rc7-mmots-2021-04-15-16-28, I'd appreciate if it 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.

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.
 
> > +
> > +	mapping = (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 cents.

Well, most if the -4.2% of the performance regression kbuild reported were
due to repeated compount_head(page) in page_mapping(). So the whole point
of this patch is to avoid calling page_mapping().

> The idea of the patch makes sense to me.

-- 
Sincerely yours,
Mike.