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

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Message-ID: <3b30ac54-8a92-5f54-28f0-f110a40700c7@redhat.com>
Date:   Mon, 19 Apr 2021 11:15:02 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Mike Rapoport <rppt@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     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 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.

> +
> +	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.

The idea of the patch makes sense to me.

-- 
Thanks,

David / dhildenb