Re: [PATCH] secretmem: optimize page_is_secretmem()

[David Hildenbrand]

On 19.04.21 10:42, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@xxxxxxxxxxxxx>
>
> 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@xxxxxxxxx>
> Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> ---
>
> @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