Re: [PATCH] secretmem: optimize page_is_secretmem()

[Mike Rapoport]

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@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.

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.