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

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Date:   Mon, 19 Apr 2021 12:43:17 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     David Hildenbrand <david@...hat.com>,
        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 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 12:36:19PM +0300, Mike Rapoport wrote:
> 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().

It's quite ludicrous how many times we call compound_head() in
page_mapping() today:

 page = compound_head(page);
 if (__builtin_expect(!!(PageSlab(page)), 0))
 if (__builtin_expect(!!(PageSwapCache(page)), 0)) {

TESTPAGEFLAG(Slab, slab, PF_NO_TAIL) expands to:

static __always_inline int PageSlab(struct page *page)
{
	PF_POISONED_CHECK(compound_head(page));
	return test_bit(PG_slab, &compound_head(page));
}

static __always_inline int PageSwapCache(struct page *page)
{
        page = compound_head(page);
        return PageSwapBacked(page) && test_bit(PG_swapcache, &page->flags);
}

but then!

TESTPAGEFLAG(SwapBacked, swapbacked, PF_NO_TAIL) also expands like Slab does.

So that's six calls to compound_head(), depending what Kconfig options
you have enabled.

And folio_mapping() is one of the functions I add in the first batch of
patches, so review, etc will be helpful.