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

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Date:   Mon, 19 Apr 2021 13:21:56 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Mike Rapoport <rppt@...nel.org>
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>,
        David Hildenbrand <david@...hat.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 02:56:17PM +0300, Mike Rapoport wrote:
> On Mon, Apr 19, 2021 at 12:23:02PM +0100, Matthew Wilcox wrote:
> > So you're calling page_is_secretmem() on a struct page without having
> > a refcount on it.  That is definitely not allowed.  secretmem seems to
> > be full of these kinds of races; I know this isn't the first one I've
> > seen in it.  I don't think this patchset is ready for this merge window.
> 
> There were races in the older version that did caching of large pages and
> those were fixed then, but this is anyway irrelevant because all that code
> was dropped in the latest respins.
> 
> I don't think that the fix of the race in gup_pte_range is that significant
> to wait 3 more months because of it.

I have no particular interest in secretmem, but it seems that every time
I come across it while looking at something else, I see these kinds of
major mistakes in it.  That says to me it's not ready and hasn't seen
enough review.

> > With that fixed, you'll have a head page that you can use for testing,
> > which means you don't need to test PageCompound() (because you know the
> > page isn't PageTail), you can just test PageHead().
> 
> I can't say I follow you here. page_is_secretmem() is intended to be a
> generic test, so I don't see how it will get PageHead() if it is called
> from other places.

static inline bool head_is_secretmem(struct page *head)
{
	if (PageHead(head))
		return false;
	...
}

static inline bool page_is_secretmem(struct page *page)
{
	if (PageTail(page))
		return false;
	return head_is_secretmem(page);
}

(yes, calling it head is a misnomer, because it's not necessarily a head,
it might be a base page, but until we have a name for pages which might
be a head page or a base page, it'll have to do ...)