Re: [PATCH] secretmem: optimize page_is_secretmem()

From: Matthew Wilcox <willy@infradead.org>
To: Mike Rapoport <rppt@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Andy Lutomirski <luto@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
	Borislav Petkov <bp@alien8.de>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Christopher Lameter <cl@linux.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	David Hildenbrand <david@redhat.com>,
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	"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
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	"Kirill A. Shutemov" <kirill@shutemov.name>,
	Matthew Garrett <mjg59@srcf.ucam.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Michal Hocko <mhocko@suse.com>,
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Subject: Re: [PATCH] secretmem: optimize page_is_secretmem()
Date: Mon, 19 Apr 2021 13:21:56 +0100	[thread overview]
Message-ID: <20210419122156.GZ2531743@casper.infradead.org> (raw)
In-Reply-To: <YH1v4XVzfXC1dYND@kernel.org>

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 ...)

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