Re: [PATCH v2] mm: option to _always_ scrub freed domheap pages

From: "Jan Beulich" <JBeulich@suse.com>
To: Eslam Elnikety <elnikety@amazon.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	Wei Liu <wei.liu2@citrix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Ian Jackson <Ian.Jackson@eu.citrix.com>, Tim Deegan <tim@xen.org>,
	Julien Grall <julien.grall@arm.com>,
	xen-devel <xen-devel@lists.xenproject.org>,
	Amit Shah <aams@amazon.com>
Subject: Re: [PATCH v2] mm: option to _always_ scrub freed domheap pages
Date: Tue, 07 May 2019 06:11:33 -0600	[thread overview]
Message-ID: <5CD175F5020000780022C7EF@prv1-mh.provo.novell.com> (raw)
In-Reply-To: <20190507113405.71851-1-elnikety@amazon.com>

>>> On 07.05.19 at 13:34, <elnikety@amazon.com> wrote:
> --- a/xen/common/page_alloc.c
> +++ b/xen/common/page_alloc.c
> @@ -214,6 +214,10 @@ custom_param("bootscrub", parse_bootscrub_param);
>  static unsigned long __initdata opt_bootscrub_chunk = MB(128);
>  size_param("bootscrub_chunk", opt_bootscrub_chunk);
>  
> + /* scrub-domheap -> Domheap pages are scrubbed when freed */
> +static bool __read_mostly opt_scrub_domheap;
> +boolean_param("scrub-domheap", opt_scrub_domheap);

Upon 2nd thought this, btw, would seem to be an excellent candidate
for becoming a runtime parameter.

> @@ -2378,9 +2382,10 @@ void free_domheap_pages(struct page_info *pg, unsigned int order)
>              /*
>               * Normally we expect a domain to clear pages before freeing them,
>               * if it cares about the secrecy of their contents. However, after
> -             * a domain has died we assume responsibility for erasure.
> +             * a domain has died we assume responsibility for erasure. We do
> +             * scrub regardless if option scrub_domheap is set.
>               */
> -            scrub = d->is_dying || scrub_debug;
> +            scrub = d->is_dying || scrub_debug || opt_scrub_domheap;

Did you consider setting opt_scrub_domheap when scrub_debug is
set? This would shorten the (runtime) calculation here by a tiny bit,
at the price of doing one more thing once while booting.

Jan

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