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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 07.05.19 at 15:15, <george.dunlap@xxxxxxxxxx> wrote:
> On 5/7/19 1:11 PM, Jan Beulich wrote:
>>>>> On 07.05.19 at 13:34, <elnikety@xxxxxxxxxx> 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.
> 
> Just for clarification Jan -- did you mean, "I'm happy for this to go in
> as it is, but if you feel like it, here are two improvements"?

Yes (maybe "I'd prefer" to replace "if you feel like it").

Jan



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