Re: [Xen-devel] [PATCH v2] mm: Don't scrub pages while holding heap lock in alloc_heap_pages()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 05.09.17 at 16:54, <boris.ostrovsky@xxxxxxxxxx> wrote:
> On 09/05/2017 10:42 AM, Boris Ostrovsky wrote:
>>>> @@ -974,13 +972,39 @@ static struct page_info *alloc_heap_pages(
>>>>           * guest can control its own visibility of/through the cache.
>>>>           */
>>>>          flush_page_to_ram(page_to_mfn(&pg[i]), !(memflags & 
> MEMF_no_icache_flush));
>>>> -
>>>> -        if ( !(memflags & MEMF_no_scrub) )
>>>> -            check_one_page(&pg[i]);
>>>>      }
>>>>  
>>>>      spin_unlock(&heap_lock);
>>>>  
>>>> +    if ( first_dirty != INVALID_DIRTY_IDX ||
>>>> +         (scrub_debug && !(memflags & MEMF_no_scrub)) )
>>> Why does scrub_debug matter here?.
>>>
>>>> +    {
>>>> +        for ( i = 0; i < (1U << order); i++ )
>>>> +        {
>>>> +            if ( test_bit(_PGC_need_scrub, &pg[i].count_info) )
>>>> +            {
>>>> +                if ( !(memflags & MEMF_no_scrub) )
>>>> +                    scrub_one_page(&pg[i]);
>>>> +
>>>> +                dirty_cnt++;
>>>> +
>>>> +                spin_lock(&heap_lock);
>>>> +                pg[i].count_info &= ~PGC_need_scrub;
>>>> +                spin_unlock(&heap_lock);
>>>> +            }
>>>> +
>>>> +            if ( !(memflags & MEMF_no_scrub) )
>>>> +                check_one_page(&pg[i]);
>>> Wouldn't this better be "else if", as checking a page just scrubbed
>>> doesn't look very useful?
>> For both of these questions --- we don't want to miss a poisoned page.
>> For example, if a page was poisoned but for some reason is not marked
>> PGC_need_scrub.
>>
>> Of course, we risk a false positive if a guest wrote the page with the
>> same pattern.
> 
> Just in case I wasn't clear --- I will remove scrub_debug test and add
> 'else' for this reason. Even though it's a debug-only feature I think we
> shouldn't do this.

Perhaps I'm dense, but it sounds to me like you agree to do a
change you think you shouldn't do.

Jan


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