Re: [PATCH] i386: fix vmalloc_sync_all() for Xen

[Jan Beulich]

>>>> +		if (!test_bit(sync_index(address), insync)) {
>>>>   
>>>>       
>>> It's probably worth reversing this test and removing a layer of indentation.
>>>     
>>
>> How? There's a second if() following this one, so we can't just 'continue;'
>> here.
>>   
>
>That second if() block seems completely redundant:
>
>		if (address == start && test_bit(pgd_index(address), insync))
>			start = address + PGDIR_SIZE;
>
>All it does it update "start", but start isn't used anywhere else in the 
>loop.

Since start is a static variable, it must be updated this way. The intention
here is to shorten the loop in later runs - since kernel page table entries
never go away, this is possible. Possibly just using the insync array would
be sufficient, but when I first coded this I wanted to avoid as much
overhead as was possible.

>>>>  			spin_lock_irqsave(&pgd_lock, flags);
>>>> +			if (unlikely(list_empty(&pgd_list))) {
>>>> +				spin_unlock_irqrestore(&pgd_lock, flags);
>>>> +				return;
>>>> +			}
>>>>   
>>>>       
>>> This seems a bit warty.  If the list is empty, then won't the 
>>> list_for_each_entry() just fall through?  Presumably this only applies 
>>> to boot, since pgd_list won't be empty on a running system with usermode 
>>> processes.  Is there a correctness issue here, or is it just a 
>>> micro-optimisation?
>>>     
>>
>> No, it isn't. Note the setting to NULL of page, which after the loop gets
>> tested for. list_for_each_entry() would never yield a NULL page, even
>> if the list is empty.
>
>Does that matter?  If pgd_list is empty, then it's in sync by 
>definition.  Why does it need special-casing?

Yes, certainly. But it would result in all insync bits set, which would be
wrong - only non-empty page directory entries can be in sync.

>>>>  			list_for_each_entry(page, &pgd_list, lru) {
>>>>  				if (!vmalloc_sync_one(page_address(page),
>>>> -						      address))
>>>> +						      address)) {
>>>> +					BUG_ON(list_first_entry(&pgd_list,
>>>> +								struct page,
>>>> +								lru) != page);
>>>>   
>>>>       
>>> What condition is this testing for?
>>>     
>>
>> This is a replacement of the BUG_ON() that an earlier patch from you
>> removed: Failure of vmalloc_sync_one() must happen on the first
>> entry or never, and this is what is being checked for here.
>>   
>
>Could you add a comment?

Sure, though there was none originally, and the intention seemed
quite clear to me.

Jan

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