Re: [PATCH 5/9] xen: introduce assign_pages_nr

[Jan Beulich <jbeulich@suse.com>]

On 30.06.2021 20:29, Julien Grall wrote:
> On 10/06/2021 10:49, Jan Beulich wrote:
>> On 07.06.2021 04:43, Penny Zheng wrote:
>>> Introduce new interface assign_pages_nr to deal with when page number is
>>> not in a power-of-two, which will save the trouble each time user needs
>>> to split the size in a power of 2 to use assign_pages.
>>
>> First of all I still don't see why in this one special case it is a
>> meaningful burden to do the count-to-order conversion in the caller you
>> mean to add,
> 
> This sort of works for one caller. However, I would expect some more 
> user in the future (we use it for Live-Update).
> 
>> and hence why we really need this new function (to keep it
>> simple, you could even have the caller not break down to arbitrary
>> power-of-2 chunks, but simply iterate over all individual [order-0]
>> pages).
> 
> The function assign_pages() will always use 1U << order (and sadly 1 << 
> order). So we would end up to convert the count in multiple order for 
> then directly converting back to a number. To me, this sounds rather 
> pointless...
> 
> There are also a slight benefits to call assign_pages() a single time 
> during boot because it will reduce the number of time we need to 
> lock/unlock d->page_alloc_lock.

Well, all of this is why I did add ...

>> The more that I'm not happy with the chosen name, despite it
>> having been suggested during v1 review. _If_ we needed two functions,
>> imo they ought to be named assign_page() (dealing with a single page of
>> the given order) and assign_pages(). Backporting confusion could be
>> helped by altering the order of parameters, such that the compiler
>> would point out that adjustments at call sites are needed.

... this. Not sure whether you not commenting on it means you agree
with the proposal.

>>> --- a/xen/common/page_alloc.c
>>> +++ b/xen/common/page_alloc.c
>>> @@ -2301,14 +2301,14 @@ void init_domheap_pages(paddr_t ps, paddr_t pe)
>>>   }
>>>   
>>>   
>>> -int assign_pages(
>>> +int assign_pages_nr(
>>>       struct domain *d,
>>>       struct page_info *pg,
>>> -    unsigned int order,
>>> +    unsigned int nr_pfns,
>>
>> Even leaving the naming aspect of "pfns" aside, I can't see why this
>> can't be simply "nr" (of appropriate type, see next remark).
>>
>>>       unsigned int memflags)
>>>   {
>>>       int rc = 0;
>>> -    unsigned long i;
>>> +    unsigned int i;
>>
>> This is not an acceptable type change, at least not as long as it's not
>> justified at all in the description. While both Arm and x86 will be
>> fine this way, the code here is supposed to be generic, and hence would
>> better remain generally correct.
> 
> I would like to point out the code is already not correct as we are 
> using 1U << order or worse 1 << order :).

Indeed there are improvements (towards being consistent) to be made. But
this is not an excuse to make things worse here. At least one of the two
loops already properly uses 1ul; sadly that's only debugging code. And
of course something like domain_tot_pages() (and the underlying field)
dealing with "unsigned int" doesn't help consistency either. As it stands
we're limiting ourselves to 8Tb VMs, as it seems, and for no good reason.

Jan