Re: [PATCH V2 2/7] xen/grants: support allocating consecutive grants

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 5/16/22 1:59 AM, Juergen Gross wrote:
> On 14.05.22 04:34, Boris Ostrovsky wrote:
>>
>>
>> On 5/13/22 1:33 AM, Juergen Gross wrote:
>>> On 12.05.22 22:01, Boris Ostrovsky wrote:
>>>>
>>>> On 5/7/22 2:19 PM, Oleksandr Tyshchenko wrote:
>>
>>>>> +/* Rebuilds the free grant list and tries to find count consecutive entries. */
>>>>> +static int get_free_seq(unsigned int count)
>>>>> +{
>>>>> +    int ret = -ENOSPC;
>>>>> +    unsigned int from, to;
>>>>> +    grant_ref_t *last;
>>>>> +
>>>>> +    gnttab_free_tail_ptr = &gnttab_free_head;
>>>>> +    last = &gnttab_free_head;
>>>>> +
>>>>> +    for (from = find_first_bit(gnttab_free_bitmap, gnttab_size);
>>>>> +         from < gnttab_size;
>>>>> +         from = find_next_bit(gnttab_free_bitmap, gnttab_size, to + 1)) {
>>>>> +        to = find_next_zero_bit(gnttab_free_bitmap, gnttab_size,
>>>>> +                    from + 1);
>>>>> +        if (ret < 0 && to - from >= count) {
>>>>> +            ret = from;
>>>>> +            bitmap_clear(gnttab_free_bitmap, ret, count);
>>>>> +            from += count;
>>>>> +            gnttab_free_count -= count;
>>>>
>>>>
>>>> IIUIC we can have multiple passes over this, meaning that the gnttab_free_count may be decremented more than once. Is that intentional?
>>>
>>> After the first pass decrementing gnttab_free_cnt, ret will no
>>> longer be less than zero, so this can be hit only once.
>>
>> Oh, yes, of course.
>>
>>>
>>>>
>>>>
>>>>> +            if (from == to)
>>>>> +                continue;
>>>>> +        }
>>>>> +
>>>>> +        while (from < to) {
>>>>> +            *last = from;
>>>>> +            last = __gnttab_entry(from);
>>>>> +            gnttab_last_free = from;
>>>>> +            from++;
>>>>> +        }
>>>>
>>>>
>>>> I have been looking at this loop and I can't understand what it is doing ;-( Can you enlighten me?
>>>
>>> It is recreating the free list in order to have it properly sorted.
>>> This is needed to make sure that the free tail has the maximum
>>> possible size (you can take the tail off the list without having
>>> to worry about breaking the linked list because of references into
>>> the tail).
>>
>>
>> So let's say we have the (one-dimensional) table of length 13
>>
>> idx    ..    2    3  ...  10  11  12
>>
>> grant       12   11        2  -1   3
>>
>>
>> and gnttab_free_head is 10. I.e. the free list is 2, 12, 3, 11.
> 
> You meant 10, 2, 12, 3, 11, I guess?
> 
>>
>> What will this look like after the 2 iterations of the outer loop?
> 
> idx    ..    2    3  ...  10  11  12
> 
> grant        3   10       11  12  -1
> 
> with gnttab_free_head being 2, i.e the free list is now 2, 3, 10, 11, 12.



OK, thanks, that helped. I couldn't link the free chunks in my head


With the error handling in gnttab_init() fixed

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>



-boris