[PATCH V2 2/7] xen/grants: support allocating consecutive grants
Boris Ostrovsky
boris.ostrovsky at oracle.com
Fri May 13 19:34:19 PDT 2022
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.
What will this look like after the 2 iterations of the outer loop?
(I am really having a mental block on this).
-boris
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