On 11/4/19 1:04 PM, Mina Almasry wrote:
> On Fri, Nov 1, 2019 at 4:23 PM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>>
>> On 10/29/19 6:36 PM, Mina Almasry wrote:
>>> static long add_reservation_in_range(struct resv_map *resv, long f, long t,
>>> - bool count_only)
>>> + long *regions_needed, bool count_only)
>>> {
>>> - long chg = 0;
>>> + long add = 0;
>>> struct list_head *head = &resv->regions;
>>> + long last_accounted_offset = f;
>>> struct file_region *rg = NULL, *trg = NULL, *nrg = NULL;
>>>
>>> - /* Locate the region we are before or in. */
>>> - list_for_each_entry (rg, head, link)
>>> - if (f <= rg->to)
>>> - break;
>>> + if (regions_needed)
>>> + *regions_needed = 0;
>>>
>>> - /* Round our left edge to the current segment if it encloses us. */
>>> - if (f > rg->from)
>>> - f = rg->from;
>>> -
>>> - chg = t - f;
>>> + /* In this loop, we essentially handle an entry for the range
>>> + * [last_accounted_offset, rg->from), at every iteration, with some
>>> + * bounds checking.
>>> + */
>>> + list_for_each_entry_safe(rg, trg, head, link) {
>>> + /* Skip irrelevant regions that start before our range. */
>>> + if (rg->from < f) {
>>> + /* If this region ends after the last accounted offset,
>>> + * then we need to update last_accounted_offset.
>>> + */
>>> + if (rg->to > last_accounted_offset)
>>> + last_accounted_offset = rg->to;
>>> + continue;
>>> + }
>>>
>>> - /* Check for and consume any regions we now overlap with. */
>>> - nrg = rg;
>>> - list_for_each_entry_safe (rg, trg, rg->link.prev, link) {
>>> - if (&rg->link == head)
>>> - break;
>>> + /* When we find a region that starts beyond our range, we've
>>> + * finished.
>>> + */
>>> if (rg->from > t)
>>> break;
>>>
>>> - /* We overlap with this area, if it extends further than
>>> - * us then we must extend ourselves. Account for its
>>> - * existing reservation.
>>> + /* Add an entry for last_accounted_offset -> rg->from, and
>>> + * update last_accounted_offset.
>>> */
>>> - if (rg->to > t) {
>>> - chg += rg->to - t;
>>> - t = rg->to;
>>> + if (rg->from > last_accounted_offset) {
>>> + add += rg->from - last_accounted_offset;
>>> + if (!count_only) {
>>> + nrg = get_file_region_entry_from_cache(
>>> + resv, last_accounted_offset, rg->from);
>>> + list_add(&nrg->link, rg->link.prev);
>>> + } else if (regions_needed)
>>> + *regions_needed += 1;
>>> }
>>> - chg -= rg->to - rg->from;
>>>
>>> - if (!count_only && rg != nrg) {
>>> - list_del(&rg->link);
>>> - kfree(rg);
>>> - }
>>> + last_accounted_offset = rg->to;
>>
>> That last assignment is unneeded. Correct?
>>
>
> Not to make you nervous, but this assignment is needed.
>
> The basic idea is that there are 2 loop invariants here:
> 1. Everything before last_accounted_offset is filled in with file_regions.
> 2. rg points to the first region past last_account_offset.
>
> Each loop iteration compares rg->from to last_accounted_offset, and if
> there is a gap, it creates a new region to fill this gap. Then this
> assignment restores loop invariant #2 by assigning
> last_accounted_offset to rg->to, since now everything before rg->to is
> filled in with file_regions.
>
My apologies!
>>> }
>>>
>>> - if (!count_only) {
>>> - nrg->from = f;
>>> - nrg->to = t;
>>> + /* Handle the case where our range extends beyond
>>> + * last_accounted_offset.
>>> + */
>>> + if (last_accounted_offset < t) {
>>> + add += t - last_accounted_offset;
>>> + if (!count_only) {
>>> + nrg = get_file_region_entry_from_cache(
>>> + resv, last_accounted_offset, t);
>>> + list_add(&nrg->link, rg->link.prev);
>>> + } else if (regions_needed)
>>> + *regions_needed += 1;
>>> + last_accounted_offset = t;
The question about an unnecessary assignment was supposed to be
directed at the above line.
--
Mike Kravetz
>>> }
>>>
>>> - return chg;
>>> + return add;
>>> }
