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SubjectRe: [PATCH 1/2] mm/hugetlb: compute/return the number of regions added by region_add()
On 05/21/2015 04:30 PM, Andrew Morton wrote:
> On Mon, 18 May 2015 10:49:08 -0700 Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
>> Modify region_add() to keep track of regions(pages) added to the
>> reserve map and return this value. The return value can be
>> compared to the return value of region_chg() to determine if the
>> map was modified between calls. Make vma_commit_reservation()
>> also pass along the return value of region_add(). The special
>> case return values of vma_needs_reservation() should also be
>> taken into account when determining the return value of
>> vma_commit_reservation().
>
> Could we please get this code slightly documented while it's hot in
> your mind?

Will do. I'll provide an updated patch to address your questions and
better document this whole region/reserve map stuff. It took me a few
days to wrap my head around how it actually works.

--
Mike Kravetz

> - One has to do an extraordinary amount of reading to discover that
> the units of file_region.from and .to are "multiples of
> 1<<huge_page_order(h)" (where "h" is imponderable).
>
> Let's get this written down?
>
> - Is file_region.to inclusive or exclusive?
>
> - What are they called "from" and "to" anyway? We usually use
> "start" and "end" for such things.
>
>
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -156,6 +156,7 @@ static long region_add(struct resv_map *resv, long f, long t)
>> {
>> struct list_head *head = &resv->regions;
>> struct file_region *rg, *nrg, *trg;
>> + long chg = 0;
>>
>> spin_lock(&resv->lock);
>> /* Locate the region we are either in or before. */
>> @@ -181,14 +182,17 @@ static long region_add(struct resv_map *resv, long f, long t)
>> if (rg->to > t)
>> t = rg->to;
>> if (rg != nrg) {
>> + chg -= (rg->to - rg->from);
>> list_del(&rg->link);
>> kfree(rg);
>> }
>> }
>> + chg += (nrg->from - f);
>> nrg->from = f;
>> + chg += t - nrg->to;
>> nrg->to = t;
>> spin_unlock(&resv->lock);
>> - return 0;
>> + return chg;
>> }
>
> Let's document the return value. It appears that this function is
> designed to return a negative number (units?) on a successful addition.
> Why, and what does that number represent.
>
>
>> static long region_chg(struct resv_map *resv, long f, long t)
>> @@ -1349,18 +1353,25 @@ static long vma_needs_reservation(struct hstate *h,
>> else
>> return chg < 0 ? chg : 0;
>> }
>> -static void vma_commit_reservation(struct hstate *h,
>> +
>> +static long vma_commit_reservation(struct hstate *h,
>> struct vm_area_struct *vma, unsigned long addr)
>> {
>> struct resv_map *resv;
>> pgoff_t idx;
>> + long add;
>>
>> resv = vma_resv_map(vma);
>> if (!resv)
>> - return;
>> + return 1;
>>
>> idx = vma_hugecache_offset(h, vma, addr);
>> - region_add(resv, idx, idx + 1);
>> + add = region_add(resv, idx, idx + 1);
>> +
>> + if (vma->vm_flags & VM_MAYSHARE)
>> + return add;
>> + else
>> + return 0;
>> }
>
> Let's document the return value here as well please.
>


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