Re: [PATCH 1/2] mm/hugetlb: compute/return the number of regions added by region_add()

[Andrew Morton <akpm@linux-foundation.org>]

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?

- 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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