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

[Mike Kravetz]

On 05/21/2015 04:30 PM, Andrew Morton wrote:
> On Mon, 18 May 2015 10:49:08 -0700 Mike Kravetz <mike.kravetz@xxxxxxxxxx> 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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