[v5,4/7] hugetlb: disable region_add file_region coalescing
diff mbox series

Message ID 20190919222421.27408-5-almasrymina@google.com
State New
Headers show
Series
  • hugetlb_cgroup: Add hugetlb_cgroup reservation limits
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Commit Message

Mina Almasry Sept. 19, 2019, 10:24 p.m. UTC
A follow up patch in this series adds hugetlb cgroup uncharge info the
file_region entries in resv->regions. The cgroup uncharge info may
differ for different regions, so they can no longer be coalesced at
region_add time. So, disable region coalescing in region_add in this
patch.

Behavior change:

Say a resv_map exists like this [0->1], [2->3], and [5->6].

Then a region_chg/add call comes in region_chg/add(f=0, t=5).

Old code would generate resv->regions: [0->5], [5->6].
New code would generate resv->regions: [0->1], [1->2], [2->3], [3->5],
[5->6].

Special care needs to be taken to handle the resv->adds_in_progress
variable correctly. In the past, only 1 region would be added for every
region_chg and region_add call. But now, each call may add multiple
regions, so we can no longer increment adds_in_progress by 1 in region_chg,
or decrement adds_in_progress by 1 after region_add or region_abort. Instead,
region_chg calls add_reservation_in_range() to count the number of regions
needed and allocates those, and that info is passed to region_add and
region_abort to decrement adds_in_progress correctly.

Signed-off-by: Mina Almasry <almasrymina@google.com>

---
 mm/hugetlb.c | 273 +++++++++++++++++++++++++++++----------------------
 1 file changed, 158 insertions(+), 115 deletions(-)

--
2.23.0.351.gc4317032e6-goog

Comments

Mike Kravetz Sept. 27, 2019, 9:44 p.m. UTC | #1
On 9/19/19 3:24 PM, Mina Almasry wrote:
> A follow up patch in this series adds hugetlb cgroup uncharge info the
> file_region entries in resv->regions. The cgroup uncharge info may
> differ for different regions, so they can no longer be coalesced at
> region_add time. So, disable region coalescing in region_add in this
> patch.
> 
> Behavior change:
> 
> Say a resv_map exists like this [0->1], [2->3], and [5->6].
> 
> Then a region_chg/add call comes in region_chg/add(f=0, t=5).
> 
> Old code would generate resv->regions: [0->5], [5->6].
> New code would generate resv->regions: [0->1], [1->2], [2->3], [3->5],
> [5->6].
> 
> Special care needs to be taken to handle the resv->adds_in_progress
> variable correctly. In the past, only 1 region would be added for every
> region_chg and region_add call. But now, each call may add multiple
> regions, so we can no longer increment adds_in_progress by 1 in region_chg,
> or decrement adds_in_progress by 1 after region_add or region_abort. Instead,
> region_chg calls add_reservation_in_range() to count the number of regions
> needed and allocates those, and that info is passed to region_add and
> region_abort to decrement adds_in_progress correctly.
> 
> Signed-off-by: Mina Almasry <almasrymina@google.com>
> 
> ---
>  mm/hugetlb.c | 273 +++++++++++++++++++++++++++++----------------------
>  1 file changed, 158 insertions(+), 115 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index bac1cbdd027c..d03b048084a3 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -244,6 +244,12 @@ struct file_region {
>  	long to;
>  };
> 
> +/* Helper that removes a struct file_region from the resv_map cache and returns
> + * it for use.
> + */
> +static struct file_region *
> +get_file_region_entry_from_cache(struct resv_map *resv, long from, long to);
> +

Instead of the forward declaration, just put the function here.

>  /* Must be called with resv->lock held. Calling this with count_only == true
>   * will count the number of pages to be added but will not modify the linked
>   * list.
> @@ -251,51 +257,61 @@ struct file_region {
>  static long add_reservation_in_range(struct resv_map *resv, long f, long t,
>  				     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;
> -
> -	/* 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);
> +			}
>  		}
> -		chg -= rg->to - rg->from;
> 
> -		if (!count_only && rg != nrg) {
> -			list_del(&rg->link);
> -			kfree(rg);
> -		}
> +		last_accounted_offset = rg->to;
>  	}
> 
> -	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);
> +		}
> +		last_accounted_offset = t;
>  	}
> 
> -	return chg;
> +	return add;
>  }
> 
>  /*
> @@ -305,46 +321,24 @@ static long add_reservation_in_range(struct resv_map *resv, long f, long t,

The start of this comment block says,

/*
 * Add the huge page range represented by [f, t) to the reserve
 * map.  Existing regions will be expanded to accommodate the specified
 * range, or a region will be taken from the cache.

We are no longer expanding existing regions.  Correct?
As an optimization, I guess we could coalesce/combine reion entries as
long as they are for the same cgroup.  However, it may not be worth the
effort.

>   * must exist in the cache due to the previous call to region_chg with
>   * the same range.
>   *
> + * regions_needed is the out value provided by a previous
> + * call to region_chg.
> + *
>   * Return the number of new huge pages added to the map.  This
>   * number is greater than or equal to zero.
>   */
> -static long region_add(struct resv_map *resv, long f, long t)
> +static long region_add(struct resv_map *resv, long f, long t,
> +		       long regions_needed)
>  {
> -	struct list_head *head = &resv->regions;
> -	struct file_region *rg, *nrg;
>  	long add = 0;
> 
>  	spin_lock(&resv->lock);
> -	/* Locate the region we are either in or before. */
> -	list_for_each_entry(rg, head, link)
> -		if (f <= rg->to)
> -			break;
> 
> -	/*
> -	 * If no region exists which can be expanded to include the
> -	 * specified range, pull a region descriptor from the cache
> -	 * and use it for this range.
> -	 */
> -	if (&rg->link == head || t < rg->from) {
> -		VM_BUG_ON(resv->region_cache_count <= 0);
> -
> -		resv->region_cache_count--;
> -		nrg = list_first_entry(&resv->region_cache, struct file_region,
> -					link);
> -		list_del(&nrg->link);
> -
> -		nrg->from = f;
> -		nrg->to = t;
> -		list_add(&nrg->link, rg->link.prev);
> -
> -		add += t - f;
> -		goto out_locked;
> -	}
> +	VM_BUG_ON(resv->region_cache_count < regions_needed);
> 
>  	add = add_reservation_in_range(resv, f, t, false);
> +	resv->adds_in_progress -= regions_needed;

Consider this example,

- region_chg(1,2)
	adds_in_progress = 1
	cache entries 1
- region_chg(3,4)
	adds_in_progress = 2
	cache entries 2
- region_chg(5,6)
	adds_in_progress = 3
	cache entries 3

At this point, no region descriptors are in the map because only
region_chg has been called.

- region_chg(0,6)
	adds_in_progress = 4
	cache entries 4

Is that correct so far?

Then the following sequence happens,

- region_add(1,2)
	adds_in_progress = 3
	cache entries 3
- region_add(3,4)
	adds_in_progress = 2
	cache entries 2
- region_add(5,6)
	adds_in_progress = 1
	cache entries 1

list of region descriptors is:
[1->2] [3->4] [5->6]

- region_add(0,6)
This is going to require 3 cache entries but only one is in the cache.
I think we are going to BUG in get_file_region_entry_from_cache() the
second time it is called from add_reservation_in_range().

I stopped looking at the code here as things will need to change if this
is a real issue.
Mina Almasry Sept. 27, 2019, 10:33 p.m. UTC | #2
On Fri, Sep 27, 2019 at 2:44 PM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 9/19/19 3:24 PM, Mina Almasry wrote:
> > A follow up patch in this series adds hugetlb cgroup uncharge info the
> > file_region entries in resv->regions. The cgroup uncharge info may
> > differ for different regions, so they can no longer be coalesced at
> > region_add time. So, disable region coalescing in region_add in this
> > patch.
> >
> > Behavior change:
> >
> > Say a resv_map exists like this [0->1], [2->3], and [5->6].
> >
> > Then a region_chg/add call comes in region_chg/add(f=0, t=5).
> >
> > Old code would generate resv->regions: [0->5], [5->6].
> > New code would generate resv->regions: [0->1], [1->2], [2->3], [3->5],
> > [5->6].
> >
> > Special care needs to be taken to handle the resv->adds_in_progress
> > variable correctly. In the past, only 1 region would be added for every
> > region_chg and region_add call. But now, each call may add multiple
> > regions, so we can no longer increment adds_in_progress by 1 in region_chg,
> > or decrement adds_in_progress by 1 after region_add or region_abort. Instead,
> > region_chg calls add_reservation_in_range() to count the number of regions
> > needed and allocates those, and that info is passed to region_add and
> > region_abort to decrement adds_in_progress correctly.
> >
> > Signed-off-by: Mina Almasry <almasrymina@google.com>
> >
> > ---
> >  mm/hugetlb.c | 273 +++++++++++++++++++++++++++++----------------------
> >  1 file changed, 158 insertions(+), 115 deletions(-)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index bac1cbdd027c..d03b048084a3 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -244,6 +244,12 @@ struct file_region {
> >       long to;
> >  };
> >
> > +/* Helper that removes a struct file_region from the resv_map cache and returns
> > + * it for use.
> > + */
> > +static struct file_region *
> > +get_file_region_entry_from_cache(struct resv_map *resv, long from, long to);
> > +
>
> Instead of the forward declaration, just put the function here.
>
> >  /* Must be called with resv->lock held. Calling this with count_only == true
> >   * will count the number of pages to be added but will not modify the linked
> >   * list.
> > @@ -251,51 +257,61 @@ struct file_region {
> >  static long add_reservation_in_range(struct resv_map *resv, long f, long t,
> >                                    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;
> > -
> > -     /* 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);
> > +                     }
> >               }
> > -             chg -= rg->to - rg->from;
> >
> > -             if (!count_only && rg != nrg) {
> > -                     list_del(&rg->link);
> > -                     kfree(rg);
> > -             }
> > +             last_accounted_offset = rg->to;
> >       }
> >
> > -     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);
> > +             }
> > +             last_accounted_offset = t;
> >       }
> >
> > -     return chg;
> > +     return add;
> >  }
> >
> >  /*
> > @@ -305,46 +321,24 @@ static long add_reservation_in_range(struct resv_map *resv, long f, long t,
>
> The start of this comment block says,
>
> /*
>  * Add the huge page range represented by [f, t) to the reserve
>  * map.  Existing regions will be expanded to accommodate the specified
>  * range, or a region will be taken from the cache.
>
> We are no longer expanding existing regions.  Correct?
> As an optimization, I guess we could coalesce/combine reion entries as
> long as they are for the same cgroup.  However, it may not be worth the
> effort.
>
> >   * must exist in the cache due to the previous call to region_chg with
> >   * the same range.
> >   *
> > + * regions_needed is the out value provided by a previous
> > + * call to region_chg.
> > + *
> >   * Return the number of new huge pages added to the map.  This
> >   * number is greater than or equal to zero.
> >   */
> > -static long region_add(struct resv_map *resv, long f, long t)
> > +static long region_add(struct resv_map *resv, long f, long t,
> > +                    long regions_needed)
> >  {
> > -     struct list_head *head = &resv->regions;
> > -     struct file_region *rg, *nrg;
> >       long add = 0;
> >
> >       spin_lock(&resv->lock);
> > -     /* Locate the region we are either in or before. */
> > -     list_for_each_entry(rg, head, link)
> > -             if (f <= rg->to)
> > -                     break;
> >
> > -     /*
> > -      * If no region exists which can be expanded to include the
> > -      * specified range, pull a region descriptor from the cache
> > -      * and use it for this range.
> > -      */
> > -     if (&rg->link == head || t < rg->from) {
> > -             VM_BUG_ON(resv->region_cache_count <= 0);
> > -
> > -             resv->region_cache_count--;
> > -             nrg = list_first_entry(&resv->region_cache, struct file_region,
> > -                                     link);
> > -             list_del(&nrg->link);
> > -
> > -             nrg->from = f;
> > -             nrg->to = t;
> > -             list_add(&nrg->link, rg->link.prev);
> > -
> > -             add += t - f;
> > -             goto out_locked;
> > -     }
> > +     VM_BUG_ON(resv->region_cache_count < regions_needed);
> >
> >       add = add_reservation_in_range(resv, f, t, false);
> > +     resv->adds_in_progress -= regions_needed;
>
> Consider this example,
>
> - region_chg(1,2)
>         adds_in_progress = 1
>         cache entries 1
> - region_chg(3,4)
>         adds_in_progress = 2
>         cache entries 2
> - region_chg(5,6)
>         adds_in_progress = 3
>         cache entries 3
>
> At this point, no region descriptors are in the map because only
> region_chg has been called.
>
> - region_chg(0,6)
>         adds_in_progress = 4
>         cache entries 4
>
> Is that correct so far?
>
> Then the following sequence happens,
>
> - region_add(1,2)
>         adds_in_progress = 3
>         cache entries 3
> - region_add(3,4)
>         adds_in_progress = 2
>         cache entries 2
> - region_add(5,6)
>         adds_in_progress = 1
>         cache entries 1
>
> list of region descriptors is:
> [1->2] [3->4] [5->6]
>
> - region_add(0,6)
> This is going to require 3 cache entries but only one is in the cache.
> I think we are going to BUG in get_file_region_entry_from_cache() the
> second time it is called from add_reservation_in_range().
>
> I stopped looking at the code here as things will need to change if this
> is a real issue.

You're right. I had been assuming that some higher level sync causes
to ban a sequence of region_chg calls without a region_add or
region_abort to resolve each region_chg call, but seems that is not
the case. I'll fix and upload another version of the patch.

> --
> Mike Kravetz

Patch
diff mbox series

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index bac1cbdd027c..d03b048084a3 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -244,6 +244,12 @@  struct file_region {
 	long to;
 };

+/* Helper that removes a struct file_region from the resv_map cache and returns
+ * it for use.
+ */
+static struct file_region *
+get_file_region_entry_from_cache(struct resv_map *resv, long from, long to);
+
 /* Must be called with resv->lock held. Calling this with count_only == true
  * will count the number of pages to be added but will not modify the linked
  * list.
@@ -251,51 +257,61 @@  struct file_region {
 static long add_reservation_in_range(struct resv_map *resv, long f, long t,
 				     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;
-
-	/* 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);
+			}
 		}
-		chg -= rg->to - rg->from;

-		if (!count_only && rg != nrg) {
-			list_del(&rg->link);
-			kfree(rg);
-		}
+		last_accounted_offset = rg->to;
 	}

-	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);
+		}
+		last_accounted_offset = t;
 	}

-	return chg;
+	return add;
 }

 /*
@@ -305,46 +321,24 @@  static long add_reservation_in_range(struct resv_map *resv, long f, long t,
  * must exist in the cache due to the previous call to region_chg with
  * the same range.
  *
+ * regions_needed is the out value provided by a previous
+ * call to region_chg.
+ *
  * Return the number of new huge pages added to the map.  This
  * number is greater than or equal to zero.
  */
-static long region_add(struct resv_map *resv, long f, long t)
+static long region_add(struct resv_map *resv, long f, long t,
+		       long regions_needed)
 {
-	struct list_head *head = &resv->regions;
-	struct file_region *rg, *nrg;
 	long add = 0;

 	spin_lock(&resv->lock);
-	/* Locate the region we are either in or before. */
-	list_for_each_entry(rg, head, link)
-		if (f <= rg->to)
-			break;

-	/*
-	 * If no region exists which can be expanded to include the
-	 * specified range, pull a region descriptor from the cache
-	 * and use it for this range.
-	 */
-	if (&rg->link == head || t < rg->from) {
-		VM_BUG_ON(resv->region_cache_count <= 0);
-
-		resv->region_cache_count--;
-		nrg = list_first_entry(&resv->region_cache, struct file_region,
-					link);
-		list_del(&nrg->link);
-
-		nrg->from = f;
-		nrg->to = t;
-		list_add(&nrg->link, rg->link.prev);
-
-		add += t - f;
-		goto out_locked;
-	}
+	VM_BUG_ON(resv->region_cache_count < regions_needed);

 	add = add_reservation_in_range(resv, f, t, false);
+	resv->adds_in_progress -= regions_needed;

-out_locked:
-	resv->adds_in_progress--;
 	spin_unlock(&resv->lock);
 	VM_BUG_ON(add < 0);
 	return add;
@@ -361,42 +355,55 @@  static long region_add(struct resv_map *resv, long f, long t)
  * as a placeholder, so that the subsequent region_add
  * call will have all the regions it needs and will not fail.
  *
+ * out_regions_needed is the number of regions added to the
+ * resv->region_cache_count.  This value needs to be provided to a follow up
+ * call to region_add or region_abort for proper accounting.
+ *
  * Returns the number of huge pages that need to be added to the existing
  * reservation map for the range [f, t).  This number is greater or equal to
  * zero.  -ENOMEM is returned if a new file_region structure or cache entry
  * is needed and can not be allocated.
  */
-static long region_chg(struct resv_map *resv, long f, long t)
+static long region_chg(struct resv_map *resv, long f, long t,
+		       long *out_regions_needed)
 {
+	struct file_region *trg = NULL;
 	long chg = 0;

+	/* Allocate the maximum number of regions we're going to need for this
+	 * reservation. The maximum number of regions we're going to need is
+	 * (t - f) / 2 + 1, which corresponds to a region with alternating
+	 * reserved and unreserved pages.
+	 */
+	*out_regions_needed = (t - f) / 2 + 1;
+retry:
 	spin_lock(&resv->lock);
-retry_locked:
-	resv->adds_in_progress++;
+
+	/* Count how many hugepages in this range are NOT respresented. */
+	chg = add_reservation_in_range(resv, f, t, true);

 	/*
 	 * Check for sufficient descriptors in the cache to accommodate
 	 * the number of in progress add operations.
 	 */
-	if (resv->adds_in_progress > resv->region_cache_count) {
-		struct file_region *trg;
-
-		VM_BUG_ON(resv->adds_in_progress - resv->region_cache_count > 1);
+	if (resv->region_cache_count < *out_regions_needed) {
 		/* Must drop lock to allocate a new descriptor. */
-		resv->adds_in_progress--;
 		spin_unlock(&resv->lock);

-		trg = kmalloc(sizeof(*trg), GFP_KERNEL);
-		if (!trg)
-			return -ENOMEM;
+		while (resv->region_cache_count < *out_regions_needed + 1) {
+			trg = kmalloc(sizeof(*trg), GFP_KERNEL);
+			if (!trg)
+				return -ENOMEM;

-		spin_lock(&resv->lock);
-		list_add(&trg->link, &resv->region_cache);
-		resv->region_cache_count++;
-		goto retry_locked;
+			spin_lock(&resv->lock);
+			list_add(&trg->link, &resv->region_cache);
+			resv->region_cache_count++;
+			spin_unlock(&resv->lock);
+		}
+		goto retry;
 	}

-	chg = add_reservation_in_range(resv, f, t, true);
+	resv->adds_in_progress += *out_regions_needed;

 	spin_unlock(&resv->lock);
 	return chg;
@@ -407,17 +414,19 @@  static long region_chg(struct resv_map *resv, long f, long t)
  * of the resv_map keeps track of the operations in progress between
  * calls to region_chg and region_add.  Operations are sometimes
  * aborted after the call to region_chg.  In such cases, region_abort
- * is called to decrement the adds_in_progress counter.
+ * is called to decrement the adds_in_progress counter. regions_needed
+ * is the value returned by the region_chg call, it is used to decrement
+ * the adds_in_progress counter.
  *
  * NOTE: The range arguments [f, t) are not needed or used in this
  * routine.  They are kept to make reading the calling code easier as
  * arguments will match the associated region_chg call.
  */
-static void region_abort(struct resv_map *resv, long f, long t)
+static void region_abort(struct resv_map *resv, long f, long t,
+			 long regions_needed)
 {
 	spin_lock(&resv->lock);
-	VM_BUG_ON(!resv->region_cache_count);
-	resv->adds_in_progress--;
+	resv->adds_in_progress -= regions_needed;
 	spin_unlock(&resv->lock);
 }

@@ -1866,9 +1875,10 @@  enum vma_resv_mode {
 	VMA_END_RESV,
 	VMA_ADD_RESV,
 };
-static long __vma_reservation_common(struct hstate *h,
-				struct vm_area_struct *vma, unsigned long addr,
-				enum vma_resv_mode mode)
+static long
+__vma_reservation_common(struct hstate *h, struct vm_area_struct *vma,
+			 unsigned long addr, enum vma_resv_mode mode,
+			 long *out_regions_needed, long in_regions_needed)
 {
 	struct resv_map *resv;
 	pgoff_t idx;
@@ -1881,20 +1891,24 @@  static long __vma_reservation_common(struct hstate *h,
 	idx = vma_hugecache_offset(h, vma, addr);
 	switch (mode) {
 	case VMA_NEEDS_RESV:
-		ret = region_chg(resv, idx, idx + 1);
+		VM_BUG_ON(!out_regions_needed);
+		ret = region_chg(resv, idx, idx + 1, out_regions_needed);
 		break;
 	case VMA_COMMIT_RESV:
-		ret = region_add(resv, idx, idx + 1);
+		VM_BUG_ON(in_regions_needed == -1);
+		ret = region_add(resv, idx, idx + 1, in_regions_needed);
 		break;
 	case VMA_END_RESV:
-		region_abort(resv, idx, idx + 1);
+		VM_BUG_ON(in_regions_needed == -1);
+		region_abort(resv, idx, idx + 1, in_regions_needed);
 		ret = 0;
 		break;
 	case VMA_ADD_RESV:
+		VM_BUG_ON(in_regions_needed == -1);
 		if (vma->vm_flags & VM_MAYSHARE)
-			ret = region_add(resv, idx, idx + 1);
+			ret = region_add(resv, idx, idx + 1, in_regions_needed);
 		else {
-			region_abort(resv, idx, idx + 1);
+			region_abort(resv, idx, idx + 1, in_regions_needed);
 			ret = region_del(resv, idx, idx + 1);
 		}
 		break;
@@ -1927,28 +1941,32 @@  static long __vma_reservation_common(struct hstate *h,
 		return ret < 0 ? ret : 0;
 }

-static long vma_needs_reservation(struct hstate *h,
-			struct vm_area_struct *vma, unsigned long addr)
+static long vma_needs_reservation(struct hstate *h, struct vm_area_struct *vma,
+				  unsigned long addr, long *out_regions_needed)
 {
-	return __vma_reservation_common(h, vma, addr, VMA_NEEDS_RESV);
+	return __vma_reservation_common(h, vma, addr, VMA_NEEDS_RESV,
+					out_regions_needed, -1);
 }

-static long vma_commit_reservation(struct hstate *h,
-			struct vm_area_struct *vma, unsigned long addr)
+static long vma_commit_reservation(struct hstate *h, struct vm_area_struct *vma,
+				   unsigned long addr, long regions_needed)
 {
-	return __vma_reservation_common(h, vma, addr, VMA_COMMIT_RESV);
+	return __vma_reservation_common(h, vma, addr, VMA_COMMIT_RESV, NULL,
+					regions_needed);
 }

-static void vma_end_reservation(struct hstate *h,
-			struct vm_area_struct *vma, unsigned long addr)
+static void vma_end_reservation(struct hstate *h, struct vm_area_struct *vma,
+				unsigned long addr, long regions_needed)
 {
-	(void)__vma_reservation_common(h, vma, addr, VMA_END_RESV);
+	(void)__vma_reservation_common(h, vma, addr, VMA_END_RESV, NULL,
+				       regions_needed);
 }

-static long vma_add_reservation(struct hstate *h,
-			struct vm_area_struct *vma, unsigned long addr)
+static long vma_add_reservation(struct hstate *h, struct vm_area_struct *vma,
+				unsigned long addr, long regions_needed)
 {
-	return __vma_reservation_common(h, vma, addr, VMA_ADD_RESV);
+	return __vma_reservation_common(h, vma, addr, VMA_ADD_RESV, NULL,
+					regions_needed);
 }

 /*
@@ -1966,8 +1984,10 @@  static void restore_reserve_on_error(struct hstate *h,
 			struct vm_area_struct *vma, unsigned long address,
 			struct page *page)
 {
+	long regions_needed = 0;
 	if (unlikely(PagePrivate(page))) {
-		long rc = vma_needs_reservation(h, vma, address);
+		long rc =
+			vma_needs_reservation(h, vma, address, &regions_needed);

 		if (unlikely(rc < 0)) {
 			/*
@@ -1983,7 +2003,8 @@  static void restore_reserve_on_error(struct hstate *h,
 			 */
 			ClearPagePrivate(page);
 		} else if (rc) {
-			rc = vma_add_reservation(h, vma, address);
+			rc = vma_add_reservation(h, vma, address,
+						 regions_needed);
 			if (unlikely(rc < 0))
 				/*
 				 * See above comment about rare out of
@@ -1991,7 +2012,7 @@  static void restore_reserve_on_error(struct hstate *h,
 				 */
 				ClearPagePrivate(page);
 		} else
-			vma_end_reservation(h, vma, address);
+			vma_end_reservation(h, vma, address, regions_needed);
 	}
 }

@@ -2005,6 +2026,7 @@  struct page *alloc_huge_page(struct vm_area_struct *vma,
 	long gbl_chg;
 	int ret, idx;
 	struct hugetlb_cgroup *h_cg;
+	long regions_needed = 0;

 	idx = hstate_index(h);
 	/*
@@ -2012,7 +2034,8 @@  struct page *alloc_huge_page(struct vm_area_struct *vma,
 	 * has a reservation for the page to be allocated.  A return
 	 * code of zero indicates a reservation exists (no change).
 	 */
-	map_chg = gbl_chg = vma_needs_reservation(h, vma, addr);
+	map_chg = gbl_chg =
+		vma_needs_reservation(h, vma, addr, &regions_needed);
 	if (map_chg < 0)
 		return ERR_PTR(-ENOMEM);

@@ -2026,7 +2049,7 @@  struct page *alloc_huge_page(struct vm_area_struct *vma,
 	if (map_chg || avoid_reserve) {
 		gbl_chg = hugepage_subpool_get_pages(spool, 1);
 		if (gbl_chg < 0) {
-			vma_end_reservation(h, vma, addr);
+			vma_end_reservation(h, vma, addr, regions_needed);
 			return ERR_PTR(-ENOSPC);
 		}

@@ -2072,7 +2095,7 @@  struct page *alloc_huge_page(struct vm_area_struct *vma,

 	set_page_private(page, (unsigned long)spool);

-	map_commit = vma_commit_reservation(h, vma, addr);
+	map_commit = vma_commit_reservation(h, vma, addr, regions_needed);
 	if (unlikely(map_chg > map_commit)) {
 		/*
 		 * The page was added to the reservation map between
@@ -2096,7 +2119,7 @@  struct page *alloc_huge_page(struct vm_area_struct *vma,
 out_subpool_put:
 	if (map_chg || avoid_reserve)
 		hugepage_subpool_put_pages(spool, 1);
-	vma_end_reservation(h, vma, addr);
+	vma_end_reservation(h, vma, addr, regions_needed);
 	return ERR_PTR(-ENOSPC);
 }

@@ -3780,6 +3803,7 @@  static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
 	spinlock_t *ptl;
 	unsigned long haddr = address & huge_page_mask(h);
 	bool new_page = false;
+	long regions_needed = 0;

 	/*
 	 * Currently, we are forced to kill the process in the event the
@@ -3897,12 +3921,12 @@  static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
 	 * the spinlock.
 	 */
 	if ((flags & FAULT_FLAG_WRITE) && !(vma->vm_flags & VM_SHARED)) {
-		if (vma_needs_reservation(h, vma, haddr) < 0) {
+		if (vma_needs_reservation(h, vma, haddr, &regions_needed) < 0) {
 			ret = VM_FAULT_OOM;
 			goto backout_unlocked;
 		}
 		/* Just decrements count, does not deallocate */
-		vma_end_reservation(h, vma, haddr);
+		vma_end_reservation(h, vma, haddr, regions_needed);
 	}

 	ptl = huge_pte_lock(h, mm, ptep);
@@ -3992,6 +4016,7 @@  vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 	struct address_space *mapping;
 	int need_wait_lock = 0;
 	unsigned long haddr = address & huge_page_mask(h);
+	long regions_needed = 0;

 	ptep = huge_pte_offset(mm, haddr, huge_page_size(h));
 	if (ptep) {
@@ -4046,12 +4071,12 @@  vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 	 * consumed.
 	 */
 	if ((flags & FAULT_FLAG_WRITE) && !huge_pte_write(entry)) {
-		if (vma_needs_reservation(h, vma, haddr) < 0) {
+		if (vma_needs_reservation(h, vma, haddr, &regions_needed) < 0) {
 			ret = VM_FAULT_OOM;
 			goto out_mutex;
 		}
 		/* Just decrements count, does not deallocate */
-		vma_end_reservation(h, vma, haddr);
+		vma_end_reservation(h, vma, haddr, regions_needed);

 		if (!(vma->vm_flags & VM_MAYSHARE))
 			pagecache_page = hugetlbfs_pagecache_page(h,
@@ -4514,7 +4539,7 @@  int hugetlb_reserve_pages(struct inode *inode,
 	struct hugepage_subpool *spool = subpool_inode(inode);
 	struct resv_map *resv_map;
 	struct hugetlb_cgroup *h_cg;
-	long gbl_reserve;
+	long gbl_reserve, regions_needed = 0;

 	/* This should never happen */
 	if (from > to) {
@@ -4544,7 +4569,7 @@  int hugetlb_reserve_pages(struct inode *inode,
 		 */
 		resv_map = inode_resv_map(inode);

-		chg = region_chg(resv_map, from, to);
+		chg = region_chg(resv_map, from, to, &regions_needed);

 	} else {
 		/* Private mapping. */
@@ -4614,7 +4639,7 @@  int hugetlb_reserve_pages(struct inode *inode,
 	 * else has to be done for private mappings here
 	 */
 	if (!vma || vma->vm_flags & VM_MAYSHARE) {
-		long add = region_add(resv_map, from, to);
+		long add = region_add(resv_map, from, to, regions_needed);

 		if (unlikely(chg > add)) {
 			/*
@@ -4636,7 +4661,7 @@  int hugetlb_reserve_pages(struct inode *inode,
 	if (!vma || vma->vm_flags & VM_MAYSHARE)
 		/* Don't call region_abort if region_chg failed */
 		if (chg >= 0)
-			region_abort(resv_map, from, to);
+			region_abort(resv_map, from, to, regions_needed);
 	if (vma && is_vma_resv_set(vma, HPAGE_RESV_OWNER))
 		kref_put(&resv_map->refs, resv_map_release);
 	return ret;
@@ -5060,3 +5085,21 @@  void move_hugetlb_state(struct page *oldpage, struct page *newpage, int reason)
 		spin_unlock(&hugetlb_lock);
 	}
 }
+
+static struct file_region *
+get_file_region_entry_from_cache(struct resv_map *resv, long from, long to)
+{
+	struct file_region *nrg = NULL;
+
+	VM_BUG_ON(resv->region_cache_count <= 0);
+
+	resv->region_cache_count--;
+	nrg = list_first_entry(&resv->region_cache, struct file_region, link);
+	VM_BUG_ON(!nrg);
+	list_del(&nrg->link);
+
+	nrg->from = from;
+	nrg->to = to;
+
+	return nrg;
+}