[PATCH 0/2] Cleanups to hugetlb code

[PATCH 0/2] Cleanups to hugetlb code

[Mina Almasry]

These couple of patches were part of my 'hugetlb_cgroup: Add hugetlb_cgroup
reservation limits' patch series, and Mike recommended that they are split off
into their own since they are generic cleanups that should apply regardless.
Hence, I upload them here as a their own patch series.

They have been already reviewed by Mike as part of the previous series, so
already hold the Reviewed-by tag.

Mina Almasry (2):
  hugetlb: region_chg provides only cache entry
  hugetlb: remove duplicated code

 mm/hugetlb.c | 180 +++++++++++++++++++--------------------------------
 1 file changed, 67 insertions(+), 113 deletions(-)

--
2.23.0.351.gc4317032e6-goog

[PATCH 1/2] hugetlb: region_chg provides only cache entry

[Mina Almasry]

Current behavior is that region_chg provides both a cache entry in
resv->region_cache, AND a placeholder entry in resv->regions. region_add
first tries to use the placeholder, and if it finds that the placeholder
has been deleted by a racing region_del call, it uses the cache entry.

This behavior is completely unnecessary and is removed in this patch for
a couple of reasons:

1. region_add needs to either find a cached file_region entry in
   resv->region_cache, or find an entry in resv->regions to expand. It
   does not need both.
2. region_chg adding a placeholder entry in resv->regions opens up
   a possible race with region_del, where region_chg adds a placeholder
   region in resv->regions, and this region is deleted by a racing call
   to region_del during region_chg execution or before region_add is
   called. Removing the race makes the code easier to reason about and
   maintain.

In addition, a follow up patch in another series that disables region
coalescing, which would be further complicated if the race with
region_del exists.

Signed-off-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>

---
 mm/hugetlb.c | 63 +++++++++-------------------------------------------
 1 file changed, 11 insertions(+), 52 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 6d7296dd11b8..a14f6047fc7e 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -246,14 +246,10 @@ struct file_region {

 /*
  * Add the huge page range represented by [f, t) to the reserve
- * map.  In the normal case, existing regions will be expanded
- * to accommodate the specified range.  Sufficient regions should
- * exist for expansion due to the previous call to region_chg
- * with the same range.  However, it is possible that region_del
- * could have been called after region_chg and modifed the map
- * in such a way that no region exists to be expanded.  In this
- * case, pull a region descriptor from the cache associated with
- * the map and use that for the new range.
+ * map.  Existing regions will be expanded to accommodate the specified
+ * range, or a region will be taken from the cache.  Sufficient regions
+ * must exist in the cache due to the previous call to region_chg with
+ * the same range.
  *
  * Return the number of new huge pages added to the map.  This
  * number is greater than or equal to zero.
@@ -272,9 +268,8 @@ static long region_add(struct resv_map *resv, long f, long t)

 	/*
 	 * If no region exists which can be expanded to include the
-	 * specified range, the list must have been modified by an
-	 * interleving call to region_del().  Pull a region descriptor
-	 * from the cache and use it for this range.
+	 * 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);
@@ -339,15 +334,9 @@ static long region_add(struct resv_map *resv, long f, long t)
  * call to region_add that will actually modify the reserve
  * map to add the specified range [f, t).  region_chg does
  * not change the number of huge pages represented by the
- * map.  However, if the existing regions in the map can not
- * be expanded to represent the new range, a new file_region
- * structure is added to the map as a placeholder.  This is
- * so that the subsequent region_add call will have all the
- * regions it needs and will not fail.
- *
- * Upon entry, region_chg will also examine the cache of region descriptors
- * associated with the map.  If there are not enough descriptors cached, one
- * will be allocated for the in progress add operation.
+ * map.  A new file_region structure is added to the cache
+ * as a placeholder, so that the subsequent region_add
+ * call will have all the regions it needs and will not fail.
  *
  * 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
@@ -357,10 +346,9 @@ static long region_add(struct resv_map *resv, long f, long t)
 static long region_chg(struct resv_map *resv, long f, long t)
 {
 	struct list_head *head = &resv->regions;
-	struct file_region *rg, *nrg = NULL;
+	struct file_region *rg;
 	long chg = 0;

-retry:
 	spin_lock(&resv->lock);
 retry_locked:
 	resv->adds_in_progress++;
@@ -378,10 +366,8 @@ static long region_chg(struct resv_map *resv, long f, long t)
 		spin_unlock(&resv->lock);

 		trg = kmalloc(sizeof(*trg), GFP_KERNEL);
-		if (!trg) {
-			kfree(nrg);
+		if (!trg)
 			return -ENOMEM;
-		}

 		spin_lock(&resv->lock);
 		list_add(&trg->link, &resv->region_cache);
@@ -394,28 +380,6 @@ static long region_chg(struct resv_map *resv, long f, long t)
 		if (f <= rg->to)
 			break;

-	/* If we are below the current region then a new region is required.
-	 * Subtle, allocate a new region at the position but make it zero
-	 * size such that we can guarantee to record the reservation. */
-	if (&rg->link == head || t < rg->from) {
-		if (!nrg) {
-			resv->adds_in_progress--;
-			spin_unlock(&resv->lock);
-			nrg = kmalloc(sizeof(*nrg), GFP_KERNEL);
-			if (!nrg)
-				return -ENOMEM;
-
-			nrg->from = f;
-			nrg->to   = f;
-			INIT_LIST_HEAD(&nrg->link);
-			goto retry;
-		}
-
-		list_add(&nrg->link, rg->link.prev);
-		chg = t - f;
-		goto out_nrg;
-	}
-
 	/* Round our left edge to the current segment if it encloses us. */
 	if (f > rg->from)
 		f = rg->from;
@@ -439,11 +403,6 @@ static long region_chg(struct resv_map *resv, long f, long t)
 	}

 out:
-	spin_unlock(&resv->lock);
-	/*  We already know we raced and no longer need the new region */
-	kfree(nrg);
-	return chg;
-out_nrg:
 	spin_unlock(&resv->lock);
 	return chg;
 }
--
2.23.0.351.gc4317032e6-goog

[PATCH 2/2] hugetlb: remove duplicated code

[Mina Almasry]

Remove duplicated code between region_chg and region_add, and refactor it into
a common function, add_reservation_in_range. This is mostly done because
there is a follow up change in another series that disables region
coalescing in region_add, and I want to make that change in one place
only. It should improve maintainability anyway on its own.

Signed-off-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>

---
 mm/hugetlb.c | 119 ++++++++++++++++++++++++---------------------------
 1 file changed, 57 insertions(+), 62 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index a14f6047fc7e..052a2532528a 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -244,6 +244,60 @@ struct file_region {
 	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.
+ */
+static long add_reservation_in_range(struct resv_map *resv, long f, long t,
+				     bool count_only)
+{
+	long chg = 0;
+	struct list_head *head = &resv->regions;
+	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;
+
+	/* 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;
+		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.
+		 */
+		if (rg->to > t) {
+			chg += rg->to - t;
+			t = rg->to;
+		}
+		chg -= rg->to - rg->from;
+
+		if (!count_only && rg != nrg) {
+			list_del(&rg->link);
+			kfree(rg);
+		}
+	}
+
+	if (!count_only) {
+		nrg->from = f;
+		nrg->to = t;
+	}
+
+	return chg;
+}
+
 /*
  * Add the huge page range represented by [f, t) to the reserve
  * map.  Existing regions will be expanded to accommodate the specified
@@ -257,7 +311,7 @@ struct file_region {
 static long region_add(struct resv_map *resv, long f, long t)
 {
 	struct list_head *head = &resv->regions;
-	struct file_region *rg, *nrg, *trg;
+	struct file_region *rg, *nrg;
 	long add = 0;

 	spin_lock(&resv->lock);
@@ -287,38 +341,7 @@ static long region_add(struct resv_map *resv, long f, long t)
 		goto out_locked;
 	}

-	/* Round our left edge to the current segment if it encloses us. */
-	if (f > rg->from)
-		f = rg->from;
-
-	/* 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;
-		if (rg->from > t)
-			break;
-
-		/* If this area reaches higher then extend our area to
-		 * include it completely.  If this is not the first area
-		 * which we intend to reuse, free it. */
-		if (rg->to > t)
-			t = rg->to;
-		if (rg != nrg) {
-			/* Decrement return value by the deleted range.
-			 * Another range will span this area so that by
-			 * end of routine add will be >= zero
-			 */
-			add -= (rg->to - rg->from);
-			list_del(&rg->link);
-			kfree(rg);
-		}
-	}
-
-	add += (nrg->from - f);		/* Added to beginning of region */
-	nrg->from = f;
-	add += t - nrg->to;		/* Added to end of region */
-	nrg->to = t;
+	add = add_reservation_in_range(resv, f, t, false);

 out_locked:
 	resv->adds_in_progress--;
@@ -345,8 +368,6 @@ static long region_add(struct resv_map *resv, long f, long t)
  */
 static long region_chg(struct resv_map *resv, long f, long t)
 {
-	struct list_head *head = &resv->regions;
-	struct file_region *rg;
 	long chg = 0;

 	spin_lock(&resv->lock);
@@ -375,34 +396,8 @@ static long region_chg(struct resv_map *resv, long f, long t)
 		goto retry_locked;
 	}

-	/* 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;
-
-	/* Check for and consume any regions we now overlap with. */
-	list_for_each_entry(rg, rg->link.prev, link) {
-		if (&rg->link == head)
-			break;
-		if (rg->from > t)
-			goto out;
-
-		/* We overlap with this area, if it extends further than
-		 * us then we must extend ourselves.  Account for its
-		 * existing reservation. */
-		if (rg->to > t) {
-			chg += rg->to - t;
-			t = rg->to;
-		}
-		chg -= rg->to - rg->from;
-	}
+	chg = add_reservation_in_range(resv, f, t, true);

-out:
 	spin_unlock(&resv->lock);
 	return chg;
 }
--
2.23.0.351.gc4317032e6-goog