[PATCH v3 1/3] mm/hugetlb: document the reserve map/region tracking routines

[Mike Kravetz <mike.kravetz@oracle.com>]

This is a documentation only patch and does not modify any code.
Descriptions of the routines used for reserve map/region tracking
are added.

Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
---
 mm/hugetlb.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 50 insertions(+), 2 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 54f129d..ad2c628 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -212,8 +212,20 @@ static inline struct hugepage_subpool *subpool_vma(struct vm_area_struct *vma)
  * Region tracking -- allows tracking of reservations and instantiated pages
  *                    across the pages in a mapping.
  *
- * The region data structures are embedded into a resv_map and
- * protected by a resv_map's lock
+ * The region data structures are embedded into a resv_map and protected
+ * by a resv_map's lock.  The set of regions within the resv_map represent
+ * reservations for huge pages, or huge pages that have already been
+ * instantiated within the map.  The from and to elements are huge page
+ * indicies into the associated mapping.  from indicates the starting index
+ * of the region.  to represents the first index past the end of  the region.
+ *
+ * For example, a file region structure with from == 0 and to == 4 represents
+ * four huge pages in a mapping.  It is important to note that the to element
+ * represents the first element past the end of the region. This is used in
+ * arithmetic as 4(to) - 0(from) = 4 huge pages in the region.
+ *
+ * Interval notation of the form [from, to) will be used to indicate that
+ * the endpoint from is inclusive and to is exclusive.
  */
 struct file_region {
 	struct list_head link;
@@ -221,6 +233,14 @@ struct file_region {
 	long to;
 };
 
+/*
+ * Add the huge page range represented by [f, t) to the reserve
+ * map.  Existing regions will be expanded to accommodate the
+ * specified range.  We know only existing regions need to be
+ * expanded, because region_add is only called after region_chg
+ * with the same range.  If a new file_region structure must
+ * be allocated, it is done in region_chg.
+ */
 static long region_add(struct resv_map *resv, long f, long t)
 {
 	struct list_head *head = &resv->regions;
@@ -260,6 +280,25 @@ static long region_add(struct resv_map *resv, long f, long t)
 	return 0;
 }
 
+/*
+ * Examine the existing reserve map and determine how many
+ * huge pages in the specified range [f, t) are NOT currently
+ * represented.  This routine is called before a subsequent
+ * 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.
+ *
+ * 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 is needed and can
+ * not be allocated.
+ */
 static long region_chg(struct resv_map *resv, long f, long t)
 {
 	struct list_head *head = &resv->regions;
@@ -326,6 +365,11 @@ out_nrg:
 	return chg;
 }
 
+/*
+ * Truncate the reserve map at index 'end'.  Modify/truncate any
+ * region which contains end.  Delete any regions past end.
+ * Return the number of huge pages removed from the map.
+ */
 static long region_truncate(struct resv_map *resv, long end)
 {
 	struct list_head *head = &resv->regions;
@@ -361,6 +405,10 @@ out:
 	return chg;
 }
 
+/*
+ * Count and return the number of huge pages in the reserve map
+ * that intersect with the range [f, t).
+ */
 static long region_count(struct resv_map *resv, long f, long t)
 {
 	struct list_head *head = &resv->regions;
-- 
2.1.0

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