[PATCH 2/2] x86/mm/pat: Change free_memtype() to free shrinking range

[Toshi Kani]

mremap() to shrink the map size of a VM_PFNMAP range causes
the following error message, and leaves the pfn range allocated.

 x86/PAT: test:3493 freeing invalid memtype [mem 0x483200000-0x4863fffff]

rbt_memtype_erase(), called from free_memtype() with spin_lock
held, only accepts to free a whole memtype node in memtype_rbroot.
Change rbt_memtype_erase() to accept a request that shrinks the
size of a memtype node for mremap().

memtype_rb_exact_match() is renamed to memtype_rb_match(),
which now performs exact match or shrink match per match_type.
Since the memtype_rbroot tree allows overlapping ranges,
rbt_memtype_erase() checks exact match first to ensure the case
of freeing a whole node, which is the normal case.  For the
shrink case, rbt_memtype_erase() proceeds in two steps, 1) remove
the node, and then 2) insert the updated node.  This allows proper
update of augmented values, subtree_max_end, in the tree.

Reference: https://lkml.org/lkml/2015/10/28/865
Signed-off-by: Toshi Kani <toshi.kani@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
---
 arch/x86/mm/pat.c        |    2 +-
 arch/x86/mm/pat_rbtree.c |   46 +++++++++++++++++++++++++++++++++++++---------
 2 files changed, 38 insertions(+), 10 deletions(-)

diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
index f3e391e..f277efd 100644
--- a/arch/x86/mm/pat.c
+++ b/arch/x86/mm/pat.c
@@ -586,7 +586,7 @@ int free_memtype(u64 start, u64 end)
 	entry = rbt_memtype_erase(start, end);
 	spin_unlock(&memtype_lock);
 
-	if (!entry) {
+	if (IS_ERR(entry)) {
 		pr_info("x86/PAT: %s:%d freeing invalid memtype [mem %#010Lx-%#010Lx]\n",
 			current->comm, current->pid, start, end - 1);
 		return -EINVAL;
diff --git a/arch/x86/mm/pat_rbtree.c b/arch/x86/mm/pat_rbtree.c
index 6393108..d6faef8 100644
--- a/arch/x86/mm/pat_rbtree.c
+++ b/arch/x86/mm/pat_rbtree.c
@@ -98,8 +98,13 @@ static struct memtype *memtype_rb_lowest_match(struct rb_root *root,
 	return last_lower; /* Returns NULL if there is no overlap */
 }
 
-static struct memtype *memtype_rb_exact_match(struct rb_root *root,
-				u64 start, u64 end)
+enum {
+	MEMTYPE_EXACT_MATCH  = 0,
+	MEMTYPE_SHRINK_MATCH = 1
+};
+
+static struct memtype *memtype_rb_match(struct rb_root *root,
+				u64 start, u64 end, int match_type)
 {
 	struct memtype *match;
 
@@ -107,7 +112,12 @@ static struct memtype *memtype_rb_exact_match(struct rb_root *root,
 	while (match != NULL && match->start < end) {
 		struct rb_node *node;
 
-		if (match->start == start && match->end == end)
+		if ((match_type == MEMTYPE_EXACT_MATCH) &&
+		    (match->start == start) && (match->end == end))
+			return match;
+
+		if ((match_type == MEMTYPE_SHRINK_MATCH) &&
+		    (match->start < start) && (match->end == end))
 			return match;
 
 		node = rb_next(&match->rb);
@@ -117,7 +127,7 @@ static struct memtype *memtype_rb_exact_match(struct rb_root *root,
 			match = NULL;
 	}
 
-	return NULL; /* Returns NULL if there is no exact match */
+	return NULL; /* Returns NULL if there is no match */
 }
 
 static int memtype_rb_check_conflict(struct rb_root *root,
@@ -210,12 +220,30 @@ struct memtype *rbt_memtype_erase(u64 start, u64 end)
 {
 	struct memtype *data;
 
-	data = memtype_rb_exact_match(&memtype_rbroot, start, end);
-	if (!data)
-		goto out;
+	/* Exact match takes precedence over shrink match */
+	data = memtype_rb_match(&memtype_rbroot, start, end,
+				MEMTYPE_EXACT_MATCH);
+	if (!data) {
+		data = memtype_rb_match(&memtype_rbroot, start, end,
+					MEMTYPE_SHRINK_MATCH);
+		if (!data)
+			return ERR_PTR(-EINVAL);
+	}
+
+	if (data->start == start) {
+		/* Exact match: erase this node */
+		rb_erase_augmented(&data->rb, &memtype_rbroot,
+					&memtype_rb_augment_cb);
+	} else {
+		/* Shrink match: update the end value of this node */
+		rb_erase_augmented(&data->rb, &memtype_rbroot,
+					&memtype_rb_augment_cb);
+		data->end = start;
+		data->subtree_max_end = data->end;
+		memtype_rb_insert(&memtype_rbroot, data);
+		return NULL;
+	}
 
-	rb_erase_augmented(&data->rb, &memtype_rbroot, &memtype_rb_augment_cb);
-out:
 	return data;
 }
 
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