[PATCH] mm: thp: Set THP defrag by default to madvise and add a stall-free defrag option -fix

[Mel Gorman]

The following is a fix to the patch
mm-thp-set-thp-defrag-by-default-to-madvise-and-add-a-stall-free-defrag-option.patch
based on feedback from Vlastimil Babka. It removes an unnecessary VM_BUG_ON for tidyness,
clarifies documentation and adds a check forbidding someone writing "defer" to the enable
knob for transparent huge pages. The ack from Vlastimil applies for the
original patch plus this fix combined.

Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 Documentation/vm/transhuge.txt | 2 +-
 mm/huge_memory.c               | 7 ++++---
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/Documentation/vm/transhuge.txt b/Documentation/vm/transhuge.txt
index a19b173cbc57..1943fe051a36 100644
--- a/Documentation/vm/transhuge.txt
+++ b/Documentation/vm/transhuge.txt
@@ -129,7 +129,7 @@ available in the near future. It's the responsibility of khugepaged
 to then install the THP pages later.
 
 "madvise" will enter direct reclaim like "always" but only for regions
-that are have used madvise(). This is the default behaviour.
+that are have used madvise(MADV_HUGEPAGE). This is the default behaviour.
 
 "never" should be self-explanatory.
 
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 206f35f06d83..9161b3a83720 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -246,6 +246,8 @@ static ssize_t triple_flag_store(struct kobject *kobj,
 {
 	if (!memcmp("defer", buf,
 		    min(sizeof("defer")-1, count))) {
+		if (enabled == deferred)
+			return -EINVAL;
 		clear_bit(enabled, &transparent_hugepage_flags);
 		clear_bit(req_madv, &transparent_hugepage_flags);
 		set_bit(deferred, &transparent_hugepage_flags);
@@ -273,10 +275,9 @@ static ssize_t triple_flag_store(struct kobject *kobj,
 static ssize_t enabled_show(struct kobject *kobj,
 			    struct kobj_attribute *attr, char *buf)
 {
-	if (test_bit(TRANSPARENT_HUGEPAGE_FLAG, &transparent_hugepage_flags)) {
-		VM_BUG_ON(test_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG, &transparent_hugepage_flags));
+	if (test_bit(TRANSPARENT_HUGEPAGE_FLAG, &transparent_hugepage_flags))
 		return sprintf(buf, "[always] madvise never\n");
-	} else if (test_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG, &transparent_hugepage_flags))
+	else if (test_bit(TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG, &transparent_hugepage_flags))
 		return sprintf(buf, "always [madvise] never\n");
 	else
 		return sprintf(buf, "always madvise [never]\n");