[PATCH] mm/madvise: allow MADV_DONTNEED to free memory that is MLOCK_ONFAULT

[Jason Baron]

In order to free memory that is marked MLOCK_ONFAULT, the memory region
needs to be first unlocked, before calling MADV_DONTNEED. And if the region
is to be reused as MLOCK_ONFAULT, we require another call to mlock2() with
the MLOCK_ONFAULT flag.

Let's simplify freeing memory that is set MLOCK_ONFAULT, by allowing
MADV_DONTNEED to work directly for memory that is set MLOCK_ONFAULT. The
locked memory limits, tracked by mm->locked_vm do not need to be adjusted
in this case, since they were charged to the entire region when
MLOCK_ONFAULT was initially set.

Further, I don't think allowing MADV_FREE for MLOCK_ONFAULT regions makes
sense, since the point of MLOCK_ONFAULT is for userspace to know when pages
are locked in memory and thus to know when page faults will occur.

Signed-off-by: Jason Baron <jbaron@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
 mm/internal.h | 18 ++++++++++++++++++
 mm/madvise.c  |  4 ++--
 mm/oom_kill.c |  2 +-
 3 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index 9e3654d..16c0041 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -15,6 +15,7 @@
 #include <linux/mm.h>
 #include <linux/pagemap.h>
 #include <linux/tracepoint-defs.h>
+#include <uapi/asm-generic/mman-common.h>
 
 /*
  * The set of flags that only affect watermark checking and reclaim
@@ -45,9 +46,26 @@ void free_pgtables(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
 
 static inline bool can_madv_dontneed_vma(struct vm_area_struct *vma)
 {
+	return !(((vma->vm_flags & (VM_LOCKED|VM_LOCKONFAULT)) == VM_LOCKED) ||
+		 (vma->vm_flags & (VM_HUGETLB|VM_PFNMAP)));
+}
+
+static inline bool can_madv_free_vma(struct vm_area_struct *vma)
+{
 	return !(vma->vm_flags & (VM_LOCKED|VM_HUGETLB|VM_PFNMAP));
 }
 
+static inline bool can_madv_dontneed_or_free_vma(struct vm_area_struct *vma,
+						 int behavior)
+{
+	if (behavior == MADV_DONTNEED)
+		return can_madv_dontneed_vma(vma);
+	else if (behavior == MADV_FREE)
+		return can_madv_free_vma(vma);
+	else
+		return 0;
+}
+
 void unmap_page_range(struct mmu_gather *tlb,
 			     struct vm_area_struct *vma,
 			     unsigned long addr, unsigned long end,
diff --git a/mm/madvise.c b/mm/madvise.c
index 4d3c922..61ff306 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -517,7 +517,7 @@ static long madvise_dontneed_free(struct vm_area_struct *vma,
 				  int behavior)
 {
 	*prev = vma;
-	if (!can_madv_dontneed_vma(vma))
+	if (!can_madv_dontneed_or_free_vma(vma, behavior))
 		return -EINVAL;
 
 	if (!userfaultfd_remove(vma, start, end)) {
@@ -539,7 +539,7 @@ static long madvise_dontneed_free(struct vm_area_struct *vma,
 			 */
 			return -ENOMEM;
 		}
-		if (!can_madv_dontneed_vma(vma))
+		if (!can_madv_dontneed_or_free_vma(vma, behavior))
 			return -EINVAL;
 		if (end > vma->vm_end) {
 			/*
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 8ba6cb8..9817d15 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -492,7 +492,7 @@ void __oom_reap_task_mm(struct mm_struct *mm)
 	set_bit(MMF_UNSTABLE, &mm->flags);
 
 	for (vma = mm->mmap ; vma; vma = vma->vm_next) {
-		if (!can_madv_dontneed_vma(vma))
+		if (!can_madv_free_vma(vma))
 			continue;
 
 		/*
-- 
2.7.4