memfd: add hugetlb support

Linux commit 749df87bd7bee5a79cef073f5d032ddb2b211de8 (v4.14-rc1)
added a new flag MFD_HUGETLB to memfd_create() that specify the file
to be created resides in the hugetlbfs filesystem.  This is the
generic hugetlbfs filesystem not associated with any specific mount
point.

hugetlbfs does not support sealing operations in v4.14, therefore
specifying MFD_ALLOW_SEALING with MFD_HUGETLB will result in EINVAL.

However, I added sealing support in "[PATCH v3 0/9] memfd: add sealing
to hugetlb-backed memory" series, queued in -mm tree for v4.16.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180201132757.23063-3-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/qemu/memfd.h

@@ -16,8 +16,8 @@
 #define F_SEAL_WRITE    0x0008  /* prevent writes */
 #endif
 
-int qemu_memfd_create(const char *name, size_t size, unsigned int seals,
-                      Error **errp);
+int qemu_memfd_create(const char *name, size_t size, bool hugetlb,
+                      unsigned int seals, Error **errp);
 void *qemu_memfd_alloc(const char *name, size_t size, unsigned int seals,
                        int *fd, Error **errp);
 void qemu_memfd_free(void *ptr, size_t size, int fd);

util/memfd.c

@@ -52,7 +52,11 @@ static int memfd_create(const char *name, unsigned int flags)
 #define MFD_ALLOW_SEALING 0x0002U
 #endif
 
-int qemu_memfd_create(const char *name, size_t size,
+#ifndef MFD_HUGETLB
+#define MFD_HUGETLB 0x0004U
+#endif
+
+int qemu_memfd_create(const char *name, size_t size, bool hugetlb,
                       unsigned int seals, Error **errp)
 {
 #ifdef CONFIG_LINUX
@@ -62,6 +66,9 @@ int qemu_memfd_create(const char *name, size_t size,
     if (seals) {
         flags |= MFD_ALLOW_SEALING;
     }
+    if (hugetlb) {
+        flags |= MFD_HUGETLB;
+    }
 
     mfd = memfd_create(name, flags);
     if (mfd < 0) {
@@ -97,11 +104,11 @@ void *qemu_memfd_alloc(const char *name, size_t size, unsigned int seals,
                        int *fd, Error **errp)
 {
     void *ptr;
-    int mfd = qemu_memfd_create(name, size, seals, NULL);
+    int mfd = qemu_memfd_create(name, size, false, seals, NULL);
 
     /* some systems have memfd without sealing */
     if (mfd == -1) {
-        mfd = qemu_memfd_create(name, size, 0, NULL);
+        mfd = qemu_memfd_create(name, size, false, 0, NULL);
     }
 
     if (mfd == -1) {