Add Transparent HugePage support on Linux.

Currently, this is unlikely to be enabled for shared memory mappings on
any user's system, so will not make a difference. If
`/sys/kernel/mm/transparent_hugepage/shmem_enabled` is set to `advise`
or higher, however, this commit should enable the use of hugepages. This
can greatly speed up startup for large windows - for a 2880x1800
fullscreen window on my laptop, enabling hugepages halves startup time.

From the kernel mailing lists, it looks like there's some work towards
allowing applications to opt-in to THP support regardless of whether
they're enabled, via `madvise(MADV_COLLAPSE)`, so this may be more
useful in future.

1 files changed, 12 insertions, 0 deletions

diff --git a/src/shm.c b/src/shm.c
index e145d4a..9e0833f 100644
--- a/src/shm.c
+++ b/src/shm.c
@@ -34,7 +34,19 @@ static int create_shm_file(void)
 
 int shm_allocate_file(size_t size)
 {
+#ifdef __linux__
+	/*
+	 * On linux, we can just use memfd_create(). This is both simpler and
+	 * potentially allows usage of Transparent HugePages, which speed up
+	 * the first paint of a large screen buffer.
+	 *
+	 * This isn't available on *BSD, which we could conceivably be running
+	 * on.
+	 */
+	int fd = memfd_create("wl_shm", 0);
+#else
 	int fd = create_shm_file();
+#endif
 	if (fd < 0)
 		return -1;
 	int ret;