[PATCH] util/mmap: optimize qemu_ram_mmap() alignment

[Steve Sistare]

Guest RAM created with memory-backend-memfd is aligned to a
QEMU_VMALLOC_ALIGN=2M boundary, and memory-backend-memfd does not support
the "align" parameter to change the default.  This is sub-optimal on
aarch64 kernels with a 64 KB page size and 512 MB huge page size, as the
range will not be backed by huge pages.  Moreover, any shared allocation
using qemu_ram_mmap() will be sub-optimal on such a system if the align
parameter is less than 512 MB.

The kernel is smart enough to return a hugely aligned pointer for MAP_SHARED
mappings when /sys/kernel/mm/transparent_hugepage/shmem_enabled allows it.
However, the qemu function qemu_ram_mmap() mmap's twice to perform its own
alignment:

    guardptr = mmap(0, total, PROT_NONE, flags, ...
    flags |= shared ? MAP_SHARED : MAP_PRIVATE;
    ptr = mmap(guardptr + offset, size, prot, flags | map_sync_flags, ...

On the first call, flags has MAP_PRIVATE, hence the kernel does not apply
its shared memory policy, and returns a non-huge-aligned guardptr.

To fix, for shared mappings, pass MAP_SHARED to both mmap calls.

Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
---
 util/mmap-alloc.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
index 5ed7d29..37a0d1e 100644
--- a/util/mmap-alloc.c
+++ b/util/mmap-alloc.c
@@ -121,7 +121,7 @@ static bool map_noreserve_effective(int fd, uint32_t 
qemu_map_flags)
  * Reserve a new memory region of the requested size to be used for mapping
  * from the given fd (if any).
  */
-static void *mmap_reserve(size_t size, int fd)
+static void *mmap_reserve(size_t size, int fd, int final_flags)
 {
     int flags = MAP_PRIVATE;
 
@@ -144,6 +144,7 @@ static void *mmap_reserve(size_t size, int fd)
 #else
     fd = -1;
     flags |= MAP_ANONYMOUS;
+    flags |= final_flags & MAP_SHARED;
 #endif
 
     return mmap(0, size, PROT_NONE, flags, fd, 0);
@@ -232,7 +233,7 @@ void *qemu_ram_mmap(int fd,
      */
     total = size + align;
 
-    guardptr = mmap_reserve(total, fd);
+    guardptr = mmap_reserve(total, fd, qemu_map_flags);
     if (guardptr == MAP_FAILED) {
         return MAP_FAILED;
     }
-- 
1.8.3.1