When a file supporting DAX is used as vNVDIMM backend, mmap it with MAP_SYNC flag in addition can guarantee the persistence of guest write to the backend file without other QEMU actions (e.g., periodic fsync() by QEMU).
By using MAP_SHARED_VALIDATE flag with MAP_SYNC, we can ensure mmap with MAP_SYNC fails if MAP_SYNC is not supported by the kernel or the backend file. On such failures, QEMU retries mmap without MAP_SYNC and MAP_SHARED_VALIDATE. Signed-off-by: Haozhong Zhang <[email protected]> --- util/mmap-alloc.c | 36 +++++++++++++++++++++++++++++++++++- 1 file changed, 35 insertions(+), 1 deletion(-) diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c index 2fd8cbcc6f..37b302f057 100644 --- a/util/mmap-alloc.c +++ b/util/mmap-alloc.c @@ -18,7 +18,18 @@ #ifdef CONFIG_LINUX #include <sys/vfs.h> + +/* + * MAP_SHARED_VALIDATE and MAP_SYNC were introduced in 4.15 kernel, so + * they may not be defined when compiling on older kernels. + */ +#ifndef MAP_SHARED_VALIDATE +#define MAP_SHARED_VALIDATE 0x3 #endif +#ifndef MAP_SYNC +#define MAP_SYNC 0x80000 +#endif +#endif /* CONFIG_LINUX */ size_t qemu_fd_getpagesize(int fd) { @@ -97,6 +108,7 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, bool shared) #endif size_t offset; void *ptr1; + int xflags = 0; if (ptr == MAP_FAILED) { return MAP_FAILED; @@ -107,12 +119,34 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, bool shared) assert(align >= getpagesize()); offset = QEMU_ALIGN_UP((uintptr_t)ptr, align) - (uintptr_t)ptr; + +#if defined(__linux__) + /* + * If 'fd' refers to a file supporting DAX, mmap it with MAP_SYNC + * will guarantee the guest write persistence without other + * actions in QEMU (e.g., fsync() in QEMU). + * + * MAP_SHARED_VALIDATE ensures mmap with MAP_SYNC fails if + * MAP_SYNC is not supported by the kernel or the file. + * + * On failures of mmap with xflags, QEMU will retry mmap without + * xflags. + */ + xflags = shared ? (MAP_SHARED_VALIDATE | MAP_SYNC) : 0; +#endif + + retry_mmap_fd: ptr1 = mmap(ptr + offset, size, PROT_READ | PROT_WRITE, MAP_FIXED | (fd == -1 ? MAP_ANONYMOUS : 0) | - (shared ? MAP_SHARED : MAP_PRIVATE), + (shared ? MAP_SHARED : MAP_PRIVATE) | xflags, fd, 0); if (ptr1 == MAP_FAILED) { + if (xflags) { + xflags = 0; + goto retry_mmap_fd; + } + munmap(ptr, total); return MAP_FAILED; } -- 2.14.1
