Re: [Qemu-devel] [PATCH V8 3/5] util/mmap-alloc: support MAP_SYNC in qemu_ram_mmap()

[Eduardo Habkost <ehabkost@redhat.com>]

On Wed, Jan 02, 2019 at 01:26:15PM +0800, Zhang Yi wrote:
> When a file supporting DAX is used as vNVDIMM backend, mmap it with
> MAP_SYNC flag in addition which can ensure file system metadata
> synced in each guest writes to the backend file, without other QEMU
> actions (e.g., periodic fsync() by QEMU).
> 
> Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> Signed-off-by: Zhang Yi <yi.z.zhang@linux.intel.com>
> ---
>  include/qemu/osdep.h | 16 ++++++++++++++++
>  util/mmap-alloc.c    | 12 +++++++++++-
>  2 files changed, 27 insertions(+), 1 deletion(-)
> 
> diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> index 3bf48bc..bb1eba1 100644
> --- a/include/qemu/osdep.h
> +++ b/include/qemu/osdep.h
> @@ -410,6 +410,22 @@ void qemu_anon_ram_free(void *ptr, size_t size);
>  #  define QEMU_VMALLOC_ALIGN getpagesize()
>  #endif
>  
> +/*
> + * MAP_SHARED_VALIDATE and MAP_SYNC are introduced in Linux kernel
> + * 4.15, so they may not be defined when compiling on older kernels.
> + */
> +#ifdef CONFIG_LINUX
> +
> +#include <asm-generic/mman.h>
> +
> +#ifndef MAP_SYNC
> +#define MAP_SYNC 0x0
> +#endif
> +
> +#else  /* !CONFIG_LINUX */
> +#define MAP_SYNC              0x0
> +#endif /* CONFIG_LINUX */
> +
>  #ifdef CONFIG_POSIX
>  struct qemu_signalfd_siginfo {
>      uint32_t ssi_signo;   /* Signal number */
> diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
> index 8f0a740..a9d5e56 100644
> --- a/util/mmap-alloc.c
> +++ b/util/mmap-alloc.c
> @@ -99,6 +99,8 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, uint32_t flags)
>      void *ptr = mmap(0, total, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
>  #endif
>      bool shared = flags & RAM_SHARED;
> +    bool is_pmem = flags & RAM_PMEM;
> +    int mmap_xflags = 0;
>      size_t offset;
>      void *ptr1;
>  
> @@ -109,13 +111,21 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, uint32_t flags)
>      assert(is_power_of_2(align));
>      /* Always align to host page size */
>      assert(align >= getpagesize());
> +    if (shared && is_pmem) {
> +        mmap_xflags |= MAP_SYNC;
> +    }
>  
>      offset = QEMU_ALIGN_UP((uintptr_t)ptr, align) - (uintptr_t)ptr;
> + 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) | mmap_xflags,
>                  fd, 0);
> +    if ((ptr1 == MAP_FAILED) && (mmap_xflags & MAP_SYNC)) {
> +        mmap_xflags &= ~MAP_SYNC;
> +        goto retry_mmap_fd;

Do we have use cases where using pmem=on without MAP_SYNC isn't
going to cause problems?  If not, shouldn't we at least print a
warning here?  Otherwise, won't we still need an option for cases
that require MAP_SYNC to be working?

> +    }

-- 
Eduardo