Re: [Qemu-devel] [PATCH v14 1/2] util/mmap-alloc: support MAP_SYNC in qemu_ram_mmap()

[Stefan Hajnoczi]

On Mon, Apr 22, 2019 at 08:48:48AM +0800, Wei Yang wrote:
> From: Zhang Yi <address@hidden>
> 
> 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).
> 
> Current, We have below different possible use cases:
> 
> 1. pmem=on is set, shared=on is set, MAP_SYNC supported:
>    a: backend is a dax supporting file.
>     - MAP_SYNC will active.
>    b: backend is not a dax supporting file.
>     - mmap will trigger a warning. then MAP_SYNC flag will be ignored
> 
> 2. The rest of cases:
>    - we will never pass the MAP_SYNC to mmap2
> 
> Signed-off-by: Haozhong Zhang <address@hidden>
> Signed-off-by: Zhang Yi <address@hidden>
> [ehabkost: Rebased patch to latest code on master]
> Signed-off-by: Eduardo Habkost <address@hidden>
> Signed-off-by: Wei Yang <address@hidden>
> Tested-by: Wei Yang <address@hidden>
> 
> ---
> v14: rebase on top of current upstream
> ---
>  util/mmap-alloc.c | 41 ++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 40 insertions(+), 1 deletion(-)
> 
> diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
> index 9713f4b960..f7f177d0ea 100644
> --- a/util/mmap-alloc.c
> +++ b/util/mmap-alloc.c
> @@ -10,6 +10,13 @@
>   * later.  See the COPYING file in the top-level directory.
>   */
>  
> +#ifdef CONFIG_LINUX
> +#include <linux/mman.h>
> +#else  /* !CONFIG_LINUX */
> +#define MAP_SYNC              0x0
> +#define MAP_SHARED_VALIDATE   0x0
> +#endif /* CONFIG_LINUX */

MAP_SHARED_VALIDATE is is from 2017:

  commit 1c9725974074a047f6080eecc62c50a8e840d050
  Author: Dan Williams <address@hidden>
  Date:   Wed Nov 1 16:36:30 2017 +0100

    mm: introduce MAP_SHARED_VALIDATE, a mechanism to safely define new mmap 
flags

This code fails to compile on Linux hosts with pre-4.15 headers.

Instead you could use the following (even on Linux!):

  #ifndef MAP_SYNC
  #define MAP_SYNC 0
  #endif
  #ifndef MAP_SHARED_VALIDATE
  #define MAP_SHARED_VALIDATE 0
  #endif

Either way:

Reviewed-by: Stefan Hajnoczi <address@hidden>

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