Re: [Qemu-devel] [PATCH v2] backends/hostmem-file: Allow to specify full pathname for backing file

[Paolo Bonzini <pbonzini@redhat.com>]

On 28/10/2015 10:54, Pavel Fedin wrote:
> This allows to explicitly specify file name to use with the backend. This
> is important when using it together with ivshmem in order to make it backed
> by hugetlbfs. By default filename is autogenerated using mkstemp(), and the
> file is unlink()ed after creation, effectively making it anonymous. This is
> not very useful with ivshmem because it ends up in a memory which cannot be
> accessed by something else.
> 
> Distinction between directory and file name is done by stat() check. If an
> existing directory is given, the code keeps old behavior. Otherwise it
> creates or opens a file with the given pathname.
> 
> Signed-off-by: Pavel Fedin <p.fedin@samsung.com>
> Tested-by: Igor Skalkin <i.skalkin@samsung.com>
> ---
> v1 => v2:
> - Changed title to more generic one
> - Do not introduce new property, check whether the given path is a
>   directory instead
> ---
>  exec.c        | 34 +++++++++++++++++++++-------------
>  qemu-doc.texi |  2 +-
>  2 files changed, 22 insertions(+), 14 deletions(-)
> 
> diff --git a/exec.c b/exec.c
> index 8af2570..3238c9a 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -1205,6 +1205,7 @@ static void *file_ram_alloc(RAMBlock *block,
>                              const char *path,
>                              Error **errp)
>  {
> +    struct stat st;
>      char *filename;
>      char *sanitized_name;
>      char *c;
> @@ -1233,26 +1234,33 @@ static void *file_ram_alloc(RAMBlock *block,
>          goto error;
>      }
>  
> -    /* Make name safe to use with mkstemp by replacing '/' with '_'. */
> -    sanitized_name = g_strdup(memory_region_name(block->mr));
> -    for (c = sanitized_name; *c != '\0'; c++) {
> -        if (*c == '/')
> -            *c = '_';
> -    }
> +    if (!stat(path, &st) && S_ISDIR(st.st_mode)) {
> +        /* Make name safe to use with mkstemp by replacing '/' with '_'. */
> +        sanitized_name = g_strdup(memory_region_name(block->mr));
> +        for (c = sanitized_name; *c != '\0'; c++) {
> +            if (*c == '/') {
> +                *c = '_';
> +            }
> +        }
>  
> -    filename = g_strdup_printf("%s/qemu_back_mem.%s.XXXXXX", path,
> -                               sanitized_name);
> -    g_free(sanitized_name);
> +        filename = g_strdup_printf("%s/qemu_back_mem.%s.XXXXXX", path,
> +                                   sanitized_name);
> +        g_free(sanitized_name);
> +
> +        fd = mkstemp(filename);
> +        if (fd >= 0) {
> +            unlink(filename);
> +        }
> +        g_free(filename);
> +    } else {
> +        fd = open(path, O_RDWR | O_CREAT, 0644);
> +    }
>  
> -    fd = mkstemp(filename);
>      if (fd < 0) {
>          error_setg_errno(errp, errno,
>                           "unable to create backing store for hugepages");
> -        g_free(filename);
>          goto error;
>      }
> -    unlink(filename);
> -    g_free(filename);
>  
>      memory = ROUND_UP(memory, hpagesize);
>  
> diff --git a/qemu-doc.texi b/qemu-doc.texi
> index 3126abd..460ab71 100644
> --- a/qemu-doc.texi
> +++ b/qemu-doc.texi
> @@ -1299,7 +1299,7 @@ Instead of specifying the <shm size> using POSIX shm, you may specify
>  a memory backend that has hugepage support:
>  
>  @example
> -qemu-system-i386 -object memory-backend-file,size=1G,mem-path=/mnt/hugepages,id=mb1
> +qemu-system-i386 -object memory-backend-file,size=1G,mem-path=/mnt/hugepages/my-shmem-file,id=mb1
>                   -device ivshmem,memdev=mb1
>  @end example
>  
> 

Queued, thanks.

Paolo