Re: [PATCH v1 06/13] util/mmap-alloc: Factor out reserving of a memory region to mmap_reserve()

[Richard Henderson]

On 2/3/20 6:31 PM, David Hildenbrand wrote:
> We want to reserve a memory region without actually populating memory.
> Let's factor that out.
> 
> Cc: "Michael S. Tsirkin" <address@hidden>
> Cc: Greg Kurz <address@hidden>
> Cc: Murilo Opsfelder Araujo <address@hidden>
> Cc: Eduardo Habkost <address@hidden>
> Cc: "Dr. David Alan Gilbert" <address@hidden>
> Signed-off-by: David Hildenbrand <address@hidden>
> ---
>  util/mmap-alloc.c | 58 +++++++++++++++++++++++++++--------------------
>  1 file changed, 33 insertions(+), 25 deletions(-)
> 
> diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
> index 82f02a2cec..43a26f38a8 100644
> --- a/util/mmap-alloc.c
> +++ b/util/mmap-alloc.c
> @@ -82,6 +82,38 @@ size_t qemu_mempath_getpagesize(const char *mem_path)
>      return qemu_real_host_page_size;
>  }
>  
> +/*
> + * 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)
> +{
> +    int flags = MAP_PRIVATE;
> +
> +#if defined(__powerpc64__) && defined(__linux__)
> +    /*
> +     * On ppc64 mappings in the same segment (aka slice) must share the same
> +     * page size. Since we will be re-allocating part of this segment
> +     * from the supplied fd, we should make sure to use the same page size, 
> to
> +     * this end we mmap the supplied fd.  In this case, set MAP_NORESERVE to
> +     * avoid allocating backing store memory.
> +     * We do this unless we are using the system page size, in which case
> +     * anonymous memory is OK.
> +     */
> +    if (fd == -1 || qemu_fd_getpagesize(fd) == qemu_real_host_page_size) {
> +        fd = -1;
> +        flags |= MAP_ANONYMOUS;
> +    } else {
> +        flags |= MAP_NORESERVE;
> +    }
> +#else
> +    fd = -1;
> +    flags |= MAP_ANONYMOUS;
> +#endif

Because this is just code movement,
Reviewed-by: Richard Henderson <address@hidden>

But is there a reason not to add MAP_NORESERVE all of the time?
It seems to match intent, in that we're reserving vma but not planning to use
the memory, anonymous or not.


r~