Re: [Qemu-devel] [PATCH] mmap-alloc: use same backend for all mappings

[Paolo Bonzini <pbonzini@redhat.com>]

On 30/11/2015 11:51, Greg Kurz wrote:
> Since commit 8561c9244ddf1122d "exec: allocate PROT_NONE pages on top of RAM",
> it is no longer possible to back guest RAM with hugepages on ppc64 hosts:
> 
> mmap(NULL, 285212672, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x3fff57000000
> mmap(0x3fff57000000, 268435456, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED, 19, 0) = -1 EBUSY (Device or resource busy)
> 
> This is due to a limitation on ppc64 that requires MAP_FIXED mappings to have
> the same page size as other mappings already present in the same "slice" of
> virtual address space (Cc'ing Ben for details). This is exactly what happens
> when calling mmap() above: first one uses native host page size (64k) and
> second one uses huge page size (16M).
> 
> To be sure we always have the same page size, let's use the same backend for
> both calls to mmap(): this is enough to fix the ppc64 issue.
> 
> This has no effect on RAM based mappings.
> 
> Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
> ---
> 
> This is a bug fix for 2.5
> 
>  util/mmap-alloc.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
> index c37acbe58ede..0ff221dd94f4 100644
> --- a/util/mmap-alloc.c
> +++ b/util/mmap-alloc.c
> @@ -21,7 +21,8 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, bool shared)
>       * space, even if size is already aligned.
>       */
>      size_t total = size + align;
> -    void *ptr = mmap(0, total, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
> +    void *ptr = mmap(0, total, PROT_NONE,
> +                     (fd == -1 ? MAP_ANONYMOUS : 0) | MAP_PRIVATE, fd, 0);
>      size_t offset = QEMU_ALIGN_UP((uintptr_t)ptr, align) - (uintptr_t)ptr;
>      void *ptr1;
>  
> 

Acked-by: Paolo Bonzini <pbonzini@redhat.com>