On Mon, 30 Nov 2015 15:12:08 +0200 "Michael S. Tsirkin" <m...@redhat.com> wrote:
> On Mon, Nov 30, 2015 at 11:53:41AM +0100, Paolo Bonzini wrote: > > > > > > 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 <gk...@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 <pbonz...@redhat.com> > > But why does this patch have any effect? > I'm worried that extra memory is still allocated > with this, even if it's not accessible. > And you are right because that is exactly what is happening with hugetlbfs_file_mmap()->hugetlb_reserve_pages() :-\ > If yes, we are better off disabling the protection for ppc. > Yes, this is the only alternative... I'll send a patch ASAP. Thanks ! -- Greg