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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Dec 01, 2015 at 04:23:11PM +0530, Aneesh Kumar K.V wrote:
> "Michael S. Tsirkin" <mst@redhat.com> writes:
> 
> > On Mon, Nov 30, 2015 at 02:46:31PM +0100, Greg Kurz wrote:
> >> On Mon, 30 Nov 2015 15:06:33 +0200
> >> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> >> 
> 
> 
> ....
> >> 
> >> On ppc64, the address space is divided in 256MB-sized segments where all pages
> >> have the same size. This is a hw limitation IIUC. I don't know if it can be
> >> fixed and I'll let Ben comment on it.
> >
> > But it's anonymous memory with PROT_NONE.  There should be no pages there:
> > just a chunk of virtual memory reserved.
> >
> 
> ppc64 use page size (called as base page size) to find the hash slot in
> which we find the virtual address to real address translation. All the
> pages in a segment should have same base page size. Hugetlb pages have a
> base page size of 16M whereas a regular linux page have 64K. mmap will
> fail to map a hugetlb mapping in a segment that already have regular
> pages mapped.
> 
> -aneesh


I see this in kernel:

       } else if (flags & MAP_HUGETLB) {
                struct user_struct *user = NULL;
                struct hstate *hs;

                hs = hstate_sizelog((flags >> MAP_HUGE_SHIFT) & SHM_HUGE_MASK);
                if (!hs)
                        return -EINVAL;

                len = ALIGN(len, huge_page_size(hs));
                /*
                 * VM_NORESERVE is used because the reservations will be
                 * taken when vm_ops->mmap() is called
                 * A dummy user value is used because we are not locking
                 * memory so no accounting is necessary
                 */
                file = hugetlb_file_setup(HUGETLB_ANON_FILE, len,
                                VM_NORESERVE,
                                &user, HUGETLB_ANONHUGE_INODE,
                                (flags >> MAP_HUGE_SHIFT) & MAP_HUGE_MASK);
                if (IS_ERR(file))
                        return PTR_ERR(file);
        }

So maybe it's a question of passing in MAP_HUGETLB and the
correct size mask.

-- 
MST