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

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

On Tue, Dec 01, 2015 at 02:31:19PM +0100, Greg Kurz wrote:
> On Tue, 1 Dec 2015 12:57:47 +0200
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > 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.
> > 
> 
> I guess you are talking about the PROT_NONE mapping here ^^.

Yes.

> How do we know that the fd points to hugepages ?

Donnu ... I guess we can just try this if the regular
mmap fails?

> And what's the difference between passing MAP_HUGETLB and passing a
> hugetlbfs backed fd + MAP_NORESERVE ?

Does MAP_NORESERVE have the desired effect?

I need to look at the kernel code, man page merely
mentions swap space use.

> I think the latter is easier
> because we don't need to guess if backend is hugetlbfs.

If this helps, that's fine by me.

It's probably a good idea to set this anyway.

-- 
MST