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

[Greg Kurz]

On Tue, 1 Dec 2015 12:57:47 +0200
"Michael S. Tsirkin" <address@hidden> wrote:

> On Tue, Dec 01, 2015 at 04:23:11PM +0530, Aneesh Kumar K.V wrote:
> > "Michael S. Tsirkin" <address@hidden> 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" <address@hidden> 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 ^^.

How do we know that the fd points to hugepages ?

And what's the difference between passing MAP_HUGETLB and passing a
hugetlbfs backed fd + MAP_NORESERVE ? I think the latter is easier
because we don't need to guess if backend is hugetlbfs.