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Re: [Qemu-devel] [PATCH] mmap-alloc: use same backend for all mappings
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [PATCH] mmap-alloc: use same backend for all mappings |
Date: |
Tue, 1 Dec 2015 12:57:47 +0200 |
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.
--
MST