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

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

"Michael S. Tsirkin" <mst@redhat.com> writes:

> 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.
>

Can you explain this more ?

If the question is do we need to pass fd and remove MAP_ANONYMOUS to map
hugetlb, we don't. A good example is
tools/testing/selftest/vm/map_hugetlb.c

If the question is whether we will loose hugepages on mmap even if the
mapping is PROT_NONE, then the answer is we do in the form of hugetlb
reservation.

-aneesh