Re: [Qemu-devel] [PATCH v2] util/mmap-alloc: fix hugetlb support on ppc64

[Rik van Riel <riel@redhat.com>]

On 12/02/2015 03:04 PM, Michael S. Tsirkin wrote:
> Since commit 8561c9244ddf1122d "exec: allocate PROT_NONE pages on top of
> RAM", it is no longer possible to back guest RAM with hugepages on ppc64
> hosts:
> 
> mmap(NULL, 285212672, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) =
> 0x3fff57000000
> mmap(0x3fff57000000, 268435456, PROT_READ|PROT_WRITE,
> MAP_PRIVATE|MAP_FIXED, 19, 0) = -1 EBUSY (Device or resource busy)
> 
> This is because on ppc64, Linux fixes a page size for a virtual address
> at mmap time, so we can't switch a range of memory from anonymous
> small pages to hugetlbs with MAP_FIXED.
> 
> See commit d0f13e3c20b6fb73ccb467bdca97fa7cf5a574cd
> ("[POWERPC] Introduce address space "slices"") in Linux
> history for the details.
> 
> Detect this and create the PROT_NONE mapping using the same fd.
> 
> Naturally, this makes the guard page bigger with hugetlbfs.
> 
> Based on patch by Greg Kurz.
> 
> Cc: Rik van Riel <riel@redhat.com>
> CC: Greg Kurz <gkurz@linux.vnet.ibm.com>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

Acked-by: Rik van Riel <riel@redhat.com>

-- 
All rights reversed