From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:47522) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1a3RnY-000134-Q4 for qemu-devel@nongnu.org; Mon, 30 Nov 2015 11:59:33 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1a3RnV-0003cb-Ga for qemu-devel@nongnu.org; Mon, 30 Nov 2015 11:59:32 -0500 Received: from mx1.redhat.com ([209.132.183.28]:60685) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1a3RnV-0003cW-9m for qemu-devel@nongnu.org; Mon, 30 Nov 2015 11:59:29 -0500 Date: Mon, 30 Nov 2015 18:59:23 +0200 From: "Michael S. Tsirkin" Message-ID: <20151130185328-mutt-send-email-mst@redhat.com> References: <20151130105044.12269.21261.stgit@bahia.huguette.org> <20151130150353-mutt-send-email-mst@redhat.com> <20151130144631.4736280b@bahia.local> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151130144631.4736280b@bahia.local> Subject: Re: [Qemu-devel] [PATCH] mmap-alloc: use same backend for all mappings List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Greg Kurz Cc: Paolo Bonzini , qemu-devel@nongnu.org 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" wrote: > > > On Mon, Nov 30, 2015 at 11:51:57AM +0100, Greg Kurz 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 due to a limitation on ppc64 that requires MAP_FIXED mappings to have > > > the same page size as other mappings already present in the same "slice" of > > > virtual address space (Cc'ing Ben for details). > > > > I'd like some details please. > > What do you mean when you say "same page size" and "slice"? > > > > 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. > Hugepage support is implemented using an abstraction of segments called > "slices". Here's a quote from the related commit changelog in the kernel > tree: > > commit d0f13e3c20b6fb73ccb467bdca97fa7cf5a574cd > Author: Benjamin Herrenschmidt > Date: Tue May 8 16:27:27 2007 +1000 > > [POWERPC] Introduce address space "slices" > > ... > > The main issues are: > > - To maintain/keep track of the page size per "segment" (as we can > only have one page size per segment on powerpc, which are 256MB > divisions of the address space). > > - To make sure special mappings stay within their allotted > "segments" (including MAP_FIXED crap) > > - To make sure everybody else doesn't mmap/brk/grow_stack into a > "segment" that is used for a special mapping > ... > > > > This is exactly what happens > > > when calling mmap() above: first one uses native host page size (64k) and > > > second one uses huge page size (16M). > > > > > > To be sure we always have the same page size, let's use the same backend for > > > both calls to mmap(): this is enough to fix the ppc64 issue. > > > > > > This has no effect on RAM based mappings. > > > > > > Signed-off-by: Greg Kurz > > > --- > > > > > > This is a bug fix for 2.5 > > > > > > util/mmap-alloc.c | 3 ++- > > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > > > diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c > > > index c37acbe58ede..0ff221dd94f4 100644 > > > --- a/util/mmap-alloc.c > > > +++ b/util/mmap-alloc.c > > > @@ -21,7 +21,8 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, bool shared) > > > * space, even if size is already aligned. > > > */ > > > size_t total = size + align; > > > - void *ptr = mmap(0, total, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0); > > > + void *ptr = mmap(0, total, PROT_NONE, > > > + (fd == -1 ? MAP_ANONYMOUS : 0) | MAP_PRIVATE, fd, 0); > > > size_t offset = QEMU_ALIGN_UP((uintptr_t)ptr, align) - (uintptr_t)ptr; > > > void *ptr1; > > > > >