[PATCH RFC] arm64/vmalloc: use module region only for module_alloc() if CONFIG_RANDOMIZE_BASE is set

[Ard Biesheuvel]

On Tue, 31 Jan 2023 at 16:07, Will Deacon <will at kernel.org> wrote:
>
> Now really adding Ard...
>
> On Tue, Jan 31, 2023 at 03:06:44PM +0000, Will Deacon wrote:
> > +Ard -- full thread here:
> >
> > https://lore.kernel.org/all/20221227092634.445212-1-liushixin2@huawei.com/
> >
> > On Sun, Jan 29, 2023 at 01:41:47PM -0800, Andrew Morton wrote:
> > > On Sun, 29 Jan 2023 10:44:31 +0800 Liu Shixin <liushixin2 at huawei.com> wrote:
> > >
> > > > Hi,
> > > >
> > > >
> > > > This patch seems to have been lost in the corner. Recently I've meet this problem again
> > > >
> > > > on v6.1, so I would like to propose this patch again.
> > > >
> > > >
> > > > Thanks,
> > > >
> > > >
> > > > On 2022/12/27 17:26, Liu Shixin wrote:
> > > > > After I add a 10GB pmem device, I got the following error message when
> > > > > insert module:
> > > > >
> > > > >  insmod: vmalloc error: size 16384, vm_struct allocation failed,
> > > > >  mode:0xcc0(GFP_KERNEL), nodemask=(null),cpuset=/,mems_allowed=0
> > > > >
> > > > > If CONFIG_RANDOMIZE_BASE is set, the module region can be located in the
> > > > > vmalloc region entirely. Although module_alloc() can fall back to a 2GB
> > > > > window if ARM64_MODULE_PLTS is set, the module region is still easily
> > > > > exhausted because the module region is located at bottom of vmalloc region
> > > > > and the vmalloc region is allocated from bottom to top.
> > > > >
> > > > > Skip module region if not calling from module_alloc().
> > > > >
> > >
> > > I'll assume this is for the arm tree.
> > >
> > > Acked-by: Andrew Morton <akpm at linux-foundation.org>
> >
> > This looks like the same issue previously reported at:
> >
> > https://lore.kernel.org/all/e6a804de-a5f7-c551-ffba-e09d04e438fc@hisilicon.com/
> >
> > where Ard had a few suggestions but, afaict, they didn't help.
> >

Thanks for the cc.

So this is a bit clunky, and I wonder whether we wouldn't be better
off just splitting the vmalloc region into two separate regions: one
for the kernel and modules, and one for everything else. That way, we
lose one bit of entropy in the randomized placement, but the default
48-bit VA space is vast anway, and even on 39-bit VA configs (such as
Android), I seriously doubt that we come anywhere close to exhausting
the vmalloc space today.