Re: [2/3] mm: fix up some user-visible effects of the stack guardpage
From: Ian Campbell
Date: Fri Aug 20 2010 - 12:33:19 EST
On Fri, 2010-08-20 at 09:07 -0700, Linus Torvalds wrote:
> Actually, thinking some more about it, that may not be a good idea.
> Why? Simply because we may want to merge the vma's back together if
> you do munlock. And it won't (and mustn't) merge if the vm_flags
> differ in VM_GROWSDOWN.
>
> So I do think we want to keep VM_GROWSDOWN (and VM_GROWSUP on PA-RISC)
> even across a vma split.
I naively hacked something together and it did seem to work, but I
shared your worries about merging.
> Of course, we could set a flag whether the vma really does have a
> guard page or not.
Bits in vma->vm_flags seems to be in rather short supply :-(
> That said, it does strike me as rather odd to do VM ops on partial
> stacks. What are you doing, exactly, to hit this?
I sent a contrived test program in my other mail.
The actual use is to mlock a buffer on the stack in order to pass it to
a Xen hypercall. The contract with the hypervisor is that the pages
passed to the hypercall must be mapped.
Ian.
--
Ian Campbell
Current Noise: Dinosaur Jr. - Green Mind
We can defeat gravity. The problem is the paperwork involved.
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