Re: [PATCH 2/9] mm: arm ready for split ptlock
From: Russell King
Date: Tue Oct 25 2005 - 02:56:11 EST
On Mon, Oct 24, 2005 at 10:45:04PM -0400, Nicolas Pitre wrote:
> On Sat, 22 Oct 2005, Russell King wrote:
> > Please contact Nicolas Pitre about that - that was my suggestion,
> > but ISTR apparantly the overhead is too high.
> Going through a kernel buffer will simply double the overhead. Let's
> suppose it should not be a big enough issue to stop the patch from being
> merged though (and it looks cleaner that way). However I'd like for the
> WARN_ON((unsigned long)frame & 7) to remain as both the kernel and user
> buffers should be 64-bit aligned.
The WARN_ON is pointless because we guarantee that the stack is always
64-bit aligned on signal handler setup and return.
> I don't see how standard COW could not happen. The only difference with
> a true write fault as if we used put_user() is that we bypassed the data
> abort vector and the code to get the FAR value. Or am I missing
pte_write() just says that the page _may_ be writable. It doesn't say
that the MMU is programmed to allow writes. If pte_dirty() doesn't
return true, that means that the page is _not_ writable from userspace.
If you write to it from kernel mode (without using put_user) you'll
bypass the MMU read-only protection and may end up writing to a page
owned by two separate processes.
Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/
maintainer of: 2.6 Serial core
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