Re: [PATCH 2/9] mm: arm ready for split ptlock

[Jesper Juhl <jesper.juhl@gmail.com>]

On 10/26/05, Russell King <rmk+lkml@arm.linux.org.uk> wrote:
> On Tue, Oct 25, 2005 at 11:00:09AM -0400, Nicolas Pitre wrote:
> > On Tue, 25 Oct 2005, Russell King wrote:
> >
> > > 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.
> >
> > Sure, but the iWMMXt context is stored after the standard sigcontext
> > which also must be 64 bits in size (which might not be always the case
> > if things change in the structure or in its padding).
> >
> > > > 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
> > > > something?
> > >
> > > 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.
> >
> > Argh...  So only suffice to s/pte_write/pte_dirty/ I'd guess?
>
> No.  If we're emulating a cmpxchg() on a clean BSS page, this code
> as it stands today will write to the zero page making it a page which
> is mostly zero.  Bad news when it's mapped into other processes BSS
> pages.
>
> Changing this for pte_dirty() means that we'll refuse to do a cmpxchg()
> on a clean BSS page.  The data may compare correctly, but because it
> isn't already dirty, you'll fail.
>
> If we still had it, I'd say you need to use verify_area() to tell the
> kernel to pre-COW the pages.  However, that got removed a while back.
>

Yes, I removed verify_area() since it was just a wrapper for access_ok().
If verify_area() was/is needed, then access_ok() should be just fine
as a replacement as far as I can see.

--
Jesper Juhl <jesper.juhl@gmail.com>
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