Subject: Re: [PATCH 1/1] Fixup write permission of TLB on powerpc e500 core

From: Peter Zijlstra <[email-masked]>
Date: Fri, 15 Jul 2011 17:24:37 +0200

On Fri, 2011-07-15 at 11:18 -0400, Shan Hai wrote:

> >> +	vma =3D find_vma(mm, address);
> > Uhm, find_vma() needs mmap_sem, and futex_atomic_cmpxchg_inatomic() is
> > most certainly not called with that lock held.
> >
>=20
> My fault, that will be fixed in the V2 patch.

But you cannot, the function isn't called _atomic_ just for kicks, its
used while holding spinlocks.

> >> +	if (likely(vma)) {
> >> +		/* only fixup present page */
> >> +		if (follow_page(vma, address, FOLL_WRITE)) {
> >> +			handle_mm_fault(mm, vma, address, FAULT_FLAG_WRITE);
> > So how can this toggle your sw dirty/young tracking, that's pretty much
> > what gup(.write=3D1) does too!
> >
>=20
> because of the kernel read only permission of the page is transparent
> to the follow_page(),  the handle_mm_fault() is not to be activated
> in the __get_use_pages(), so the gup(.write=3D1) could not help to fixup
> the write permission.

So why do you need the vma? Is it like I wrote earlier that you don't
have spare PTE bits and need the vma flags to see if it may become
writable?

gup(.write=3D1) not triggering this is a serious problem though, not
something you can just paper over. I wouldn't be at all surprised to
find there's more things broken because of that.