Re: [git pull] changes for tip, and a nasty x86 page table bug

Linus Torvalds
2009-02-20T04:18:16Z

On Thu, 19 Feb 2009, Steven Rostedt wrote:
>
> Is this something worthy of 29? I could whip up a patch against your
> latest tree.

I think it's a real issue, but I do have to admit that I don't see why it
would only trigegr for you. Is it just because the trace stuff ends up
setting pages to RW, and you have to have had a lot of read-only stuff to
get a whole read-only PMD to begin with?

So there's two things that make me nervous:

- I do think the KERNPG_TABLE thing is the right thing, and I _think_
that code is just confused, and we should just do KERNPG_TABLE rather
than play with confused bits one by one (PRESENT, RW, NX) to the point
of just making for more confusion.

But I'd like some of the people involved with that code confirm that.
Either a "Yeah, we were just confused" or "No, there's this really
subtle thing going on, liek this: ..."

- The fact that apparently you're the first one to hit this. I realize
that you do odd things with ftrace. Was it the fact that you made the
"set_memory_ro()" area larger, and then more dynamically mark it back
to read-write that you hit it? Haven't we done things like that before?

But that said, I'd love to fix this for 2.6.29, especially if somebody
can resolve the two worries above. I do _not_ want to take your patch that
makes confused code even more confused, unless somebody really explains
why a pure KERNPG_TABLE isn't right.

Linus