Re: [patch 4/6] mm: merge populate and nopage into fault (fixes nonlinear)

Blaisorblade
2007-03-21T19:46:10Z

On Tuesday 20 March 2007 07:00, Nick Piggin wrote:
> On Mon, Mar 19, 2007 at 09:44:28PM +0100, Blaisorblade wrote:
> > On Sunday 18 March 2007 03:50, Nick Piggin wrote:
> > > > > Yes, I believe that is the case, however I wonder if that is going
> > > > > to be a problem for you to distinguish between write faults for
> > > > > clean writable ptes, and write faults for readonly ptes?

> > > > I wouldn't be able to distinguish them, but am I going to get write
> > > > faults for clean ptes when vma_wants_writenotify() is false (as seems
> > > > to be for tmpfs)? I guess not.

> > > > For tmpfs pages, clean writable PTEs are mapped as writable so they
> > > > won't give any problem, since vma_wants_writenotify() is false for
> > > > tmpfs. Correct?

> > > Yes, that should be the case. So would this mean that nonlinear
> > > protections don't work on regular files?

> > They still work in most cases (including for UML), but if the initial
> > mmap() specified PROT_WRITE, that is ignored, for pages which are not
> > remapped via remap_file_pages(). UML uses PROT_NONE for the initial mmap,
> > so that's no problem.

> But how are you going to distinguish a write fault on a readonly pte for
> dirty page accounting vs a read-only nonlinear protection?

Hmm... I was only thinking to PTEs which hadn't been remapped via
remap_file_pages, but just faulted in with initial mmap() protection.

For the other PTEs, however, I overlooked that the current code ignores
vma_wants_writenotify(), i.e. breaks dirty page accounting for them, and I
refused to even consider this opportunity, even without knowing the purposes
of dirty pages accounting (I found the commits explaining this however).

> You can't store any more data in a present pte AFAIK, so you'd have to
> have some out of band data. At which point, you may as well just forget
> about vma_wants_writenotify vmas, considering that everybody is using
> shmem/ramfs.

I was going to do that anyway. I'd guess that I should just disallow in
remap_file_pages() the VM_MANYPROTS (i.e. MAP_CHGPROT in flags) &&
vma_wants_writenotify() combination, right? Ok, trivial (shouldn't even have
pointed this out).
--
Inform me of my mistakes, so I can add them to my list!
Paolo Giarrusso, aka Blaisorblade
http://www.user-mode-linux.org/~blaisorblade
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